home
search
categories
the physicist
groundrules
answers
older answers
donate

FAQ

 

Old Questions and Answers

Here are older questions and answers processed by "Ask the Physicist!"; older still answers may be found at http://www.askthephysicist.com/ask_phys_q&a_old.html and http://www.askthephysicist.com/ask_phys_q&a_older.html  . If you like my answer, please consider making a donation to help support this service.


QUESTION: 
Why do we only see 'red hot' and 'white hot' radiation? Why doesn't it run through the spectrum showing 'green hot' or 'blue hot'?

ANSWER: 
A hot body does not emit one color but a continuous spectrum of wavelengths. An object a room temperature radiates most of the energy in the infrared part of the spectrum so you cannot see it in a dark room. But, as the temperature is increased (to around 1500-30000) the radiated energy shifts so that a significant amount of the energy begins to be emitted at the lowest energy part of the visible spectrum, red, and not much at higher energies. At higher temperatures, (up to about 60000 or 80000) the energy is spread out over all the parts of the visible spectrum so the source appears white. At higher yet temperatures the spectrum is skewed toward the ultraviolet part of the spectrum and the source will take on a bluish hue. A nice little calculator where you can see the spectrum in comparison to the visible spectrum can be seen here.


QUESTION: 
Working on an invention and I have a question please. Envision a pingpong ball with a 3 ounce of weight attached. Dropped in water by itself the weight will drop immediately to the bottom. Attach the pingpong ball to the weight and drop the set in the water and the weight will fall more slowly because of the buoyancy of the ball. Now is the question. If one could somehow inject more air, helium, nitrogen, whatever...into the hard plastic ball, without exploding it, would this actually increase air pressure inside the ball making it more buoyant than a ball that was just produced normally with the air just trapped inside. I'm truly looking forward to your response and I greatly appreciate your time.

ANSWER: 
There is a slight semantic issue here. Buoyant force on something is equal to the weight of the displaced fluid (Archimedes' principle). A lead ball and a ping pong ball, both fully submerged, are equally buoyant in that they have equal buoyant forces on them. But I think that you are asking whether your pressurized ball will be less likely to sink, right? The answer is that it will be more likely to sink because its weight increases.


QUESTION: 
When you shoot an arrow at a target, the arrow eventually reaches the halfway point between you and the target. Then it eventually reaches the halfway point of the distance between the previous halfway point and the target. It keeps doing this as it travels toward the target; the distance between the arrow and the target continues reducing by half. So why isn't the arrow's path asymptotic, never reaching the target? It may seem like I am asking an off-the-wall question, but if the distance between the arrow and the target keeps reducing by half infinitely, why does the arrow eventually reach the target?

ANSWER: 
As you may know, this is often called "Zeno's paradox". Suppose that the speed of the arrow is 100 m/s and the target is 100 m away. Then the time it takes to go the first half way is 1/2 s, the second half time takes 1/4 s, the third half time takes 1/8 s, etc. So the total time it takes is the infinite series (1/2+1/4+1/8+1/16+1/32+
…). Although this is an infinite series it has a finite sum and that sum is, guess what, 1 s. So, when viewed this way, you take an infinite number of steps but in a finite amount of time. You can read more about the history and details of Zeno's paradox on Wikepedia.


QUESTION: 
I have a question about finding the distance that a spring has been stretch using Hooke's Law vs. conservation of energy and the elastic potential energy equation. When hanging a known mass from a spring, I am able to calculate the distanced stretched by determining the force due to gravity of the mass and using d = F/k (derived from k = F/d). I can also determing the distanced stretched by determining the energy stored through gravitational potential energy before the mass settles and transferring it to energy stored through elastical energy after the mass settles. For this I use mgh = PEgravity = PEelastic = 0.5kx^2, or mg = 0.5kx. The problem that I am finding is that the estimated distance using Hooke's Law is always twice the estimated distance using the conservation of energy. Which am I doing wrong? Which one is right?

ANSWER: 
Your first one is right, k=mg/d. In your second determination, if you put the mass at its equilibrium position (at x=d) you must do work on the system and you cannot use energy conservation; it will not go there on its own. If you let it drop from the unstretched spring position, the mass will have its maximum kinetic energy as it passes throught x=d and it will continue going down until it gets to x=2d where it will be momentarily at rest, then go back up, etc. So, in your second equation is correct except x should be 2d as you found.


QUESTION: 
my question is regarding inductors i recently studied about inductors and i got really confused it was given that when a voltage is applied across an inductor an induced emf was generated which opposed the applied voltage also if the resistance of the coil and the wires was zero then the induced emf was exactly equal in magnitude to the applied voltage and thus the applied voltage and the induced emf were equal and opposite to each other .now if the two voltages are equal and opposite in magnitude their vector sum should be zero and no current should flow through the inductor however a current was shown flowing through the inductor i am really confused please help me

ANSWER: 
The essential part which you are missing is that the induced emf depends on the rate of change of the current through the inductor. So, if you just hook it up to a battery there will be a constant current after a short time. When you first attach the battery, current will start to flow and so there will be an induced emf (but smaller than the emf of the battery) and as the current gets bigger it will change more slowly until, eventually, a constant current will flow. (The preceding assumes that the inductor itself has some resistance or else a battery would cause infinite current to flow.) Now, if you connect the inductor to an AC source the current will always be changing and so there will always be an induced back emf which will impede but not stop current flow. The higher the frequency of the AC source, the faster the current will be changing, so the smaller the current through the inductor will be.


QUESTION: 
Einstein's famous E=Mc2 doesn't seem to hold for a photon which is massless but has energy. It doesn't even hold for the creation of energy since the photon is not created by annilating mass, but rather by an electron shifting orbit. Further, the photon doesn't convert into mass when absorbed. What am I missing?

ANSWER: 
I often get this question. It originates with taking a famous equation and not understanding when it is applicable. E=mc2 is the energy of a particle of mass m at rest; a photon is never at rest and therefore this equation is not applicable to it. The energy of any particle is E=
√[m2c4+p2c2] where p is the linear momentum. Note that if p=0, the particle is at rest and indeed E=mc2. If m=0 then E=pc. Massless particles have momentum. The only massless particle we know is the photon which has an energy E=hf where h is Planck's constant and f is the frequency. So the momentum of a photon is hf/c. Regarding the rest of your question, when an electron drops to a lower orbit the mass of the atom decreases by exactly the right amount to supply the energy to the photon. When a photon is absorbed by an atom, conversely, the atom becomes excited and therefore more massive.

FOLLOWUP QUESTION: 
In a previous answer you stated the mass of an electron decreases as it shifts to a lower orbit and releases a quantum (hf) of energy. How does this mass change occur? Does the electron divide itself and become smaller in order to shed the mass required for energy release? What's the mechanism at work here?

ANSWER: 
I did not say that the electron mass decreases. I said that the mass of the atom decreases. A bound system always has a mass less than the sum of the masses of its parts. Consider a system of two bound masses, each of mass m when alone. Their bound mass is less than 2m. You can say this because it takes work to pull them apart. Where does this work go? Into mass. In atoms, it is almost impossible to measure the mass difference because it is so small. But in nuclei, where the binding is much stronger, it is measurable. The mass of a 4He nucleus is measurably smaller than the masses of two protons and two neurtons.


QUESTION: 
If a spacecraft travelling at half the speed of light had a laser beam shot at it by another spacecraft that was following at half the speed of light, then a stationary observer would see this laser beam shoot passed them at the speed of light. Also the observers on both spacecraft would see the laser beam shoot between them at the speed of light. It is said that time has slowed to half the speed for those travelling at half the speed of light, so this difference in the perceived speed of light is caused by this time difference. However if the spacecraft in front shot back what speed would everyone see the laser beam travel? I heard Einstein said everyone would perceive it as been the speed of light, but this doesn’t make sense with the explanation that time slows for those travelling at speed. The stationary observer would see the laser beam shooting passed him at the speed of light, but would also see the craft in front travelling in the opposite direction at half the speed of light. This gives a perceived difference between the two from his position as one and a half times the speed of light. For those on the spacecraft to also perceive the laser beam shoot back at the speed of light, their time must be accelerated by one and a half times. So which is right has time been halved for those on the spacecraft, or has it been accelerated by one and a half times. Or do the people on the spacecraft experience both depending whether the laser beams are been shot forwards or backwards?

ANSWER: 
You have it backwards: time slows down because the speed of light is the same to all observers, not the other way around. Also, time has not slowed down (relative to a stationary observer) by half for the observer moving at half the speed of light, it is only about 13% slower. All the rest of your question is simply thrashing around trying to find a situation where somebody will see light traveling at a different speed, and that simply does not happen. To see my earlier discussions on the constancy of the speed of light, go here.


QUESTION: 
A generator produces electricity by using magnetic fields to cause the flow of electrons down a wire, if that energy is used, how do the electrons get replenished in the system? Where do they come from? Do they just spontaneously manifest into existence inside the coiled wire? They have to come from somewhere, if there is a flow going out, there must be a flow coming in right?

ANSWER: 
The wires are conductors. A conductor is usually a metal which conducts electricity easily. Because of the atomic structure and the way the solid forms, there is typically one electron for each atom which is free to move around in the wire. If it is just a piece of wire sitting in your hand, there are just as many electrons going one way as the other and the net flow of electrons (current) is zero. But, if you cause there to be voltage across the wire (by attaching it to a battery or by having a time changing magnetic field as in a generator), more electrons will move in one direction than the other and you will have a current. In the case of a generator, no current will flow unless there is a complete circuit, for example by connecting the ends of the wire to some device or to each other; that obviates the necessity to as where are they coming from or going to since they simply circulate round and round the circuit.


QUESTION: 
I have lived near a railroad track crossing for 2.5 years. Recently, the train whistles seem to be louder. I seem to remember this happening about a year ago, and then it seemed to get better. It occurs to me that there may be a scientific explanation for this. Is it possible that since the weather here in Houston, TX has cooled off that the sound, as observed from my house, seems louder? I know that cooler, denser air will transmit sound (pressure waves) more efficiently, but I am wondering of a decrease of 20 degF would make a significant difference at an approximate distance of 500 feet from the tracks.

ANSWER: 
The speed of sound in cooler air is higher, but what you are interested in is the attenuation of the sound. There are two things to think about, temperature and humidity. There is a nice little calculator at this link. If you mess around with it a little, you will see that the attenuation of the sound is not very dependent on the temperature but it more dependent on the humidity. For example, if the frequency is 4 kHz, attenuation is about 26 dB/km for 50% humidity at 350C (about 950F) and about 30 dB/km for 50% humidity at 200C (about 680F); this is not very different but should be slightly louder at higher temperatures, not what you observe. However, humidities tend to be higher at higher temperatures (particularly in Houston), so if we calculate at the extremes, attenuation is about 29 dB/km for 100% humidity at 350C (about 950F) and about 4 dB/km for 0% humidity at 200C (about 680F); this is a significant difference and in accord with your observations. For overall comparison, a train whistle at 500 feet is about 80 dB, so 30 dB/km is not an insignificant loss rate. Now, I cannot guarantee that this is the explanation. Sometimes other atmospheric conditions can affect the sound. For example, if there is a temperature inversion with a warm layer of air above the cooler ground air, sound may be reflected off the boundary and reach you where it would not have without the inversion.


QUESTION: 
what happens if a human being can travel the speed of a bullet and how will it effect the human ?

ANSWER: 
The speed of a bullet is about 2000 miles/hour. The speed of the space shuttle is about 18,000 miles/hour. Traveling at a high speed is not harmful to a human, only large acceleration is harmful (see question just below).


QUESTION: 
hello, i recently had an idea for a spacecraft, pretty much an object is fired at 3/5 the speed of light, this is attached by cables to the spacecraft itself. By The Way this is all done in space, Back To The Point, anyway would there be G-forces involved and if they were would they be strong enough to kill you?

ANSWER: 
I do not really get the picture, but if your "object" is attached to slack cables also attached to the spececraft, you are in for some pretty big trouble. When the cables become taught they have to be strong enough to accleerate the mass of the spacecraft in a very short time to a very large speed, so they probably will not be strong enough. But suppose that they were and suppose that they caused the space craft to accelerate to 0.6c in, say, 1 second. Anyone in the spacecraft would be crushed to a mush. The greatest acceleration a person can tolerate without blacking out is about 5g. Accelerating to 0.6c at 5g would take you about 7 years. And don't forget: at 5g you would be miserably uncomfortable. One of the biggest obstacles to traveling at speeds comparable to light speed is our physiology which cannot tolerate large accelerations.


QUESTION: 
Which car will suffer the most damage? A stationery vehicle, hit from the back or the vehicle that drove into the stationery vehicle? Given that both vehicle are of the same material strength.

ANSWER: 
Because of Newton's third law (N3), each should experience the same force, the same impulse, the same damage, etc. For this case, N3 would state that if the moving car exerts a force on the stationary car, the stationary car exerts an equal and opposite force on the moving car. There is one catch, however, not really having to do with physics. The moving car has its radiator, engine, steering, etc. where the impact occurs and the stationary car just has the trunk, so the cost of repair will likely be bigger for the moving car.


QUESTION: 
Im trying to help my daughter on her 7th grade science fair project. She is testing several insulation types. We used a temp logger and have the data but because the outside air temps were constantly increasing or decreasing throughout the logging process, I am struggling to find a way to conclude which is better. We built a box with an inner box separated by 3 1/2 inches on all sides to represent stud wall cavity. We insulated the 3 1/2 inch void and heated the inside cube with a heatlamp sealed within, along with a data logger. Ex: Fiberglass batting inside temp went from 98f to 68f in 2 hrs with outside temp starting at 55f and ending at 61f. Data for others vary in time range and starting/ending outside temps. I feel stupid. Can you help with a formula?

ANSWER: 
With the data you have, the best you can do is to compute an average rate of change of temperature and state the average external temperature; 150/hr at an average lower temperature of 580. This will surely not win any blue ribbons, though! So, making one measurement tells you very little. What I would recommend would be to measure the temperatures every 10 minutes or so, over which the temperature difference would not be changing too much. What matters is the rate of change of temperature as a function of temperature difference. So there would be two interesting things you could graph. The first would be temperature difference (between inside and outside) as a function of time; the second would be rate of change of inside temperature as a function of temperature difference. The first graph should show a slower change for better insulators but all graphs should approach zero at large time (eventual equilibrium). The second graph should show that the rate of change is proportional to the temperature difference (it should be a straight line, approximately) and the slope of that line should be bigger for poorer insulators (they leak faster at a given temperature difference). I also have a 7th grader and he is studying graphs and slopes, so I am assuming that your daughter could get what I am talking about.


QUESTION: 
Consider centripetal force of a tetherball. This would be provided by the tetherball's string. My physics teacher tells us that since kinetic friction is orthogonal to centripetal force, it may be ignored in centripetal force's calculation. However, it is obvious that centripetal force depends on the ball's velocity: F=m(v squared)/r, and that velocity depends upon kinetic friction. So if the tetherball were to rub against the ground, wouldn't it require are greater centripetal force?

ANSWER: 
The tetherball is a particularly tricky example because of the thickness of the pole. (First, ignore friction.) At any instant the ball is rotating about the point where the rope is tangent to the pole and the ball is moving perpendicular to the rope. But you would calculate centripetal acceleration relative to the center of the pole. Therefore, the tension in the rope is not the centripetal force, only its component Tr along the radius R is (see first picture). Similarly, the centripetal acceleration is not V2/R but Vt2/R where Vt is the tangential component of the velocity (see second picture). Finally, if there were friction present, it would contribute to the centripetal force because the the frictional force will be opposite the direction of the velocity and will therefore have a component fr which is along R (see third picture). So, your teacher is wrong because the velocity V is not perpendicular to the centripetal force Tr-fr (compare all three figures). On the other hand, you also are wrong because the reason is not because the friction is changing the velocity of the ball (which, of course, it is); note that all my arguments are made at an instant, not over a time when the ball will slow down because of friction.

A couple of other things about the tetherball (without friction): Angular momentum is not conserved because there is an external torque (-TtR). Energy is conserved because there is no work being done by the tension (T and v are perpendicular).


QUESTION: 
In the absolute absence of gravity (if such a thing were possible) would time move infinitely fast?

ANSWER: 
Although time runs more slowly as the gravity increases, it does not follow that time runs inifinitely fast at zero gravity. In fact, it takes an enormous gravitational field for time to run noticably slowly and, for most purposes, a clock on earth runs with approximately the same rate as it would if there were no gravity.


QUESTION: 
Do particles falling into a black hole at event horizon have an infinite amount of energy?

ANSWER: 
No.


QUESTION: 
Particle 1 experiences a perfectly elastic head-on collision with a stationary particle 2 which has a mass of 1 kg. Determine the mass of  particle 1, if after the collision the particles fly apart with the same speeds.

ANSWER: 
(I have been assured that this is not a homework problem.) For particle 2 at rest, the equations resulting from applying momentum and energy conservation are v1=u(m1-m2)/(m1+m2) and v2=2m1u/(m1+m2) where u is the speed of approach of m1. Set v1=-v2 and solve for m2.


QUESTION: 
Can Dark matter particle or neutrino have size? Could those particles be smaller then the planck length?

ANSWER: 
As far as anybody knows there is no such thing as a "dark matter particle" because no such thing has ever been observed. For the size of the neutrino, see an earlier answer.


QUESTION: 
I was reading that an electron must act as a wave in order to produce an interference pattern in the double slit experiment. I believe there explanation is that an electron arrives at the two different slits at the same time (wave nature). Seems a little far fetched, would it not be simpler to think that the inteference pattern is a result of electrons that are travelling along certain straight lines pass close enough to each other to repulse each other to provide the dark bands. Hence no need for them to be waves, just particles with charge.

ANSWER: 
Sorry, but there is no escape from the fact that particles may behave as waves (no matter how "far fetched" you find it!). It would be impossible to come up with a scenario like you suggest because you could send the electrons through at a rate of one every day and, after a large number of days, the interference pattern would still appear.


QUESTION: 
A question regards projectile motion principles. If I'm on a railroad flatcar and am travelling along the horizontal in x direction only with speed relative to the earth = 10 mph.There's no friction. Now I toss a ball forward @ some angle theta to the horizontal with a some initial launch velocity (Vo). Is not the horizontial velocity component of the ball now Vo = (10 mph)*cos(theta) by definition?

ANSWER: 
Who is measuring it? If you measure it, the speed of the train is irrelevant; the horizontal component is v0cos
θ. If someone on the ground measures it, the horizontal component is 10+v0cosθ.


QUESTION: 
Based on Physics, is a 90 MPH Fastball Slower or Faster than a 95 MPH. At work we are trying to determine if the 95 MPH fastball loses energy faster than a 90 MPH fastball. Your answer is greatly appreciated.

ANSWER: 
You are asking two questions; if a 95 mph ball loses energy faster than a 90 mph fastball (it does) you cannot conclude that it "is faster" (by which you mean, I presume, when it passes over the plate). For the details of the following, see the earlier lacrosse ball answer. Following the (exact) solution in that earlier answer, I find that the 95 mph ball reaches the plate in 0.47 s and arrives at the plate with a speed of about 80.8 mph. The 90 mph ball reaches the plate in 0.50 s and arrives at the plate with a speed of about 76.3 mph. So, each loses about 14 mph with the faster ball losing a bit more. This surprised me but I found another reference saying that something like 10 mph is what is lost, so my calculations are reasonable. So they do not lose energy significantly differently (the faster pitch lost more speed in a shorter time so its average rate of change of speed was indeed bigger). (I used 3 inches for the diameter, 0.145 kg for the mass, and 60'6" for the distance to the plate.) There is certainly no way that one could characterize a 95 mph fastball as slower than a 90 mph fastball.


QUESTION: 
So we are launching rockets; soda bottles filled with water and pumped up with air. If a student's rocket goes up for 4 seconds it will then fall for four seconds, right? and according to the chart in this site it is going 87 miles perhour; but that's just something like a marble dropped for four seconds, not something with all the surface area of a bottle, right?

ANSWER: 
So the question is: is air resistance important? My guess is that it is important because 87 mph is a pretty large velocity and air resistance is roughly proportional to the square of the speed. The force due to air resistance is opposite the direction of the velocity, so on the way up both the air resistance force and the weight are down, but on the way down the weight is down and the air resistance force is up. So up and down are not symmetrical. So I would propose measuring the time up and the time down; if they are so close that you cannot see any significant difference, you may assume air resistance is negligible and 87 mph is a good estimate of the initial speed. As a double check on that, measure the height to which it goes (a good exercise in trigonometry if you measure the angle of inclination at some distance). If air resistance is negligible and the initial speed is v=87 mph=39 m/s, then the height should be h=
½v2/g=78 m=256 ft.


QUESTION: 
Why does the wavelength of light change but the frequency doesn't when it moves into a different medium?

ANSWER: 
Frequency is a property given the electromagnetic radiation at its source. If you have an electron oscillating with a particular frequency, then that will be the frequency of the radiated wave. What the wavelength will be depends on what the speed of the wave is. Take a simple example: the frequency is 5 cycles per second and the velocity is 10 meters per second. Then the wavelength will be the distance the wave travels in one cycle (called the period) which is, in this case, 1/5 of a second. Hence the wavelength will be 2 meter. If the the speed were instead 5 m/s, the wavelength would be 1 meter. When light enters another medium, what changes is the speed and, since the frequency is characteristic of the source rather than the medium, it does not change. Hence, the wavelength must change.


QUESTION: 
If an iron ball is dropped from a train moving at a constant velocity of 60mph, its acceleration on the x axis is 0, hence it has 0 force. How can it break a mirror standing on the embankment as it hits it at 60mph ?

ANSWER: 
Let's just simplify the problem: an iron ball is moving through empty space with a constant velocity of 60 mph. You are right, as long as it moves with constant velocity there is no force on it. But that does not mean it is incapable of exerting a force. Suppose the ball hits a wall which stops it in a very short time. The ball, in other words, will experience a very large acceleration. This means the wall must have exerted a very large force on the ball. But Newton's third law tells us that if the wall exerts a force on the ball then the ball exerts an equal and opposite force on the wall. So, if you replace the wall with a mirror, the ball will exert a large force on the mirror which will likely break it.


QUESTION: 
Does light ever end or stop. For instance im aware that the light of a star which blew up many years ago, still travels towards us. The great distance giving the impression that the star is still stable. Do the photons just get converted to other things?

ANSWER: 
Light consists of electric and magnetic fields. All normal matter consists of electrical charges. Therefore, light interacts strongly with matter. Shine a light on a wall and the light either reflects or disappears. It is absorbed by atoms and molecules in myriad ways. The energy it carries, of course, does not disappear but is absorbed by the matter by exciting the atoms. The reason that light reaches us over vast distances in the universe is that there is very little matter in space for the light to interact with.


QUESTION: 
what is the upward acceleration of the earth if a student falls toward the earth at 9.8 m/S2?

ANSWER: 
It depends on the mass of the student. Assuming a mass of about 100 kg, the weight is about 1000 N. So the earth, with mass 6x1024 kg, experiences a force of 1000 N. The acceleration is then F/m=(1/6)x10-21 m/s2.


QUESTION: 
My father recently experienced some strange activity in his freezer. Approximately six hours after putting an ice cube tray in the freezer (right after filling it with water) he noticed there were thin pieces of ice coming out of it. They were about an inch long and on a bit of an angle. These pieces of ice seem to be defying gravity and we cannot come up with an explanation.

ANSWER: 
I have answered this question before.


QUESTION: 
I have a question about why the shape of a window affects the structure of the object it's in. For example, when Britain was producing their commercial airplane "The Comet", they started tearing apart and falling out of the sky over time. The engineers discovered that the fuselage was getting cracks and tears due to the square-shaped windows on the plane. I know that the cabin in the plane is pressurized so that the passengers can breathe normally, and this extra pressure pushes on the inside of the plane. But why does the air pressure cause cracks around the square windows as opposed to the newer round windows? I heard that it's because the corners of square windows act as some sort of stress point or something, and over time they cause the cracks, but this is where my understanding gets fuzzy. Can you please explain how those square windows differ from the round windows of today, and how the corners of the square windows caused the cracks/tears in the fuselage?

ANSWER: 
The causes of the crashes of the Comet was actually a lot more complicated than you suggest. The main problem was an underestimation of the importance of metal fatigue in the overall engineering of an aircraft. It is like if you bend a paperclip back and forth repeatedly, eventually it breaks; than is metal fatigue. Every time the aircraft is pressurized it is like bending the paperclip. It is well known that sharp corners are more vulnerable to failure under stress than gentler curves; I have spent some time trying to think how to make this plausible. I think a good analogy is the arch used in architecture. Compare an arch to a "pointy arch" like an inverted V. The V will be able to hold much less total weight because it will fail at its apex. The arch distributes the load carrying over its whole length. Or, imagine that you have a V-shaped object and a semicircular object of similar material and you grasp the ends of each and try to break them; I think it is probably clear that the V will break first, right at its apex, whereas the semicircle will require much more force to break because no one part of it carries the brunt of the force. Another example is in glass cutting. I have made many stained glass windows and one of the things you learn early on is that it is almost impossible to cut a sharp indentation into the glass without breaking it.


QUESTION: 
On the subject of light waves - (1) are the instantaneous amplitudes shown on the waveform in units of energy, oscillating around zero ? If so, how can there be instantaneous values of negative energy? (2) since quantum refers to restricted values of energy equal to nhf, why is a photon depicted as a burst of energy (a packet) but not a continuous waveform of energy nhf? Is there a difference between a quantum of light and a photon of light?

ANSWER: 
The amplitude of a light wave is usually specified by the maximum magnitude of the electric field, units of N/C (newtons per coulomb) and does not specify an energy. The intensity of a light wave is proportional to the time-average (over one cycle) of the square of the magnitude of the electric field. The time-averaged energy transported is proportional to the intensity. The instantaneous energy transported by the wave is proportional to the square of the electric field, so no negative energies are introduced. A photon has no particular size, rather size is determined by how you prepare or observe it. The important thing is that the photon is the smallest possible amount of light of a particular frequency, you can't cut a photon in half. A photon and a quantum of light are synonymous.


QUESTION: 
I am confused about how gravity works throughout the universe. Does everything pull on everything else, or do objects have to be within a certain distance of each other for a gravitational effect to be apreciable between them? I guess the real question that has me puzzled has to do with something I read about black holes eventually swallowing up everything in the universe. I had read your recommendation to go to "ask an astrophysicist" in search of answers to such questions, and found that mine had already been asked there. The answer was that black holes would not consume the entire universe because they only swallowed matter which moved across their event horizons. That brings me to my question. If everything in the universe were suddenly gone except for two stars (or particles of sand, for that matter) floating at opposite ends of the known universe and not moving relative to one another...would they eventually attract one another and merge? Would that attraction happen regardless of how many billions or trillions of light years separated them?

ANSWER: 
As best we know, gravity falls off like 1/r2 where r is the distance to the object causing the gravitational force. So, the force between two objects is only exactly zero if they are separated by an infinite distance. From that perspective, the answer to your last question is yes; but the time for them to come together would be awfully big. However, be sure to note that my answer is qualified by "as best we know"; there are indications that maybe we do not understand gravity as well as we think we do; examples of this are so-called dark matter and dark energy, both of which are really totally puzzling to scientists. Regarding the swallowing up of the universe by black holes, the event horizon statement is true but you also have to argue that everything will not eventually find its way inside some black hole's event horizon. You could at least argue that, if there is a giant black hole in the center of our galaxy (as is thought to be the case), it would eventually consume all of this galaxy.


QUESTION: 
is there any way to read the earths magnetic field and somehow reverce polarity in a magnet to make it ,or the craft its mounted on hover if you will using polarization for instance an electro magnet with increasing power supply to it. And if so what kind of range of power would be required for such a task.

ANSWER: 
There are any number of reasons why your idea is not practicable. First, the earth's magnetic field is far too weak to be able to cause any weight to hover. But, even if the field were strong, your idea would not work. At any particular place on earth the field is nearly uniform over the size of any magnet you are likely to use; that is, if you measure the magnitude and direction of the field here and then remeasure anywhere within a few miles of here, you would get the same field. In a uniform magnetic field a magnet may experience a net torque (which is what causes a compass to align with the field) but zero net force.


QUESTION: 
hypothetically you have a hand crank mechanism attached to a generator to produce electricity to light a light bulb. This device works fine at room temperature. The parts are all still functional but are at (or nearly at) absolute zero. (thus, in my limited knowledge I assume devoid of excess energy, thermal and otherwise) If you crank the mechanism will the bulb light? Will it light-up but be at a different intensity than when it was at a much more energized room temperature?

ANSWER: 
(Near absolute zero is ok, at zero is impossible.) Any change in temperature will result in a change of resistance of both the wire and the light bulb filament. Usually a temperature decrease results in a decrease in resistance, so equivalent cranking (generating equal EMF) would result in increased current in the circuit as the temperature goes down. Since the power dissipated in a resistance R with an EMF of V across it is P=V2/R, more power will be dissipated in the light bulb so it should burn brighter. Of course, the problem is that the light bulb, by virtue of its operation, becomes very hot, it cannot be cold. So we have a sort of "feedback" situation here; more current through the lightbulb will cause it to have a higher temperature increasing its resistance. So there is a sort of "tug of war" between the light bulb and the rest of the circuit. If the conductors become superconducting, it is a more complicated situation but if the resistance of the filament is zero it will not shine at all regardless of the current through it.


QUESTION: 
What are some specific examples of how quantum mechanics and relativity conflict with each other? I've heard that they do but the conflict seems vague to me...

ANSWER: 
I would say that there is no incompatibility between quantum mechanics and special relativity. Relativistic quantum mechanics is just usual quantum mechanics written to include special relativity. The relevant equations of this theory are the Dirac equation (for fermions) and the Klein-Gordan equation (for bosons). However, the theory of general relativity, which is essentially the accepted theory of gravity, is incompatible with quantum mechanics. We say that no one has yet been able to devise a theory of quantum gravity. You might be interested in a couple of my earlier answers (1 & 2).


QUESTION: 
why do galaxies and solar systems have the galactic or solar plane? ie, why to things seem to balance out along a linear plane in both these instances?

ANSWER: 
I will give the solar system as an example; a galaxy would follow from a similar argument. A star starts out as a very large, essentially homogeneous cloud. It begins getting smaller under the mutual gravitational attraction of all the pieces. Such a huge collection of matter is bound to have some net angular momentum so that the whole cloud has a very small net spin around the center as it collapses. But, angular momentum must be conserved and as the cloud gets smaller it spins faster; an everyday example of this kind of angular momentum conservation is a figure skater who, when she is spinning and then pulls her arms in close to her body, spins faster. So now the cloud gets smaller and smaller and spins faster and faster. Now, envision a drop of fluid which spins very fast; what does it do? It flattens out.


QUESTION: 
Suppose you hold a small ball in contact with, and directly over, the center of a large ball. If you then drop the small ball a short time after dropping the large ball, the small ball rebounds with surprising speed. To show the extreme case, ignore air resistance and suppose the large ball makes an elastic collision with the floor and then rebounds to make an elastic collision with the still-descending small ball. Just before the collision between the two balls, the large ball is moving upward with velocity and the small ball has velocity . (Do you see why?) Assume the large ball has a much greater mass than the small ball.

ANSWER: 
I will not give all the details but, to understand this problem, you need to understand one-dimensional elastic collisions between two masses. There is a derivation in a Wikepedia article. If a mass with speed v collides elastically with a very much larger mass with speed V in the opposite direction as the smaller mass, the smaller mass rebounds with a speed approximately equal to v+2V. In the case you note, since the larger ball rebounded from the floor elastically, both balls have about the same speed when they collide, i.e. v=V, so the smaller ball rebounds with speed 3v.


QUESTION: ;
Why, if light always travels at the same speed, does a galaxy that's moving away from us appear red-shifted and one that's moving towards us appear blue-shifted? Is it because the light wave is traveling at the same SPEED but the FREQUENCY is what actually gets shifted?

ANSWER: 
For the same reason sound is Doppler shifted by a moving source. The stationary source emits two consecutive wavefronts a distance of
λ apart. If the source moves away, the distance will be λ'>λ and if it moves forward, the distance will be λ'<λ.


QUESTION: 
What would be the total kinetic energy of a long, thin rod of length L and mass m which rotates with an angular velocity w and simultaneously has a constant translational velocity v. The rod rotates around an axis which is at the end of the rod, not through the center of mass of the rod. Would it just be the sum of the translational kinetic energy of the center of mass of the rod and the rotational kinetic energy of the rod with respect to the rod's rotation around the axis through its end? Another words, expressed mathematically, would it be: E total = 1/2 m v (sq) + 1/2 I w (sq) Where: I = the moment of inertia of the rod with respect to rotation at one end of the rod

ANSWER: 
The case you describe is purely rotational. The kinetic energy is
½2 where I is the moment of inertia about the end. It is only if you cannot locate a stationary point about which the object is actually rotating that you need to include translational kinetic energy. For example, a pitched baseball is spinning about its center of mass and the center of mass is moving.


QUESTION: 
In my independent studies of physics (purely interest, not for school) I seem to repeatedly encounter an "infinite cylinder". Now I have a general understanding of what is meant by an infinite cylinder within the many contexts its used, but I'm daft as to its exact meaning. What is an "infinite cylinder"?

ANSWER: 
An infinite cylinder is a cylinder which is infinitely long but with a finite radius. You will also encounter infinite planes and infinitely long wires. There is obviously no such thing as any of these. However, they are very useful mental constructs to help us understand many real physical problems. Suppose we have a cylinder which has an electric charge distributed uniformly throughout its volume. To find what the electric field due to this charge distribution would be extremely difficult and possibly impossible in closed form. However, if the cylinder were infinitely long, it is a trivial problem. What makes the problem very hard is the effects of the ends of the cylinder. If the cylinder were much longer than its radius, and if you stayed fairly far from the ends, the field there would be very well approximated by the field due to the (fictitious) infinitely long cylinder. If you are much closer to a wire than you are to either end, it looks like an infinitely long wire. If you are much closer to a plane than you are to its edges, it looks infinitely large.


QUESTION: 
My husband and I are having a lively discussion: You are on a train going forward at 200 mph, you shoot a bullet from a gun on the train - going in the same direction of the train: does the velocity of the bullet equal the velocity of the muzzle of the gun PLUS the velocity of the train or does it equal the muzzle velocity?

ANSWER: 
You have to stipulate who is measuring it. If you are on the train, you measure muzzle velocity; if you are on the ground, you measure muzzle velocity plus train velocity.


QUESTION: 
If an object, like a block of metal, were suspended in a vacuum with supports of aerogel, would it gradually release energy in the form of radio waves/gradually disperse energy through the super-high insulation of aerogel and become a superconductor since it has emitted its energy? Or will it remain room temperature?

ANSWER: 
An object radiates energy. The rate is proportional to the fourth power of its temperature. That means that as it gets colder the rate it loses energy gets smaller. You talk about a perfect vacuum, but first of all let's realize that there is no such thing. The object will eventually come into equilibrium with its environment. Now we assume that it is in some kind of container and the container is at some temperature and radiating itself; the same goes for your aerogel supports. So, you might rather want to imagine it to be in the middle of ingalactic space where there is no other "stuff" nearby. Nevertheless, the space is filled with radiation from all the rest of the universe and the object will eventually be absorbing as much energy as it radiates. It will get very cold, but I do not know how cold because it depends on where it is; if far from any stars where there is only the background microwave radiation, the temperature is about 3K. Of course, whether it becomes superconducting depends on the material and what its transition temperature is.


QUESTION: 
Imagine a varying magnetic field is confined to an area smaller than that of a conducting loop surrounding it. We know there is an induced emf in the loop. Will the induced emf be the same if the same magnetic field is used but the loop is now as big as the solar system?

ANSWER: 
Yes. The emf is proportional to the rate of change of flux. Flux is essentially the magnetic field times the area. So although the flux itself will be greatly reduced if you greatly increase the area but keep the field the same, the rate at which the flux changes will remain the same because in both cases the area remains constant. If the area remains constant then the rate of change of flux is proportional to the rate of change of field.

FOLLOWUP QUESTION: 
your answer is a big SURPRISE because many teachers and "experts" explained what causes the emf is the cutting action of magnetic flux and the loop. Since in this case the changing field is unable to cut the loop, therefore there is no emf induced. Did they misinterpret Faraday's Law?

ANSWER:
I do not know what you mean by "the changing field is unable to cut the loop". Your question implied to me that there is a field which "pierces" the area of the little loop and then this identical field pierces the area of the big loop. There must be no other fields which are changing in the second case for my answer to be correct. Now, if you stipulate the mechanism for producing the original field, it may be that enlarging the loop will change the flux. For example look at the figure to the right. Here the changing flux through the loop causes the current to flow. But if the size of the loop is increased, the flux will change because the lines from the north pole eventually return to the south pole and would "pierce" back through the loop. So, if the loop were huge, there would be no flux through it.


QUESTION: 
Something which has always boggled my mind is that electron behavior changes if it is observed. So, to put it crudely, how do electrons "know" that they are being observed? I'm aware that the Schrodinger wave equation explains it mathematically, but qualitatively, I'm at a loss.

ANSWER: 
In order to observe an electron you need to interact with it; when you interact with something, you change it. For example, to observe an apple you have to shine light on it so that you can observe the reflected light. For the apple, the changes due to all that light bouncing off it are so tiny as to be unmeasurable. For an electron, a photon bouncing off it is a big deal. This is related to the uncertainty principle and Heisenberg's microscope.


QUESTION: 
I have a question regarding dust and static electricity. This may seem like a weird question but its important because my daughter has Asthma and is highly allergic to dust and I am planning on purchasing an air purifier like the Ionic Breeze. Well let me get to the question. At what point would dust become charged? At the point of contact with the unit (rods) or at the time it enters its electric field. If the dust is charged before making contact is the rest of the surrounding field charged as well? See the reason I ask is because I own a small home based company that cleans and repairs boards for hobby toys. And I am worried that if the dust or the surrounding area holds a charge it will damage the electrical components. When I work on these things I always have to have my antistic band on so I don't fry the boards so obviously I am worried that this could damage the units. However my daughters saftey is more important to me than the need to work on this business.

ANSWER: 
I don't think you need to worry at all. The way these work is to first filter the air, then ionize dust particles not caught by the filter, then collect the ionized dust on charged plates, and then, often, add a second filter. So very few charged dust particles will escape and those that do will quickly neutralize in the air. The amount of charge on any given dust particle would be trivial with regard to harming the boards. If you have a central forced air heating/cooling system, you might consider a whole-house electrostatic cleaner; my wife has asthma and that is what we have.


QUESTION: 
Because when we look deep into space we are looking farther and farther back in time, it follows logically that if we could see far enough we could see the beginning. I am having trouble understanding this paradox, especially how it might be possible for me to look in any direction and see the birthplace of the universe if I was able to look far enough. I know this isn't a question, but I'd just like to get some kind of formal explanation of this phenomenon (if there is an accepted one currently).

ANSWER: 
I do not agree that "it logically follows
" If you were standing ten billion light years away from the location of the big bang when it happened, you would see it in ten billion years (assuming space and time even existed before the big bang). But you were part of the big bang, and so your being able to see it later is out of the question since the information showing the big bang is receding away from you with the speed of light. When you see distant objects you are seeing them delayed, you are not actually turning back time which is what you would have to do to see the big bang. Every piece of the universe was there at the time of the big bang and so look at anything and you are looking at the birthplace.


QUESTION: 
I am trying to put the very low voltages detectable by sharks in a more familiar context. Could a voltage strength of 1 nanovolt/cm be reached by separating the poles of a AA battery a certain long distance apart, such as 7500 miles?

ANSWER: 
No. The poles of a AA battery have a potential difference of about 1.5 volts regardless of their separation. Potential difference (voltage) is a measure of the work it takes to move an electric charge from one pole to the other; if the poles are farther apart the field between them is weaker but the distance you must move the charge is farther and the work turns out to be the same. Imagine two poles separated by 1,000,000,000 m and with a voltage across them of 1 V. Then 1 nV would be the voltage between two points 1 m apart along a line between the poles.


QUESTION: 
In radioactive decay, if one starts with x number of a particular kind of redioactive isotope, then x/2 of the original isotope remain after the first half-life. And, x/4 remain after the second half-life and so on.
a) I understand that individual atoms of the same isotope is exactly identical in every way. (eg. one Carbon 14 atom is exactly the same as any other Carbon 14 atom). So, how does nature decide which carbon 14 atoms decay during the first half-life, and which decay during the second half-life and which ones stay around until the 10th half-life.
b) Which of the following is the case:
i) Exactly half of the original number of radioactive atoms remains after the first half-life.
ii) Approximately half of the original number of redioactive stoms ramains after the first half-life.
and why?

ANSWER: 
Nature does not "decide", it is random when any given nucleus decays. This was the aspect of quantum mechanics which so bothered Einstein when he said "God does not play dice with the universe". There will be approximately half left after the first half life. How close to "exactly half" depends on how many there were to start since it is a statistical process and its accuracy depends on large numbers. If there were only one you would end up with either all or none. If there were 1020 to start, you would be hard-pressed to determine a difference from exactly 0.500x1020 afterwards.


QUESTION: 
Has there been an experiment done along the following lines?: there are two sedentary synchronized clocks ((a.) & (b.)) and you place clock (a.) as close as possible to a mass continually traveling as fast as possible, perhaps in a circle around the clock, and you place clock (b.) far away near nothing moving at all. The idea is to see whether clock (a.) and clock (b.) remain synchronized or whether clock (a.) slows down, suggesting that the mass continually traveling as fast as possible "absorbs" time from the surrounding space.

ANSWER: 
In general relativity gravity itself affects the rate of clocks. The stronger the gravitational field, the slower time goes. So you do not need to have mass traveling fast for the effect to occur, just have mass there; traveling fast, though, would increase the effect since it would increase the effective mass causing the field. With a mass even the size of the earth the effect is almost immeasurably small. It is, however, built into GPS software because the time is very important in determining position and noticable errors are made due to altitudinal changes in clock rates.


QUESTION: 
I have been reading about parabolic trough solar collectors. There is a youtube video showing one made from a PVC pipe cut in half: http://www.youtube.com/watch?v=kXXOwfZA2Rk&NR=1&feature=fvwp I am wondering if you could estimate for me, how much efficiency is lost with this design due to the fact that the trough is actually a semi-circle and not a parabolic trough? I have read that some troughs gain efficiencies up to 23 times versus a flat collector. If the loss in efficiency is not too great from using the semi-circle it is certainly a cheaper and easier method of building than trying to create a true parabolic trough. Also, is there a formula or rule of thumb for the size of the collector tube in relation to the size of the trough that will produce the highest efficiency?

ANSWER: 
My estimate would be that there would be no significant gain by making a parabolic reflector of similar size. The reason is that the pipe carrying the water has a much bigger diameter than the "fuzziness" of the focus due to the circular rather than parabolic reflector (which has a sharp focus). Hence, in either case all the reflected light will be utilized. Of course, the bigger the reflector, the more energy you can harvest.


QUESTION: 
Why do sub atomic particle have a magnetic moment? Are they spining? If placed in a magnetic field would a proton or electorn align themselves similiar to the way a compass reacts? Does a neutron have a magnetic moment even though it has no charge? Are all magnetic fields caused by moving charges?

ANSWER: 
Most subatomic particles have both an intrinsic angular momentum (as if they were spinning on their axes) and a magnetic moment (as if they were behaving like a spinning charge distribution). I emphasize the "as if" since classical mechanics is not applicable for this small scale and one must use quantum mechanics. If you try to make a mechanical model for, say, an electron and try to figure out its rotational speed based on any reasonable values for its size, mass distribution, etc., you get ridiculous, unphysical answers
—like the surface having a speed much larger than the speed of light. Many physicists still think about little spheres of charge spinning as a rough model, but it is not really accurate. But relativistic quantum mechanics (Dirac equation) automatically predicts the spin of particles and so you should just think of it as an intrinsic property of particles. Putting a particle with magnetic moment in a magnetic field does cause it to want to align but quantum mechanics forbids having it line up exactly, that is, there is a maximum component of the magnetic moment which can occur. So a better way to visualize it is like a gyroscope in a gravitational field precessing around the direction of the field but not perfectly aligned. Yes, neutrons do have magnetic moments; they have a magnetic moment opposite their angular momentum, similar to negatively charged particles. It is most accurate to say that magnetic fields are caused by current densities, but that is a fancy way to say moving charges. The term moving charges, though, might inspire one to be too literal in trying to describe intrinsic magnetic moments as I warn above. Magnetic fields may also be caused by changing electric fields. [In future, please comply with website groundrules stipulating "single, concise, well-focused questions".]


QUESTION: 
Please forgive me, I don't know where this question falls in the realm of physics, but I have just watched a video on M Theory and string theory and have a question in regards to the other dimensions mentioned there. My best friend is a psychic, not unlike Allison Dubois of the Medium show, being able to see people who have died, experiences of people's past, present and future, and things like this. I have witnessed this phenomenon over the last 6 years, time and time again, to the point that I cannot intelligently ignore the reality that my friend is able to see things 'of another dimension'. My question is, are physicists willing to look at these kinds of experiences of the legitimate psychic (I understand people's skepticism, but please bear with me) and perhaps equate what they are seeing as part of one or many of the unseen dimensions mentioned in M theory? Is there a physicist out there who considers such things and are they contactable?

ANSWER: 
I ought to discard this question as falling into the "off the wall" category, but I could not resist the temptation to weigh in. I will just make a few points, state a few opinions. I would be a poor scientist if I tried to say we know everything. Therefore, let us just say that there may be such things as mediums, psychics, paranormal experiences, etc. Personally, I believe it to all be hogwash, but if it isn't, something is happening which cannot be understood using existing physics. So, maybe it's some kind of voodoo happening in the extra dimensions which appear in string theory? Although I have a slightly higher regard for string theory than for the paranormal, I do not consider it to be physics (yet) because it is unable to make verifiable predictions of how nature behaves. Therefore, if you want to make a connection between two highly speculative topics, be my guest. But be sure to understand that you are not doing science, you are doing speculative metaphysics
*. No self respecting physicist would consider studying such a thing (my opinion).

*definition: A priori speculation upon questions that are unanswerable to scientific observation, analysis, or experiment.


QUESTION: 
I know you hate question about ftl.... however I was researching the web and there are a lot of papers about superluminal phonomena involving: pulsars, blazars, and accelerating photons in cesium vapors. Is it me or is real evidence for the superluminal emerging

ANSWER: 
There is nothing which forbids particles traveling faster than the speed of light
…just not faster than the speed of light in a vacuum. In water, for example, a particle may travel faster than the speed of light in water. With charged particles there is a shock wave similar to a sonic boom for particles moving faster than the speed of sound called Čerenkov radiation.

FOLLOWUP QUESTION: 
The following is a link from cbs news showing an experiment involving cesium vapor traversing over 300 times the speed of light in a vacumn. Why doesnt this experiment contradict nothing faster than light. http://www.cbsnews.com/stories/2000/07/19/tech/main216905.shtml

ANSWER: 
This becomes quite technical, now that I understand your question better. It is an example of the wave velocity being different from the group velocity. This is explained pretty well at
http://www.theness.com/speed-of-light-repealed/ (although it is pretty tough going if you do not know much physics). You may be assured that no information or energy may be transmitted at a greater speed than c.


QUESTION: 
I have a question on the physics of a spinning or swinging object. If I spin a ball, does the top of the ball spin faster than the outside?

ANSWER: 
That all depends on what you mean by "spin faster". A physicist would probably mean the angular velocity of a point, the number of rotations per second, for example. In that case, all points in the object have the same angular velocity. But if you mean the linear velocity, the speed with which a point is moving, miles per hour, for example, then points have speeds proportional to their distance from the axis of rotation. So, the earth, for example, has the poles at rest and the equator having the highest speed.


QUESTION: 
If it were possible to dig through the center of the earth and someone were to fall through the hole, would gravity take effect on both sides of the earth and keep the person in the middle or would the velocity of the person be greater than the force of gravity allowing them to fall to the other side?

ANSWER: 
I have answered this question several times before.


QUESTION: 
what is the size of neutrino? How big is a neutrino?

ANSWER: 
Suppose you had asked the same question about a photon. Essentially the question would be meaningless since the photon has neither charge nor mass and so what would you measure the size of? A neutrino has almost no mass and no charge; it is conceivable that a measurement could measure a mass distribution but since it interacts only via the weak interaction, such an experiment is, for all practical purposes, impossible. In terms of making a measurement of where it is at a particular time, apart from the fact that any measurement on a neutrino is extremely difficult, would give a "size" depending on how the measurement was made (because of the Heisenberg uncertainty principle). The bottom line is that a size is just not a meaningful concept for neutrinos.


QUESTION: 
how can the photoelectrons have different energy?

ANSWER: 
Not every photoelectric event is exactly at the surface and the electrons lose energy getting to the surface. Also other mechanisms, like Auger electrons and Compton scattering, can contribute to the electron spectrum.


QUESTION: 
This has to do with the conservation of energy. A spring is compressed and kept compressed by binding it with wire. The whole assembly is tossed into an acid bath and is completely dissolved. What happens to the potential energy stored in the spring?

ANSWER: 
The potential energy has its origins at the atomic level. When you compress a spring you cause the atoms to be more closely spaced than they "want to be" and it takes work to scrunch them up like that, hence the potential energy. When the spring atoms come apart, they start a little closer to each other than they would if the spring were not compressed, so they move a little faster when they leave and so the average kinetic energy of all the atoms is a little larger than it would have been for the uncompressed spring. The potential energy ends up as kinetic energy (thermal energy).


QUESTION: 
When my husband told me that the leak in our Westerbeke diesel was caused by capillary action and surface tension, I had no idea what he meant. I understand the concept of surface tension- the water molecules are more attracted to each other than a different subsstance. But I don't get the "adhesive" force. Is the water molecule on the top attracted to a charge coming from the wall of the tube? or hole in the paper towlel. What gives.? I am a writer and trying to differentiate between the two for a metaphor.

ANSWER: 
Capillary action is the result of surface tension. The answer is a bit involved, but I will try to spell it out below. I will try to keep it as free from math and jargon as I can.

  • Any two atoms or molecules will exert forces on each other. There is no general way you can summarize the details of this other than the forces are always electrical in nature. Put two oxygen molecules in proximity with each other and they will form an O2 molecule. Put many water molecules near each other (at not too high a temperature) and they will hold each other in a liquid because of attractive forces among them. Put a water molecule near a "glass molecule" (mainly SiO2) and the force of attraction will be greater than between two water molecules. The force between similar (different) molecules is referred to as cohesion (adhesion).
  • Now, if you compare water molecules inside the liquid with those on the surface, those inside interact with more of their neighbors and therefore have a lower energy than those on the surface who interact with only about half as many. (An attractive interaction lowers energy; you can see this because two things which are stuck together can be pulled apart by adding energy, doing work, so it takes more energy to remove a molecule from the volume than from the surface.) A physical system will seek its lowest energy state, so the liquid (if isolated) will minimize the number on the surface, that is the surface area will be minimized. So a small volume of water will tend to become a sphere and a large body of water will tend to have a flat surface to achieve this. This tendency to have the surface push into its mimimum area is what surface tension is.
  • But, what happens if the liquid is in contact with another material, say glass? Look at the figures to the right where I try to represent a molecule (circle) where the surface of the liquid meets the surface of the container. The molecule experiences a force down, its weight, (green); an average force due to all its water neighbors, down and to the right (black); and a force due to its interaction with the glass (blue). In figure A to the right I have chosen the force due to the glass to be exactly equal to the sum (red) of the weight and water forces. So all the forces add up to zero, the molecule is in equilibrium, and the surface will be perfectly flat; there is no meniscus and there would be no capillary action. But, suppose the glass force were a little bigger than the water+weight force. In that case the forces will not be balanced but instead there would be a net force (water+weight+glass) straight up (pink). This is the force which drags up the edges of the surface to make a concave meniscus and this upward force is what drags up the column of water in a thin tube, the capillary action. If the glass force were weaker than the water force, the force would have been down and we would have a convex meniscus curving down at the edges; this is what happens with mercury in glass and the capillary action is opposite, the surface is pushed down at the edges. You can read more in the Wikipedia article on capillary action.
  • What happens is that the capillary force keeps pulling the water up but this means that more and more water has to be held up. Eventually, the edge of the meniscus will supply the same force as the weight of the water and it will stop rising. It is fairly easy to convince yourself that the height to which the water will rise in the tube is inversely proportional to the diameter of the tube. So if the capillary action causes a fluid to rise 1 cm in a 1 mm tube, the fluid will rise 10 cm in a 0.1 mm tube.
  • A paper towel is just like a huge tangle of tiny, tiny worm holes and the water is drawn into them by capillary action.

QUESTION: 
This has to do with the conservation of energy. A spring is compressed and kept compressed by binding it with wire. The whole assembly is tossed into an acid bath and is completely dissolved. What happens to the potential energy stored in the spring?

ANSWER: 
The potential energy has its origins at the atomic level. When you compress a spring you cause the atoms to be more closely spaced than they "want to be" and it takes work to scrunch them up like that, hence the potential energy. When the spring atoms come apart, they start a little closer to each other than they would if the spring were not compressed, so they move a little faster when they leave and so the average kinetic energy of all the atoms is a little larger than it would have been for the uncompressed spring. The potential energy ends up as kinetic energy (thermal energy).


QUESTION: 
Is there a practical and efficient method to transfer thermal energy to electrical energy without an electro-mechanical intermediary. i.e.. steam driven turbine ?

ANSWER: 
I won't judge how practical or efficient it is, but two dissimilar metals which form a junction will generate a potential difference across the two when heated. This is called a thermocouple.


QUESTION: 
Can a massless particle have or carry energy?

ANSWER: 
The energy of any particle is E=
√[m2c4+p2c2] where p is the linear momentum. If m=0 then E=pc. Massless particles have momentum. The only massless particle we know is the photon which has an energy E=hf where h is Planck's constant and f is the frequency. So the momentum of a photon is hf/c.


QUESTION: 
If a proton were to collide with a mini black hole about the size of a proton, and mass of 10^15 grams what will happen to the proton, will it be sucked into the black hole?

ANSWER: 
Yes, if it goes inside the event horizon.


QUESTION: 
I don't understand quantum entanglement properly. If no local connection between two photons exists, how can one photon know what state the other is in? Are force carrying particles involved, or is it some weird, quantum voodoo? I read somewhere that some scientists believe there might be particles involved that travel through unseen dimensions of space-time which are much smaller, sparing the need for a theory that permits the light constant to be exceeded. What do you think? Surely there must be some form of physical relationship, or a change in photon A wouldn't be reflected in photon B. Does anyone know for sure what is going on here?

ANSWER: 
The reason is that the two photons are in the same quantum system and one does not need to know what the other is doing, it only needs to know the rules which pertain to the whole system. Like many things in quantum mechanics, this is difficult to get your head around. I personally do not believe that there are particles connecting the two photons and traveling faster than light speed through "unseen dimensions", whatever that means. I will point out that quantum entanglement has been observed experimentally.


QUESTION: 
When it come to the UNCERTAINTY PRINCIPLE and the SLIT experiment, is it possible that the electrons or photons of light from a laser are hitting the walls of the slit? and that causes the horizontal uncertainty if the slit were horizontal the uncertainty line would be virtical?

ANSWER: 
Certainly not. A well designed experiment shapes the slit edges to not contribute to the diffraction. If it were the case, then different shaped slit edges would give rise to different diffraction patterns.


QUESTION: 
Could you answer a question to solve an argument I'm having regarding Kinetic Energy? On a web forum, a participant is claiming that an object has an absolute amount of Kinetic Energy, and that this is dependent on all the accelerations it has ever undergone. He claims it is measured with regard to "Inertial space". I and others claim Kinetic Energy is a relative value, as is Velocity, and that the same object will have different Kinetic Energy depending on the Frame Of Reference of the observer.

ANSWER: 
You and your friends are correct.


QUESTION: 
In Disney's the The Three Little Pigs, when the big bad wolf can't blow the brick house down he tries to get in via the chimney. At minute 7:34, noticing mortar falling into his boling pot of water the smart (brick laying) pig opens the vat of hot water boiling over the fire and pours in turpentine. http://www.youtube.com/watch?v=VHJ0L6DftGg I immediately assumed that the pig did this to raise the Boiling Point of water. However, per google, turpentine would effectively lower the boiling point of the two liquids to @ 95 degrees Celsius. So why does the smart pig add turpentine to the liquid?

ANSWER: 
As you note, adding room temperature turpentine to boiling water will initially cool it. And, since the wolf is on his way down, there is not really time for it to heat up, but if the pig had planned ahead, the boiling point of turpentine is above 1500C, so that would have helped make the pot hotter. However, I did a little research and found a situation where a blacksmith put turpentine in a horse's hoof nail hole with the idea of cauterizing it. The horse apparently went almost insane with the pain, so maybe that is what got the wolf? He sits in almost boiling water, gets a blister, and then gets a horrible irritant on that
—well, that would explain it, wouldn't it? Not physics, really, but fun to figure out.


QUESTION: 
How does a weight on a string spinning about a vertical axis (like a helicopter blade) appear to defy gravity? Slowly spinning it makes the weight swing wide and out, producing a cone-shaped trajectory with the string, but faster speeds causes it to become more disc shaped. Can a FBFD be drawn for the weight in this instance, or what vertical force (or force component) supplies the force that cancels the weight's weight. My only guess is that the tension on the string supplies enough force to not only supply the centripetal requirement, but also to equalize the weight, thus putting the weight vertically in equillibrium. Would this not mean that the string is always at a slight angle, even if w cannot perceive it?

ANSWER: 
I fail to see how this problem is like a helicopter blade. But, you have more or less analyzed the problem (this problem is called the conical pendulum) correctly. It goes like this: there are (neglecting air friction) two forces on the object, its own weight (which points straight down) and the tension in the string (which points along the string and toward the pivot point. The object is in equilibrium in the vertical direction and accelerating (centripetally) in the horizontal direction. The vertical component of the tension balances the weight and holds it up and the horizontal component provides the centripetal acceleration. And, yes, it is impossible for the plane of motion to be such that the string is perfectly horizontal since the tension could then have no vertical component.


QUESTION: 
it is concerning an experiment i did in my college lab i shorted the two ends of a voltage source after setting some value of voltage .after shorting the two ends of the voltages source i observed that the voltage displayed by the voltage source became zero and it indicated some high value of current.how is this possible if there is no potential difference the there could be a current in the circuit.isnt A VOLTAGE SOURCE SUPPOSED TO MAINTAIN THE SAME VOLTAGE ACROSS ITS ENDS NO MATTER WHAT WE CONNECT ACROSS ITS END?also please explain what will be the potential at the two ends of the voltage source if its two ends are shorted(considering ideal situation) and the potential drops across wire if any?

ANSWER: 
Quite simply, your instrument was not sensitive enough to measure the voltage, but if a current was flowing and the wire was not a superconductor, there was a potential difference but it was likely small. Contrary to what you seem to believe (
given your CAPITAL LETTER SHOUT ABOVE), a power supply can provide some maximum current at a given voltage and if you cause more current to flow (by choosing a very small resistance), the output voltage will drop. But it will not drop to zero (unless you exhaust the battery or blow a fuse in the power supply) without the current dropping to zero. If you short with a true zero resistance (superconductor), either the potential difference must drop to zero or the wire must go nonsuperconducting.


QUESTION: 
How can an electron have mass but no radius?

ANSWER: 
The size of an electron has never been measured. There is a nice answer on WikiAnswers which essentially says that it is probably not measureable and does not matter anyway.


QUESTION: 
In the "twin paradox", the twin that "moves" ages more slowly. But don't they both "move", since movement is relative? So, using the same logic, wouldn't the twin at home age less than the twin in the spaceship since the twin at home (relatively) "moves" away from the spaceship? How can time be slower for one than the other since they both move, relative to each other?

ANSWER: 
You have hit on why they call it a paradox! However, there is really no paradox at all because there is an inherent asymmetry between the two twins. Think of the "distance" to the destination star as a stick between the earth and the star. The moving twin sees this stick contracted because of his motion whereas the earthbound twin does not. Hence, the moving twin sees a shorter distance he must travel and so it takes him less than the (classically) expected time. For a detailed discussion of the twin paradox, see an earlier answer.


QUESTION: 
Why does time slow down at higher speeds and could this mean that we don't all perceive the same moment at the same time?

ANSWER: 
What you are referring to is called time dilation
—moving clocks run slow. And yes, this certainly means that we need a whole new idea of what is meant by "the same moment at the same time". Particularly the idea of simultaneaty—two events simultaneous to one observer will not necessarily be simultaneous to another. And I do not mean appear to be not simultaneous, I mean they really are not simultaneous. The answer to your "why" is best understood using the light clock which I explained in an earlier answer.


QUESTION: 
I was wondering if: Someone was shinning a flashlight toward you. And someone else, say at 90 degrees, emitted a light beam or a beam of a certain frequency. Could the light from the flashlight be blocked? Can you cancel out a light frequency, with another frequency?

ANSWER: 
For starters, the flashlight has a continuum of frequencies (white light) so each frequency would have to be dealt with individually, clearly impractical. In addition, the flashlight light is incoherent, that is the light is a complete hodgepodge of light waves having no particular phase relationship among themselves; to get interference effects you need coherence (waves all in lockstep with each other). Finally, if you were able to do this, it would certainly not be from 900 for the following reason. Suppose you had a flashlight with one single wavelength and the light was coherent (you would call that a laser). In principle, if you shone an identical wave in the opposite direction you would establish a standing wave where you would have a dark spot every half wavelength of the light. But, since the wavelength of visible light is like 600 nm=6x10-7 m, you would not be able to observe these dark spots without very special instrumentation. Note that you can never make the light go away completely, you can never cancel it all out.


QUESTION: 
What are bosons and fermions?

ANSWER: 
There are two broad classes of particles, those you state. They are classified by what "statistics" they obey. In a particular quantum system, say an atom, there might be many particles. If those particles are fermions, no two of them may be in identical quantum states. Electrons are fermions and, if all the electrons in an atom could be in the same state, then every element would be chemically identical to hydrogen and chemistry would be completely different (and you would not exist). The reason different elements have different chemical properties is that as the electrons are added, they go into different quantum states. The other type of particle is the boson. They have no restrictions as to what quantum state they may occupy and in a given system you can have a thousand bosons all in the lowest state of that system. An easy way to distinguish fermions from bosons is their intrinsic angular momentum quantum number (spin). Particles with spin
1/2, 3/2, 5/2… are ferminons; particles with spin 0, 1, 2… are bosons.


QUESTION: 
If it is true that there is no cold, only the absence of heat - my question is 'why'? As in why is there no heat, only the absence of cold?

ANSWER: 
First of all, heat is energy transfer, not energy content. For example, heat flows from a hot object to a cold one but you do not say that the hot object has more heat than the cold one. It is important in science to clearly distinguish between qualitative concepts and quantitative concepts. Cold is an adjective, not a noun, and it is a qualitative concept which refers to something with a relatively low temperature but it has no quantitative meaning. Similarly, hot or warm are qualitative adjectives which refer to something with a relatively high temperature. The temperature of something is a quantitative measurement of how much internal energy the object has. Temperature measures the average energy per constituent. In a gas this is the average kinetic energy per molecule. So if one thing is hot and another cold, the hot one has a higher average energy per atom. In order to increase the temperature of something you have to add energy and this is done by causing heat to flow to it.


QUESTION: 
I'm a first year BSc student, and we have just covered electromagnetism. We were shown the formula for the speed of light in terms of the permeability and permittivity of free space, and how it showed that light is a self-propagating electromagnetic wave which abides by Maxwell's equations. Was this formula derived in any way, or was it found by fiddling around with the constants? If it was derived, where can I find the derivation? I know that the units conveniently cancel out to get m/s, and that it is quite aesthetically pleasing in the manner in which it unites electricity with magnetism, but understanding how this formula came about at a deeper level would really help me get a more rounded understanding of this fascinating subject.

ANSWER: 
This is definitely the result of what could arguably be called one of the most important derivations in the history of physics. In the latter half of the 19th century it was known that light was a wave but it was unknown what was waving. Maxwell's triumph was, by taking his four equations he was able to show that a solution was a wave which traveled with exactly the speed 3x108 m/s
—some coincidence, eh? If you are a first year student, you are probably not ready to understand Maxwell's equations and the derivation since they involve both vector calculus and partial second-order differential equations. In case you are, I have attached an abbreviated derivation (showing only electric fields); Maxwell's equations written here are for empty space with no charges or currents. A little more detail can be found here. You might also want to read a more qualitative explanation I gave in an earlier answer and also to read a little about electromagnetic waves I wrote.

At the risk of getting a little long-winded here, let me add that Maxwell's equations are laws of physics and Einstein's principle philosophical belief was that all laws of physics must be the same for all observers in the universe. Since the speed of light is 1/√[μ0ε0] for the observer who wrote down Maxwell's equations here on earth, that must be the speed for all observers. This is one of (unexpected) cornerstones of the theory of special relativity. Again, see an earlier answer.


QUESTION: 
Why is there an absolute freezing point, but not an absolute boiling point?

ANSWER: 
There is not an absolute freezing point
—the freezing temperature depends on the pressure. As the pressure is increased, the temperature at which liquid water freezes (turns to ice) decreases. See the phase diagram for water in an earlier answer.


QUESTION: 
Textbooks say that since the earth's curvature can be estimated at 5m for each 8000 m horizontally, an object moving with a horizontal speed of 8000 m/s with no air resistance would go into orbit. This makes sense for the first second because the object falls about 5 meters. But, after the 1st second, the ball continues to accelerate and therefore will fall more than 5 meters each second, so how could it become a satellite? I ran into this dilemia teaching HS physics. The Paul Hewitt Conceptual Physics book we use and many other resources (like http://www.physicsclassroom.com/mmedia/vectors/sat.cfm) cite this 8000 m/s and it sounds good at first, but when we tried to analyze it in class saw that it only seems to hold up for the first second. Am I missing something?

ANSWER: 
During that first second the surface of the earth also "falls" so the height of the object above the surface stays the same. Therefore, although it is accelerating it is not falling. This is an example of centripetal acceleration where the object accelerates by changing the direction of its velocity but not the magnitude of its velocity.


QUESTION: 
We currently use photons and electrons to send 1s and 0s to other places. Would neutrons work too? Could one send these neutrons to places where electrons or photons could not go? Like through the Earth? Google says there are portable neutron sources available. Are they strong enough and can they be modulated and detected at long distances?

ANSWER: 
Neutrons have numerous things going against them:

  • They are unstable outside a nucleus and beta decay after about 15 minutes.
  • Although they do not interact electromagnetically, they do interact with nuclei via the strong force so their range, while longer than that of charged particles, is short and they would not pass through the earth, only go a few inches in.
  • Because they have no charge, they are impossible to manipulate, steer, speed up, or slow down.
  • They are difficult to detect, again because of their lack of charge.

QUESTION: 
Consider an exited electron.how does it emit a photon during spontaneous emission?

ANSWER: 
It is easiest to think about it classically. If an electron is slowing down it is losing energy and this energy appears as electromagnetic radiation (this is how a radio antenna works, electrons being accelerated inside the antenna). So we conclude that an electron losing energy radiates electromagnetic energy. So, certainly an electron dropping to a lower state in an atom is losing energy and so we expect to see photons come out. Think of a radiating atom as a tiny antenna.


QUESTION: 
Frequency = Herz
Cycles / Second
Can a cycle be defined equally as "Distance" or "Velocity" or "Time" ? Or can a Herz only be "Cycle" ?

ANSWER: 
A cycle is a dimensionless quantity. 1 Hz=1 s-1. And Herz does not need to be "cycles" per second, it could be the rate of anything which is dimensionless
raindrops/s, apples/s, bullets/s, etc.


QUESTION: 
How do you make something radioactive? Can you simply hold it next to a radioactive item?

ANSWER: 
Holding something close to a radioactive source will not cause it to become radioactive. Suppose you start with something stable (as most naturally occuring things are) and you want to make it radioactive. You have to change the nucleus to one which is unstable. The most common way is to expose the sample to slow neutrons. The reason is that neutrons have no electric charge and so they are not repelled from the nucleus and, if they move slowly, they take a relatively long time to pass through and therefore have a relatively high probability of being captured. Usually the sample to be activated is put into a nuclear reactor where there are copious amounts of neutrons. One example would be 60Co, a radioactive isotope of cobalt which is commonly used for cancer treatment. Stable cobalt which is composed of 59Co absorbs a neutron and becomes 60Co which has a half life of about five years.


QUESTION: 
I am fascinated by particles, black holes, quantum mechanics, string theory, and especially gravity but hate doing math. Is there any hope of my being a scientist that studies any of those fields?

ANSWER: 
I am sorry to have to tell you, but, no, there is no hope of succeeding in physics if you hate doing math. Math is the "language" of physics. Although it is possible, as many gifted writers have shown, to appreciate the beauty and symmetry of nature qualitatively, there is no way you could be a physicist without a facility for and love of mathematics.


QUESTION: 
Does gravity have a speed? For example, if a distant star suddenly (somehow) increased its mass, would the resultant increase in attraction reach earth instantaneously? Or would it act like light, which would reach us only when the photons have traveled that large distance.

ANSWER: 
I have previously answered this question. As you will see in my earlier answer, it is believed (not observed) that gravity propogates with the speed of light and the force is transmitted by a hypothetical massless particle called a graviton.


QUESTION: 
I have a question that has me somewhat baffled and I'm wondering if you might have any explanation to. String theorists predict of the existence of a "graviton" particle which supposedly carries the force of gravity. And yet I have trouble figuring out how this is compatible with general relativity, which says that gravity is not a force at all but rather the warping of space and time. My question is, is there a way in which they go together; a way which I've yet to grasp?

ANSWER: 
First of all, I am not aware of string theory predicting a graviton; the problem with string theory is that it doesn't predict anything. I have wondered the same thing that you are wondering
—if gravity is just geometry, why treat it like a force? I have been informed that theorists believe that any theory must be properly quantized at small distances and the attempt to develop a theory of quantum gravity is one of the holy grails of theoretical physics. Such a theory would inevitably include the quantum which transmits the force and this, a purely hypothetical and unobserved particle, has been dubbed the graviton. A sort of parallel example is electromagnetism which was seemingly a fine theory at the end of the 19th century once Maxwell's equations had been fully understood. But eventually the electromagnetic field had to be quantized which is, essentially, where the notion of a photon comes from. Although photons were known to be an alternative to an electromagnetic wave, their role as the quantum carrying the electromagnetic force was not fully understood until Feynman and others developed quantum electrodynamics (QED).


QUESTION: 
Is an energy level the same as an orbital in an atom? If not, what is the difference?

ANSWER: 
Normally an orbital is a reference to a specific state, that is its specification includes the specifiction of all its quantum numbers. However, there are usually many different orbitals with the same energy; this is called degeneracy. An energy level usually refers to a specific allowed energy of the atom. So, if there is degeneracy, there may be several orbitals which comprise the same energy level.


QUESTION: 
How is it possible for the electron in a hydrogen atom to be at all places at all times around the nucleous? I know we cant measure where it is due the uncertanty principle, but that doesnt mean it isnt in one place at any one given time does it?

ANSWER: 
This is more a question of philosophy or of semantics than physics. If we acknowledge that we cannot measure its position with certainty, does that not mean that it is not in a particular place. To my mind, something you can't measure isn't. I think of the electrons in an atom as being "smeared out" over the volume with a specific distribution which is determined by the solution to Schr
ödinger's equation and which tells me the probability that I will find it somewhere if I make a measurement there.


QUESTION: 
I was wondering if there is anything as empty space? To be more specific.... What is in the space between a nucleous of an atom and the electrons circling around it?

ANSWER: 
This is really two questions. Regarding what is in "the space between a nucleous of an atom and the electrons circling around it", the question is incorrectly phrased because the electrons are not really circling around it but rather smeared out over the whole volume of the atom. That is, electrons do not move in well defined orbits. Your other question, is there such a thing as empty space is one of those questions whose answer is "yes and no". We can remove all real particles from a volume and end up with what is called a vacuum. However, there is something in field theory referred to as vacuum polarization; here, particles can "pop into and out of" existence as long as they pop out quickly enough that the apparent violation of energy conservation does not violate the uncertainty principle. So you can think of a vacuum as a swarm of virtual particles popping into and out of existence.


QUESTION: 
Capacitive reactance is inversely proportional to the frequency of the current flowing in the circuit. Thus it should completely block "Direct Current". But in reality it blocks D.C. only after it is completely charged by it. Why?

ANSWER: 
The plates of the capacitor need to charge up and during this time the current must flow to achieve this. However, the current is not a constant during this time and so I would not call it direct current, so maybe this semantics makes you feel better about the no DC rule?


QUESTION: 
I was wondering if you could clear up a conundrum for me. Obviously, creationist claim the earth to be somewhere around 6000 years old, or at least human existence on the earth to be somewhere in that range. How does the speed of light weigh in on this theory? I thought I read somewhere that we could use the speed of light to disprove the creationist claim about the age of human life. Any truth to that? Also, I understood light to be the only constant speed in the universe. However, I believe on Science Channel I heart that light moves outward from the core of the sun and a significantly slower rate than 186,000 mps. Does light always remain constant or can it's speed be affected by things like water or in the case of the sun possibly gravity?

ANSWER: 
The age of the earth or the time of mankind's existence has nothing to do with the speed of light as far as I can imagine. You heard wrong if you heard that light moves from the sun with a speed other than the speed of light. Gravity certainly does affect light but only to change its wavelength (a large mass will shift the wavelength slightly longer if the light moves away from the mass) or, if the light is passing by a large mass, it will be bent. If light passes through a material it will move more slowly than through a vacuum. But that is not because of the speed actually getting smaller; it sort of jumps from atom to atom causing its average time to get through the medium to increase.

FOLLOWUP QUESTION: 
I am still unclear about one aspect of light speed. If the speed is always constant, how can it take thousands of years for light to travel from it's core to the corona? Maybe it like Corona with a lime? J/k... Howcome light takes longer to travel the same distance in water than in the air?

ANSWER: 
The important feature about light speed is that the speed of light in a vacuum is constant. It is hard to imagine anything farther from a vacuum than the interior of a star! This constancy of the speed of light is the main ingredient of the theory of special relativity. And, it is totally unexpected since, if there is a source of light and you move toward it with a speed of half the speed of light, you will still measure the speed of the light as the same as if you were not moving. Moving through the sun you may think of photons being absorbed, then re-emitted, etc. until they get out which essentially means they are bouncing around inside which is why it takes so long to get out; see this link for a demonstration. Regarding the speed of light in materials like water or glass, this is spelled out in detail in an earlier answer. You might also need to read an earlier answer about electromagnetic waves which describes what light waves are composed of.


QUESTION: 
what is planet X?

ANSWER: 
See the Wikepedia article.


QUESTION: 
what is the state of sub-atomic particles at absolute zero?

ANSWER: 
Nothing can exist at absolute zero.


QUESTION: 
I understand that some particles have electrical charges (e.g. the electron has a negative electrical charge and the proton has a positive electrical charge) but I do not understand exactly what electrical charge is, nor why some particles have electrical charge and others do not. So my question is "What is electrical charge and why do some particles have it and others do not?"

ANSWER: 
I have answered this question previously. You may be disappointed that science cannot answer every "why"!


QUESTION: 
Hi, my sister and I were having a debate last night that I'm hoping you can settle for us. It seems to be a fairly rudimentary physics problem, but I'm hoping you'll tackle it. It started when she claimed that the water coming out of a shower is falling at a slower rate near the ground than it is when it first comes out of the shower head. As evidence, she said that she couldn't rinse out her hair as effectively if she were sitting down in the shower than if she put her head right near the shower head...that the pressure isn't the same. I said that was largely because the water just isn't as concentrated-- as it falls that 4-5 feet, it spreads out so the total force is less. Also, the angle of the shower head (probably not pointed straight down) contributes to the "spreading out". That factor aside, she claimed the real reason was that gravity actually slows down the water as it falls. Air resistance (and terminal velocity) aside, her argument was that the water comes out of the shower head at a greater force than gravity, and once it's no longer under that pressure (having left the pipes and shower head), the speed actually backs off to meet the force of gravity. I wasn't buying it. My simplified analogy is: if you were standing on the top of a building, pointed a gun straight down at the ground, and fired it, is the bullet travelling as fast when it hits the ground as when it first leaves the gun? Again, air resistance aside, I say yes-- it's travelling at least as fast...no way is it slowing down!

ANSWER: 
I will address what I see as the main question, does the water slow down, below. But let me make a few comments about your remarks. First, you are right that the water spreads and so less water per second hits her head if she is sitting down and this may very well be the main reason it is slower to rinse. The argument that gravity slows the water down is totally wrong unless the water is going up. But, your insistence in neglecting air resistance throws out the whole possibility of the water slowing down since that is the only thing which could do it. So let us discuss terminal velocity. We know that if we drop something it will continue accelerating until the force down (its weight) equals the force up (air resistance) at which point it will fall with constant speed (terminal velocity). But what if we don't drop it but rather throw it down with a speed larger than the terminal velocity? Initially the air resistance will be larger than the weight and so it will slow down but continue doing so until it slows down to the terminal velocity. So, your contention that a bullet fired from the top of a building will not slow down is wrong; it will if the initial velocity is greater than the terminal velocity. So, how about the shower water? I looked up the terminal velocity of a raindrop, which should be at least comparable to the droplets from the shower: about 9 m/s. Now, the exit velocity at the shower head will vary depending on the model and I could not find any reference to actual speeds on the internet. So, I went outside and turned on the hose with a spray nozzle attached to it and it appeared to go maybe 20 feet high, about 7 meters; this would correspond to an initial speed of about 15 m/s (just a rough estimate including an estimate of the effect of air resistance). I think it is quite likely that the speed out of the nozzle is greater than 9 m/s and so your sister is probably right but definitely for the wrong reason. The reason her hair rinses better at the top, though, is probably mainly for the reason you suggest which is essentially greater flux, that is more water per second over her scalp, not any reduction in speed.


QUESTION: 
Scientists say there may be extra compacted dimensions wrapped up at the Hubble length (~1.6E-35mtrs), but could not there be just ONE dimension wrapped up at the reduced Compton wavelength (about 3.86E-13mtrs for the electron)? Couldn't a fermion's spin be in that single extra dimension, which merges into the reduced De Broglie wavelength at relativistic velocities?

ANSWER: 
I have very little regard for string theory in general and consider speculations about extra dimensions as just that
speculative. I have never heard the phraseology "dimensions wrapped up at the ______ length" and do not know what it means. I think you must mean Planck length, not Hubble length which is a huge length something like a third the size of the universe. And, I am very sorry that I have no idea what you mean by a fermion's spin being "in an extra dimension". Spin is an angular momentum and intrinsic angular momentum of fermions is not a mystery but is a natural consequence of doing quantum mechanics relativistically (Dirac equation).

FOLLOWUP QUESTION: 
Fermion spin (or intrinsic angular momentum) was stated by Planck, Kronig and others in 1927, as a "4th degree of freedom" implying a 4th hidden real axis perpendicular to the 3 normal axes. Why is it that that 4th degree of freedom is always explained away as "intrinsic", ie not having any actual presence, when the De Broglie wavelength which relates to the Compton wavelength at relativistic velocities, is definitely NOT intrinsic (ie hidden) otherwise there would be no electron microscopes, etc. ?

ANSWER: 
I guess I don’t see that a fourth degree of freedom implies a fourth spatial dimension. Here is an example: When applying the equipartition theorem in thermodynamics, a degreee of freedom is any mode which can have energy. So, for example, a monotomic gas has three degrees of freedom. But a diatomic molecule has also vibrational and rotational degrees of freedom, but we do not associate these with new spatial dimensions. And using the word intrinsic should not be viewed as “explaining something away”. If I say that an O2 molecule has an intrinsic ability to vibrate because of its structure, this is not sweeping something under the rug, it is just realizing that this is a property of a diatomic molecule. In quantum mechanics, quantities which can be quantized play an important role. In a bound physical system, energy is quantized, and angular momentum of the system is also quantized. Discovery of spin was simply discovery that there is angular momentum in a system which is other than that due to the particles moving around, angular momentum which is intrinsic to the particles and they posess even if their orbital angular momentum is zero.


QUESTION: 
My friend tells me positrons travel "backward in time." I say this is total nonsense, a positron is a particle in anti-matter that would be the equivalent of an electron in matter, only with a positive charge. I also say that something traveling backwards in time, could not exist in out universe. So do positrons travel backwards in time?

ANSWER: 
When you draw Feynman diagrams, used to analyze quantum electrodynamics (QED), positrons are often represented as electrons propogating backward in time. An electron going backward in time is mathematically equivalent to a positron going forward in time. Positrons do not travel backwards in time.


QUESTION: 
Light has no mass. Yet light (photons) have energy. E=mc2 says energy and mass are different manifestations of the same thing. Then the energy from light (electromagnetic) should be able to be converted to mass, correct? Wouldn't that mean that light has mass then (m=E/c2)?

ANSWER: 
I get these questions often. The question of whether a photon has mass because it has energy is addressed in an earlier answer. Likewise, the possiblity of converting electromagnetic energy into mass is addressed in another earlier answer.


QUESTION: 
Can a sphere steel ball have a single magnetic charge? (either + or -, as apposed to axialy magnetized)

ANSWER: 
One of the fundamental laws of electromagnetism is that there are no magnetic monopoles. The answer to your question is therefore no. The reason is that the sources of all magnetic fields are electric currents and the simplest current is a small circular loop. The magnetic field of a current loop resembles not the electric field of an electric charge alone but rather the electric field of equal positive and negative charges separated by a small distance, called a dipole.


QUESTION: 
I want to know that when electricity is due to movement of electrons in definite circuit, then how electricity(energy) is exhausted, even when electrons are still in that circuit?

ANSWER: 
Electrons are "pushed" through the circuit by the power source (battery or other power supply). An electron collides with atoms and transfers some of its energy to them thereby heating up the material. The power source gives back the lost energy to the electrons. So, you see, the electrons carry energy from the power source to the material; the ultimate source of energy is the power source. When the battery has given all the energy it can, the current stops flowing.


QUESTION: 
Given the effect gravity has on light as it travel galactic distances isn't it possible we observe more than one image of the galaxies? Light, propagating as a wave, could be "bent" along its route thus presenting us with images arriving out-of-sync and increasing the number of observed galaxies?

ANSWER: 
There is an earlier answer which addresses this point. What you are suggesting does happen, but the multiple images are close to each other and pretty easily recognizable. In terms of your reference to the answer to a recent question, astronomers estimate based on average counts, they do not try to count all galaxies.


QUESTION: 
Place a small rubber ball on top of a basketball or soccer ball and then drop them together. If vertical alignment nicely remains as they fall to the floor, you'll see that the small ball bounces unusually high. Can you reconcile this with energy conservation?

ANSWER: 
The large ball, which has much more kinetic energy than the small ball upon impact with the floor, transfers some of its kinetic energy to the small ball. The large ball compresses and acts like a spring.


QUESTION: 
The P in PET scan stands for positron. Positrons are antimatter. How are the positrons transported and injected without destroying the instruments and how is the positron selecting the correct tissue to destroy?

ANSWER: 
The patient is not bombarded by positrons. What happens is that she is injected with radioactive nuclei which emit positrons. The positrons, very close to where they decay, encounter an electron and they annihilate resulting in two photons which are detected and their trajectories traced back to where the annihilation took place. Read the Wikepedia entry on PET.


QUESTION: 
how much does a lacrosse ball (2 inch diameter) slow down (horizontal velocity only) if thrown at 80 mph from the instant it is released until it reaches a point 10 meters away. Taking into account air resistance.

ANSWER: 
I prefer to work in metric units so 80 mph is about v0=35 m/s and the diameter is about D=6 cm=0.06 m. I will also need the mass of a lacrosse ball which I looked up to be about m=0.15 kg. Now, for a ball of this size traveling through air with this velocity, the air resistance force is proportional to the square of the velocity. Therefore Newton's second law is of the form -Cv2=ma=m(dv/dt) where C is a constant which can be calculated approximately as C=0.22D2 for a sphere in air. Therefore we must solve the differential equation (dv/dt)+0.00079v2=0. (I completely ignore gravity because the ball starts with zero velocity in the vertical direction and flies for only a very short time.) If you know differential equations, then this is not particularly difficult to solve. I will do that later. For starters, however, it is instructive to make a reasonable approximation and see what we get. I am going to say that I expect, over so short a distance as 10 m and starting with such a large initial velocity, that the acceleration will not change much. So I will say that the acceleration at the beginning, a0=-0.00079x352=-0.97 m/s2, does not change much over the flight. So we have a uniform acceleration problem and we can say x=v0t+
½a0t2=10 and solve for t; I find that t=0.29 s. Finally, we can get the estimated final velocity, v=v0+a0t=35-0.97x0.29=34.7 m/s. So the ball loses about 0.9% of its initial velocity.

For anyone interested in the exact solution of the differential equation, here it is. The solution to the equation is v=v0/(1+kt) where k=Cv0/m. And, x=(v0/k)ln(1+kt). Solving these I find that t=0.29 s and v=33.2 m/s. So, only about 5% of the velocity is lost.


QUESTION: 
I had a discussion with my physics teacher the other day about how everything is in motion. I was wondering, is there anything that is not in motion? Is time in motion? Is space in motion? If so why and if not why?

ANSWER: 
If you are talking about motion, you have to talk about something you can measure the speed of. Time is out of the question since you cannot measure the distance it travels. Similarly, you cannot measure the distance space travels since distance is space. So we are confined to objects with mass and photons, neither of which can be perfectly still. Photons are known to always go at the speed of light, so we can dismiss them in this discussion. Any object with mass will have two things which we can measure: position and momentum (which is mass times velocity). There is a very important law of physics known as the Heisenberg uncertainty principle which states, essentially, that you cannot know both the position and the momentum of an object with arbitrary precision. What that implies, then, is that if you know one of them precisely you lose all knowledge of the other. So, if an object were at "rest", then you would not be able to know where it is, that is it could be anywhere in the universe! So, certainly, an atom in a piece of iron (if it is in that piece of iron, you know something about where it is) cannot be at rest.


QUESTION: 
My question relates the emergence of photons from the sun. Four hydrogen nuclei fuse to form one helium nucleus. A helium atom has two protons, two neutrons and two electrons. If we count all four hydrogen atoms which are involved in the production of one helium atom,then we have four protons and four electrons. If we equate two protons and two neutrons of helium with four protons of four hydrogens then what about four electrons. What happens to them? How are photons formed in the sun?

ANSWER: 
You should first look at the a detailed description of the fusion processes actually going on in a star. Although it is not as simple as you suggest, the net result is to fuse four hydrogens to one helium. However, your suspicion that something is wrong is correct because electric charge is not conserved if those two electrons just disappear. Essentially (not in detail) two of the protons and two of the electrons combine to make two neutrons. How this is actually achieved is that two hydrogens fuse to one deuterium (heavy hydrogen consisting of a proton and a neutron) and a positron and a neutrino. The positron annihilates with the extra electron and two photons result. So you end up with a deuteron, an electron, a neutrino, and two photons, all having a net charge of zero which is what you started with. Next you would think you have to get two deuteriums to fuse into one helium; this, however, does not happen but there are several ways that you can end up with a helium (helium 4, that is) which are described in the link above.


QUESTION: 
I know a little about the bohr model of the atom and I am curious about how the nuclear charge of an atom affects it's radii. If the nuclear charge increases I would think that the radii would decrease but I'm not sure of the math and if that model holds up.

ANSWER: 
If you do the Bohr model except for a central charge of Ze instead of e (i.e. magnitude of the force on the electron is keZe2/r2, you will find that the radius is a0/Z where a0 is the radius of the hydrogen orbit in the ground state.


QUESTION: 
This is a slightly odd question, but what would happen if you threw a boomerang in space? For example, if you were an astronaut and you drifted away from a satellite, but you threw a boomerang to give you velocity in the direction of you wanted to go, would it come back, enabling you to throw it again and accelerate even more?

ANSWER: 
The boomerang comes back because of aerodynamic effects, that is it would not come back if there were no air. If it did come back you would also get an acceleration when you caught it.


QUESTION: 
Dear sir, in momentum conservation law apply when there is no external force acting on system . Why it apply in vertically motion presence in gravity ?

ANSWER: 
This is a good question. One reason it is good is that students seldom ask it although they are told that you must have zero external force for momentum conservation. It is actually the impulse which must be zero for momentum conservation, and it is possible for impulse to be zero without force being zero. Let us review where momentum conservation comes from. Write Newton's second law in the form
Δp/Δt=F, so Δp=FΔt. The product FΔt is called the impulse and change in momentum is equal to impulse. How can we make impulse be (approximately) zero? One way is to make F=0, but, even if there is a force present, if the time during which the force is applied is very small, then impulse is approximately zero and the momentum is approximately conserved.


QUESTION: 
About how many galaxies are there?

ANSWER: 
You cannot know for certain since we cannot see the whole universe. Astronomers estimate there are 100-200 billion galaxies.


QUESTION: 
Concerning the twin paradox: one twin remains on earth and the other leaves by rocket that (shortly after liftoff) continuously accelerates at one g, then reverses thrust and continuously deccelerates at one g for an equal time period. Then the astronaut returns to earth using the same means. Does the astronaut experience time dilation as predicted by General Relativity? If so, I'm a little confused, because I thought that Einstein used the Equivalence Principle as part of the foundation of General Relativity, and since both twins experience one g during the trip, how can you say that one accelerated frame of reference is different from the other?

ANSWER: 
There are two kinds of time dilation. The first is that due to motion relative to an "at rest frame". This is the one you usually associate with the twin paradox and it is negligibly small unless the velocity is not small compared to the speed of light. The second is gravitational time dilation which says that the stronger the gravitational field the slower the clock runs. And, you are right, the astronaut's clock would run slower at the same rate that the earthbound clock would due to this gravitational time dilation. But there would still be the motional time dilation; it would just be more complicated to calculate because the velocity would be constantly changing. (Incidentally, note that your stipulation of constant acceleration relative to the earth might not be possible; see an earlier answer.) Finally, gravitational time dilation in a field as small as that at the surface of the earth is extremely small and if that were the only time dilation going on here there would be a very small aging difference between the twins if the astronaut traveled at a constant speed so that his gravitational/acceleration time dilation were zero. In other words if both effects are taken into account and accelerations were only on the order of g and if speeds got comparable to the speed of light, the general relativity effects would be negligible.


QUESTION: 
What is the principle or reason for a glass breaking when a certain pitch is reached- or two like waves meeting and cancelling each other out? Are they the same principle and does it show up in other areas of physics?

ANSWER: 
The cancellation is called destructive interference of waves; it results from what is called the superposition principle which states that if two or more waves are in the same medium the net disturbance is the sum of the individual disturbances. The glass breaking is the result of resonance where the pitch is just right so that the sound waves bouncing around in the glass are in phase with each other and add up (called constructive interference) resulting in a large enough amplitude to break the glass. So the two phenomena both result from the superposition principle.


QUESTION: 
I am wondering. With the uncertainty principle stating that we cannot "know" both the position and momentum of a particle at the same time, and the fact that photons behave with both wave-like and particle-like properties, is the speed of light an actual constant or is it our best approximation? Can we know both the position and momentum of photons?

ANSWER: 
Your second question first
—no, we cannot know precisely both the position and momentum of a photon. But, just because we cannot precisely know the momentum does not mean we cannot precisely know the velocity of a photon. For a photon the momentum is proportional to the energy so uncertainty in momentum implies uncertainty in the energy of the photon, not its speed. One of the cornerstones of modern physics is that the speed of light is a universal physical constant.

FOLLOWUP QUESTION: 
I must admit to being a bit confused about the difference between Momentum and Velocity. Both are vector quantities (from what I can find) and seem to be expressions of the same value.

ANSWER: 
You are right that velocity and momentum are both vectors. And, both point in the same direction. But they are very different things. In classical physics, velocity is just what you think it is, rate of change of position. It has the dimensions of length/time, for example miles/hour or feet/second. But momentum is the object's mass times its velocity. It has the dimensions of mass times length/time, for example kilogram meters/second. So a car and a baseball, both going 100 mph north have the same velocities but very different momenta. If classical physics were true then, since the car or baseball have the same mass no matter what the velocity, an uncertainty in the momentum would automatically mean an uncertainty in the velocity. But when we do things relativistically things get more tangled up and the momentum is much more complicated to calculate, but it obviously cannot be mass times velocity anymore because a photon has momentum but not any mass. The momentum may be written for a particle of mass m and speed v as p=mv/
√[1-(v/c)2] where c is the speed of light; it may also be written in terms of the energy E of the particle as p=[√(E2-m2c4)]/c. For a massless particle, m=0, you can see that the momentum is particularly simple: p=E/c. And, all massless particles must move with exactly the speed of light.


QUESTION: 
this is something which came in my mind.the question is........ we know that : let a book is lying on the table means its in rest so the weight of the book or downward force must be equal to reaction force or upward force. weight of the book=force given by the table on the book. Now if we put another book on this book the equation goes: weight of first book+weight of second book=force given by the table on the books. since from above equations: force given by the table on the book=force given by the table on the book or weight of the first book=weight of first book+weight of second book then weight of the second book is zero(0) how come its possible. plz plz make me clear.

ANSWER: 
There are lots of clues in your question that you misunderstand Newton's first and third laws. You refer to the upward force of the table on the book as the "reaction force" which implies that you think that Newton's third law (N3) is the reason it must be equal to the weight. The "reaction" force, that referred to in Newton's third law, is never on the same body as the "action" force; for a more thorough discussion of Newton's third law see an earlier answer. The weight of the first book is the force which the earth exerts on the book; the reaction force is the force which the book exerts on the earth. The reason the force which the table exerts on the book is equal and opposite to the book's weight is that the book, being in equilibrium, must have the sum of all forces on it equal to zero; this is Newton's first law (N1). I will try to go through each of your two questions individually:

  • The book has two forces on it, its own weight and a force from the table. Those are the only two forces. Because of N1, the force from the table must be up and equal in magnitude to the weight.
  • I will call the top book T and it has a weight WT. I will call the bottom book B and it has a weight WB. The table exerts a force on the bottom book and I will call that F. Book B exerts a force up on book T and I will call that FTB. Book T exerts a force down on book B and I will call that FBT. Focus your attention on book T. There are only two forces on it, its own weight and the force from book B. It looks exactly like the one book problem (it is the one book problem!) and so the force FTB must be equal in magnitude to WT and pointing upward because of N1. Now, focus your attention on book B. There are three forces on book B, its weight WB, the force from the table F, and the force from book T FBT. These three must add up to zero because of N1. But we do not know FBT. But wait, aha! We know that FBT is the force which T exerts on B and FTB is the force which B exerts on T, so N3 tells us that these must be equal and opposite. Therefore the magnitude of FBT must be equal in magnitude to WT and point down. Putting it all together and using N1, the magnitude of F must be WT+WB and F, of course points up.

A final word of warning: never say something like "the weight of the top book is a force down on the bottom book" because the weight of anything is a force on that thing, not something else. Just because it works out here that the magnitude of the force the top book exerts on the bottom book is equal in magnitude to the weight of the top book does not mean the "forbidden statement" above is true. If you had a book on the floor of an upward accelerating elevator, the magnitude of the force of the book on the floor would not equal the magnitude of the weight of the book.


QUESTION: 
Ohm's Law states that V=IR. The greater the resistance for a given voltage, the less the current density. However, what if there is no resistance (for instance, in a superconductor), then the voltage should be maintained at its original value conceptually. However, if V=I*0, then V is also zero. In another case, I=V/R, I would be undefined. Could you explain this scenario?

ANSWER: 
Ohm's law is not a law at all. It is an equation which approximately describes how some materials, called ohmic materials, behave over some range of voltages applied and currents flowing. A superconductor is not an ohmic material and Ohm's law does therefore not apply.


QUESTION: 
where fission occurs is the mass energy released in discrete quantum units?

ANSWER: 
There is no one fission process, but rather a huge number of possibilities. And fission is not the last thing to happen but is followed by ejection of many neutrons and there being many radioactive decays of the fission products. But certainly every event following a specific fission event carries a discrete amount of energy.


QUESTION: 
I understand that the all existing electrons were created in the “big bang” as various scientists think. Can you tell me what we would observe if a large amount of matter existed with no electrons present? Sort of like a galaxy size collection of protons and neutrons but no electrons present since the start of the universe?

ANSWER: 
There would be no stars, no planets, no galaxies. The electrostatic repulsion would be hugely bigger than the gravitational attraction and nothing could stick together.


QUESTION: 
I've been reading about reletivity lately and I'm not quite sure I get it. My question is that if all perspectives are equal then if you had two objects moving in opposite directions at just over half the speed of light would they have broken the universal speed limit with respect to each other?

ANSWER: 
Good question and one which is often asked here. In fact, I recently did an interview with Popular Science magazine on this question; an article is scheduled in an upcoming issue. What you are doing is using what is called Galilean velocity addition to get the answer. For example, two cars going in opposite directions each going 60 mph each see the other approaching with speed of 120 mph, right? Well, it is actually not right but it is so close to being right that no measurement we could do would detect the error. In general, Galilean velocity addition is v'=u+v where u is the speed of one car, v the speed of the other, and v' the speed one sees the other approaching. As I said, this is only approximately true. I have written down the pertinent equations is an earlier answer and, you will see, v'=u+v is not even close to being true if the speeds are, as in your question, not very small compared to the speed of light.


QUESTION: 
Two guys are on a train, this train is fast moving (say, 7/8ths the speed of light) they are several meters apart. They each have a stopwatch, which they agree to start when they see the light of a light bulb turn on. The mediator on the train turns on the light, and, as relativity states, they start their watches at the same time from the perspective of the mediator. The second mediator, standing on the train station that the train happens to pass by, however, disagrees, stating that the clocks were not started at the same time, as the light took longer to reach the guy on the front of the train, as it had to catch up to him at difference of 1/8th the speed of light, whereas the guy on the back approached the light at a combined speed of 15/8ths of the speed of light. Now, I understand that simultaneity, and time are relativistic, and each point of view has equal merit, and is considered true. For the first mediator, the clocks have the same time. For the second mediator, the clock at the front of the train will be behind than that of the back of the train, as I understand it. My question is, what happens when the 1st mediator and his two clock bearing friends hop off the train, and meet the second mediator in the town between the two stations for some lunch and tea? Will the clocks be in synchronization, or will one be slower than the other?

ANSWER: 
There are the same issues of simultaneaty in when the two guys leap from the train as when they synchronize their clocks. It is too complicated to go into in detail here, but the result is that there ends up no discrepancy regarding what is observed by the guys and the person at the station.


QUESTION: 
In Einstein's thought experiment where a man is in a chest being pulled through space where there are no nearby large masses, Einstein implies the man would believe he was in a gravitational field and could not tell the difference between that situation and his actual situation. My question is, why couldn't he measure the acceleration at the floor of the chest and again at the ceiling of the chest and see that they were the same, whereas if he were in a gravitational field the acceleration would be less at the ceiling of the chest since gravitational acceleration decreases with altitude? Of course I'm assuming he has instruments sensitive enough to detect the very small differences that would show up if he were in a gravity field.

ANSWER: 
This thought experiment says that the experimenter would not be able to perform any experiment where he could distinguish between the uniformly acclerating frame and a uniform gravitational field. You are right, the earth's field is not exactly uniform, only approximately uniform. But this is not the issue in the thought experiment.


QUESTION: 
I'm trying to figure out how magnets work exactly. I have that the individual atoms inside a magnet line up according to their poles. how do atoms have poles?

ANSWER: 
The real source of magnetism is electric currents. As you doubtless know, you can make a bar magnet by wrapping an electric current carrying wire around a cylinder. So, what is a bar magnet which (ostensibly) does not have currents? As you say, the atoms are themselves magnets. How can they be? The electrons running around in little orbits are currents; each electron behaves as if it were spinning and this is a current since an electron is charged; protons and neutrons also have magnetic moments but much smaller. In most materials all these atomic moments add up to nothing because all the "poles" are randomly oriented. In a very few special materials (called ferromagnetic materials) there is a tendency of the atomic electrons to couple up with their nearest neighbors and all align. It is the intrinsic (due to spin) magnetism of the electrons, not their orbital motion, which is at the root of ferromagnetism.


QUESTION: 
I know that any matter that has mass generates gravity. Furthermore, I also know that in Einstein's E=MC squared that as any massive particle approaches the speed of light its mass will become infinite at the speed of light. With that being said, I imagine that a hydrogen atom (arbitrary massive object) if accelerated to (X) percentage of the speed of light and maintained at that velocity. It would then generate substantial mass, therefore substantial gravity, to fit our needs to use artificial gravity. My first question is would this accelerated hydrogen atom disrupt other massive objects described in Eistein's General Theory of Relativety in manner that may cause shifting planetary orbits and that sort of thing?

ANSWER: 
I am not sure what you want "artificial gravity" for, but let's think about the numbers. If you would accelerate a hydrogen atom to an energy of 100 TeV (greater than any accelerator on earth can achieve) you would increase its mass by a factor of about 100,000, so it would have a mass on the order of 10-20 kg. The gravitational force due to such a mass on a 1 kg object at a distance of 1 mm would be about 10-24 N. And, it would only last for an instant as the atom passed by. I am afraid I have to conclude that this would not be a practical source of gravity.


QUESTION: 
Can a particle accelerator create more energy than is needed to operate it? youir answer is very much appreciated.

ANSWER: 
In principle, yes. This is the basis for all proposed fusion reactors. In the tokomak type reactors, a hot gas is confined (heating it up hot qualifies as an accelerator, I would say) and nuclei fuse to create energy, hopefully more than the input energy although this has never been done yet. In the laser reactors, laser pulses cause a hydrogen pellet to implode (acceleration) and subsequently the hydrogen fuses into helium releasing energy. As a further illustration, take a simpler scenario
just accelerating a deuteron (heavy hddrogen) and causing it to collide with a helium-3 nucleus, you get a helium-4, a proton, and more released energy than the kinetic energy the deuteron had coming in; so that should illustrate why I say yes in answer to your question.


QUESTION: 
I have things that has bothered me for a while and I have searched but never found the answer. Here it is:
1) Why is gravity created ?
2) Why does gravity act on matter?
3) Does gravity pull towards the centre of any matter ?
I have theories but since I am not very knowledgeable in Physics, I haven't a clue if I am on the right path. So I thought I would ask someone more in a position to do so. My assumption is that the atomic structure of matter is the creator of gravity! Thus you would have denser matter like stuff on a neutron star exhibiting huge gravitational forces. Assuming all of that makes any sense, what I don't understand is how the atomic structure manages to create this gravity. Is it the energy of the atoms ?

ANSWER: 
The best explanation of gravity is called the theory of general relativity. In essense, the idea is that the presence of mass causes the space/time around it to deform and the result is that objects with mass are seen to attract each other. It has nothing to do with atomic structure. There are numerous answers to earlier questions which discuss general relativity (a, b, c, for example).


QUESTION: 
What is the proper formula for calculating the inside temp of an aluminum container sitting outside (temp 100 degrees)

ANSWER: 
There is no such formula. It depends on numerous things, the initial temperature of the aluminum and whatever is inside, the time it has been sitting outside, the geometry of the container, and whatever it is inside it. After a sufficiently long time the formula is T=1000.


QUESTION: 
What effect would extreme gravity have on a sound wave? If i were calling your name from across the room on Jupiter, would you be able to hear me? Would my words fall right to my feet (ignoring the fact that I'd be crushed by the gravity we're discussing)?

ANSWER: 
Sound is a disturbance which moves through the medium in which it is propogating. It is not something which has mass and would therefore experience a force. So it would not be bent downward in a strong gravitational field.


QUESTION: 
If Electrons can turn into gamma rays, then could you reverse this process, and turn gamma rays into Electrons?

ANSWER: 
An electron does not "turn into gamma rays", but an electron and a positron (its antiparticle) can annihilate and that results in two gamma rays. A gamma ray, provided it has enough energy to create twice the electron mass, can create a positron-electron pair; this is called pair production. However, this does not happen spontaneously but the gamma ray must be perturbed somehow to sort of "trigger" the pair production. This trigger is usually a very strong electric field, near the nucleus of an atom the gamma ray is passing through.


QUESTION: 
electron and photon have same energy 105ev.what is the ratio of their momentum if any.My friend asked me this question but i doubt its validty.

ANSWER: 
I don't know, this sounds like a homework problem to me. I will not work out this specific case, rather will sketch out the general case. Since the kinetic energy is much less than the rest mass of either particle, the problem may be treated nonrelativistically. So, kinetic energy is
½mv2 and momentum is mv. Knowing that the energies are equal, we can find the ratio of the velocities of the two particles as a function of the ratio of their masses; knowing this, we can write the ratio of their momenta as a function of the ratio of their masses.


QUESTION: 
If a point fixed on the circumference of a rotating wheel (physical or theoretical) has an ever increasing velocity as the radius increases, does the perfect center (singularity) have any velocity at all? i.e., Given the shaft of an electric motor with a radius of 3 cm, a point at r=3 has a higher speed than point at r=1.5, what is the velocity at exactly r=0 (if any at all)?

ANSWER: 
Since v=r
ω, where ω is the angular velocity, if r=0 then v=0.


QUESTION: 
My question has to do with diagnostic medical x-ray production. I have investigated several sources (textbooks, and the web), but have not found the answer. In the production of characteristic x-rays several texts give the same table of energies indicating the energy of the characteristic x-ray produced for each of the various electron transitions. For example if a k shell electron is ejected from the target material (let's say tungsten) and then if an L shell backfills the void, the ensuing x-ray is 57.4 keV(just the difference in electron binding energies), if M shell fills the void, then 66.7 keV, etc. Then they always give an "effective" energy value, which for k shell is 69 keV. They never explain where this "effective" value comes from. My question is how is this effective value arrived at? I have tried to use weighted averages based on the number of electrons in each shell, but never get the same value presented in the table. I emailed one author (whose name I won't mention) of such a textbook, and he wasn't sure how it was calculated. He was not a pure physicist but a radiographic technologist turned author.

ANSWER: 
This is a pretty technical question because in specialized fields like radiology, there are often "jargony" things which become part of the lore of the field. Some quick research on my part indicates that effective energy is only semiquantitative and depends on a measurement. The energy would seem to be an instrumental average of some sort over all x-ray energies in the spectrum. Where I can provide, as a physicist, some insight is into the nature of the x-ray spectrum. It seems puzzling, at first, why the two most likely characteristic x-rays are both less than the effective energy. However, x-rays coming from atomic transition are a relatively small part of the overall spectrum. Look at a spectrum and you will see that there is a very strong continuum under the transition lines. This continuum is called bremsstrahlung (German for braking radiation) and results from the fact that electrons are slowing down and hence radiating energy. The highest this energy can be is the energy of the electron beam. Although this may not look as big as the much higher transition peaks, it is the greater part of the whole spectrum when integrated. It is easy to see how a weighted average of this spectrum could be higher than the highest transition peak.


QUESTION: 
Hey..so the question is an MCQ...and it hopefully will not violate your ground rules...:p...here goes... With a rise in the boiling point of water, the latent heat of steam:
A) decreases
B) increases
C) does not change
D) may increase or decrease depending on the actual temperature
Please explain, it will help end a lot of differences with my physics teacher.

ANSWER: 
I assume you mean latent heat of vaporization of steam. The correct answer for the range of boiling points 17-2340C is (A), it decreases. This corresponds to a pressure range of 0.02-30 bar. I could not necessarily rule out (D) as the answer for points outside the range I quote. My reference is a table I found on the web. (What's an MCQ?) (If you meant the specific heat of the steam, the answer is (B) as you can see from the table.)


QUESTION: 
I understand field lines in magnetism and electric fields, but i always thought they were theoretical for the purpose of analysing problems. However there are the simple experiments that show fields lines are a physical phenomena (like the iron filings on paper for magnets, or grass seeds in oil for electric fields). What I don't understand is this: What's in between the field lines? I would have thought the fields were continuous and that the iron filings wouldn't form into specific lines.

ANSWER: 
Your gut feeling at the end of your question, "
I would have thought the fields were continuous" was right. The field fills the whole space around a magnet. So, what is going on with the iron filings? The field causes each little filing to become a magnet itself and it lines up with the field of the big magnet. But nearby filings also become little magnets and they get attracted to and attached to their neighbors so they make chains. These "chains" get made rather randomly but their exact location is not relevant, is not an indication of some line of field. If you shook everything up and started all over again, the chains would not appear in exactly the same places. The same is true in the electric field except the seeds become little electric dipoles by virtual of electric polarization. (I just noticed that I answered this question a long time ago. Maybe that answer is more lucid to you.)


QUESTION: 
My 5-year-old son Alex asked me why gravity still effects people and objects inside of a house, why the house does not block the force from acting on us. I thought it was an interesting question and wasn't sure how to explain it. Any suggestions?

ANSWER: 
Well, Alex, gravity is just something you cannot block. If you could figure out how to block gravity you would be the youngest Nobel prize winner ever! Any two objects attract each other; for example, the sun pulls on the earth with gravity (which is why we are able to orbit around the sun). But suppose that we look at the earth when the moon is between the earth and the sun which happens during a solar eclipse; does the moon block the sun's gravity, even a little bit? The answer is no, since the moon also pulls on the earth, the earth actually feels more gravity during an eclipse. When you are in the house the floor pulls down on you and the roof pulls up on you but these are such tiny forces that you never see them. The earth is so heavy compared to your house that the house makes no difference at all.


QUESTION: 
I have been trying to understand how particles work and interact though I am not a physicist, but rather a computer scientist, the result being that I understand algebra and calculus but I am not so good at vectors, the “meat” of Quantum Physics. I am trying to solve a “simple’ Schrödinger equation for an ultra-simple system, such as a fictitious particle with simplified properties. I see examples but those posted on the internet skip steps which a classically trained physicist or mathematician would already know, but I apparently do not. Is my question too outlandish, or is there something amiss in my reasoning which I am unaware of in my naivety of the subject? I choose the criteria of question because the example I refer to is used in the Quantum Physics book I am reading, by Alastair I. M. Rae. (A great book for a beginner!) [My Question] I would am trying to understand how a Schrödinger equation describes the physical states of a system, but I am having trouble solving the equation. Would you show me a simple step-by-step solution for an ultra-simplistic system using a Schrödinger equation? Perhaps the famous particle in a box would be simple unless you had a better idea? A box, or number line, with a maxima denoted by a and minima denoted by b and the rule V=infinity when a > x > b.

ANSWER: 
Calculus is much more the "meat" of quantum mechanics than vectors. To do QM in three dimensions you need to know some vector calculus, but the requested problem is a one dimensional one so we just need calculus and some simple differential equations. I will supply you with the detailed description you want for the particle in a box, but I am quite surprised that you can't find the equivalent in any QM book or a dozen places on the web. You ask for the solution between x=b and a but, since the choice of coordinate system is never really relevant, I will choose x to be between 0 and a because the math is much more transparent. Here is the solution.


QUESTION: 
Regarding Einsteins general relativety theory, and the speed of something relative to something else. What if your flying an airplane at 100mph and you fire a gun backwords, the bullet leaves the gun at a 100mph. The bullet then stands totally still relative to the earth, and since the earth is our main gravitation, what happens next? Does the bullet just stand still in the air for a while, or does it instantly start dropping towards the earth?

ANSWER: 
We are not talking about Einstein's general relativity here, just classical Galilean relativity since the speeds are so small. The answer is that the bullet will drop (as seen by someone on the ground) exactly as if you dropped it from a hovering helicopter. You can read a complete discussion of classical velocity addition in one of my recent answers. For a discussion of velocity addition in special relativity, see an earlier answer.


QUESTION: 
The unanswered question of the big bang is how did gravity weaken so that everything didn't collapse in on itself. My question is- If the big bang started from a point smaller than an atom and the laws of physics do not apply to things of this size then how was there gravity at the point the big bang happened?

ANSWER: 
First of all, your statement that the "laws of physics do not apply to things of this size" is flawed. By definition, laws of physics always apply or else they would not be proper laws. It is just that in different limiting situations, the true laws take on different approximate forms so that it appears that there are different laws. But the holy grail of physics remains to find expression for the one law or set of laws which applies universally. This is what the so-called theories of everything, TOEs, are about. In essence, you ask the wrong question since collapsing in on itself is determined by initial conditions, not the strength of the forces. That is, if the universe is flying apart fast enough it can escape any force. Besides, maybe it will eventually fall back on itself; this is one of the open questions of cosmology. There is a very nice article on Wikipedia which outlines how the fundamental forces developed over time after the big bang.


QUESTION: 
differnce betwwen kinetics and kinematics

ANSWER: 
Kinematics studies the motion of objects without reference to the causes of the motion, that is it does not include the forces. Kinetics (and this I just get from the dictionary) is the study of motion including the causes of that motion, but most physicists I know use the term dynamics for this.


QUESTION: 
If there were a black hole with the exact same mass as Earth, would an object 8000 km away (any distance greater than Earth's radius) experience the same acceleration that it would if it were 8000 km away from Earth's center? More generally, if you have a constant amount of mass confined to a sphere of radius R, and you compress the matter into a smaller sphere, will the gravitational field be exactly the same as it was before (at distances greater than R)? If so, would this also apply to charge and electric field? That is, if you had a constant amount of charge confined to a sphere of radius R, and you compressed it into a smaller sphere, would the new electric field be exactly the same as the original one (at distances greater than R)?

ANSWER: 
I have read that Newton, worried about this problem, delayed publishing his laws of gravity for 20 years. We know that the gravitational force is inversely proportional to the square of the distance between two point masses, but what is it between two spheres? Newton had to invent integral calculus to find that the gravitational force due to a (spherically symmetric) sphere is exactly the same as that due to a point mass of the same mass if you are outside the sphere. The same holds for electrostatics since the force law is also an inverse square law. Modern mathematics allows easy solution of this problem using what is known as Gauss's law.


QUESTION: 
According to Albert Einstein's equation "E=MC2", If you accelerate an object at twice the speed of light, will it become energy? And does it matter what type of physical composition the object has? The last question I ask. Is it possible for scientists to to get an object to such a high speed without being in the vacuum of space??

ANSWER: 
You have this completely wrong. What this equation says is that a mass M, at rest, has an energy of Mc2. It is, in fact, impossible for a material object to go even as fast as the speed of light c, let alone twice that speed.

FOLLOWUP QUESTION: 
I do not understand E=MC squared. How does an object having mass, such as a pencil at rest, store that much potential energy? I understand that an object in motion has kinetic energy. Also, how fast can energy actually move?

ANSWER: 
It is not a question of the mass "storing up" potential energy. It is simply a statement about how nature works. It may be derived from special relativity. And, most important, it is experimentally verifiable. Maybe the reason that you are having trouble understanding is that we normally do not see the whole mass of something suddenly converted into energy; if your pencil suddenly converted all its mass to energy it would be 2000 times the energy of the most powerful H-bomb ever detonated. In an H-bomb, and in the sun, the process is essentially taking two hydrogens and combining them to make one helium (many times over, of course). The mass of the helium is slightly less than the sum of the hydrogen masses, and this mass is where the energy comes from. On a tiny scale, we do see complete conversions of mass to energy; when an electron meets its antiparticle, called a positron, they both disappear and pure energy (in the form of gamma rays) appears. Regarding your second question, nothing can propogate faster than the speed of light. Light itself carries energy at the speed of light; anything else carries its energy at a slower speed.


QUESTION: 
If Iron-56 is the most stable type of nucleus, why is it that most radioactive elements decay to lead rather than iron?

ANSWER: 
You are apparently thinking of radioactive elements heavier than lead, e.g. radium, uranium, etc. Most of these decay by alpha decay (removing two protons and two neutrons) or beta decay (changing a neutron (proton) to a proton (neutron) and ejecting an electron (positron)). These decay until they get to lead or near by because lead is about the heaviest stable element and has many isotopes to decay to. The only way to make a big jump down is by means of spontaneous fission which is very rare. Nearly all decays of elements lighter than lead are beta decay which keeps the mass about the same but changes the ratio of protons and neutrons. Overall, even though there is one most stable nucleus, there are many stable nuclei and a decay generally finds the shortest route to the nearest stable nucleus.


QUESTION: 
Why hasn't the asteroid belt condensed or at least started to condense itself into a planet? I was always taught that most planets start as spinning rings of dust and matter (much like the Kuiper Belt) and over time, bada bing, you got yourself a new planet. Has the fact that the Kuiper belt has kept itself apart for so long switched to conventional thinking of how planets are formed, or is there some other explanation as to why it has been stuck in the first planetary formation phase?

ANSWER: 
This is well explained in the Wikepedia article on the asteroid belt. I quote: "gravitational perturbations from the giant planet [Jupiter] imbued the planetesimals with too much orbital energy for them to accrete into a planet. Collisions became too violent, and instead of sticking together, the planetesimals shattered. As a result, most of the main belt's mass has been lost since the formation of the Solar System. Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets."


QUESTION: 
I have a question about how electric guitar pick-ups work. I understand that the string's vibrations induce a current in a coil (with a magnet in the middle) by alternating the magnetic flux through the coil (I have a general understanding of how inductors and transducers work); but what's confusing me is: if the guitar string isn't magnetic, then how does it's movement change the magnetic flux through the coil and induce a current? Does the presence of the string warp the shape of the magnet's field? I can't really find anything online about how conductors interact with magnetic fields. I can't remember if the conductor displaces the field or if the field passes right through it. Anyway, I'd appreciate it if you could help cure my confusion.

ANSWER: 
The pickup consists of a magnet and a coil. The magnet provides a strong, static field where the string is. If a conducting material moves through a magnetic field, a current is induced in the conductor. Hence, the vibrating string carries a time varying current. Now, this current causes a time varying magnetic field which is superimposed on the static field. The varying magnetic field induces a current in the coil which is then amplified. The guitar pickup was invented by the pioneering guitarist Les Paul who just died last week.


QUESTION: 
Einstein found that if a person could travel at the speed of light, time would stand still. Their mass would also be infinite but what can you do? The question then is, "how does the universe look from light's point of view?" Is time a factor? If yes, then how? And if not, then what would the universe be like from that perspective?

ANSWER: 
Einstein found no such thing because no material object can travel at the speed of light. And, light does not have a "point of view". I have answered this question several times before; see one of my earlier answers.


QUESTION: 
Does the Sun's gravitational field cause the moon's orbit to become more elliptical over time? If that's true, how much of an effect does the Sun have? I visualize, in two dimensions, the Earth's warping of spacetime (leaving out the sun for the moment) to cause the moon to orbit it. But by throwing the Sun into the picture, my logic leads me to think that the moon would gradually get closer to the Sun because of it warping spacetime so much more than the Earth. Is there a problem with my visualization? Or is there something more that I haven't considered?

ANSWER: 
There is nothing to be gained by trying to visualize warped spacetime. The "bowling ball on a trampoline" picture is useful primarily as a qualitative picture to illustrate what we mean by "warping" but it is by no means a true picture (it is, after all, only a two-dimensional space being warped). The effect of the sun on the moon is rather small but not totally negligible. Certainly the result is not to make its orbit more elliptical over time. What actually happens is very complicated but can be calculated using just good old classical Newtonian gravity. The main effect is that the earth's orbit becomes wobbly because the sun acts on the earth-moon system as a single body and sometimes the moon is closer to the sun and sometimes farther. A pretty readable explication may be seen at http://library.thinkquest.org/29033/begin/earthsunmoon.htm


QUESTION: 
Regarding the de Broglie wavelength and simple model of a particle bouncing back and forth in a closed box (and no force field therein), I am trying to understand where the energy comes from if I do the following conceptual experiment. Assume a particle that was moving to the left has just rebounded off of the left box wall (perfectly elastic collision), and now is proceeding (with constant velocity) toward the right wall. While it is in transit, you quickly move the left box wall inward a short distance (before the particle has time to return from the right) thus shortening the separation between the two walls, and reducing the associated de Broglie wavelength. de Broglie's equation mv=h/lambda requires that the particle's velocity must increase. This implies that the kinetic energy of the particle must also increase . My question is, where did the energy come from? My moving of the wall did not add energy to the system.

ANSWER: 
If you are to think about the system quantum mechanically, you cannot say "
before the particle has time to return from the right" Because of the uncertainty principle, or simply because the "particle" is wavelike, you do not know where it is precisely. Hence, it is not possible for you to move the wall in without doing work thereby increasing the energy of the particle/wave.

FOLLOWUP QUESTION: 
I still am having difficulty with the energy source of a shorter de Broglie wavelength. Let me modify my thought experiment a little if I may. Assume the box has a slot in the center of one side whereby I can insert a reflecting plane surface. As the particle is bouncing back and forth between the now fixed ends of the box, I believe the probability of the particle being located somewhere in the space either to the right or left of the box center at any point in time is 0.5 (because of the symmetry of the de Broglie wave shape). I don't need to know where it is at any particular point in time, nor how fast it is going. All I know is that at all times it is somewhere inside the box between the two end walls. I now insert my reflecting plane surface in the slot, thus creating two spaces, each being 1/2 the length of the original space. The particle is now confined to rebound in one of those two now smaller spaces, thus its de Broglie wavelength is now half of the original, its velocity therefore greater along with its kinetic energy. I have done no work on the system which would have provided the additional energy, and I don't need to determine the particle's position-velocity uncertainty profile at any time point. Am I making an erroneous assumption here?

ANSWER: (Thanks to a helpful discussion with S. P. Lewis and W. M. Dennis)
I have consulted with several physics professors and here is the concensus:
The simplest case to understand is if you move the wall in "adiabatically", that is very slowly. Here you keep the particle in its ground state and slowly increase its energy so that you must do work on the particle. If you move it in rapidly, you end up with the particle not in its ground state but in an admixture of many states. But, still, you must do work because the energy of the system increases; you are the source, again, of the added energy. Now, we come to the part that got me most hung up, moving the wall instantaneously to the center (which is, in most ways, equivalent to your dropping the barrier into the middle). The real problem here is that the potential walls are infinitely high and if you instantaneously move the wall to the center then there is half of the original wave function outside the well at t=0, and this is forbidden due to the height of the wall being infinite and so the implication is that an infinite amount of work has been done to get some of the wave function in that forbidden region; this means, of course, the particle now has infinite energy. This is, obviously, not a possible situation; one eventually gets into conundrums like this when assuming infinite (i.e. unphysical) potential barriers. So, the bottom line is that the increased energy the particle will have will come from whoever moves the wall and that wall cannot be moved instaneously. Similarly, your "mirror", which I presume is an infinite potential barrier on either side cannot instantaneously drop in without doing infinite work for the same reason the wall cannot move instantaneously in. In other words, you need to give the wave function the opportunity to obey the boundary condition (
Ψ=0 at the wall) to keep it inside the box.

An interesting variation is the reverse process of expanding the box. If you go out slowly, so the particle remains always in its appropriate ground state, the particle will do work on you. But if you go out instaneously you leave (at t=0) the original wave function sitting there so the energy remains the same, no work done at all. This will then evolve into a superposition of states of the new well.


QUESTION: 
How newton used to calculate the force of gravitation between two objects before value of G was known?

ANSWER: 
To calculate forces and orbital properties you do not need to know G, just the product MG where M is the mass of the body which is the source (like the earth or the sun) and this product is relatively easy to get from data. For example, MG=gR2 where g=9.8 m/s2,
M is the mass of the earth, and R is the radius of the earth. You can also get the product from orbital data; for example, MG=4π2R3/T2 where M is the mass of the sun, R is the distance from the sun to the earth, and T is the period of the earth's orbit (one year).


QUESTION: 
is it possible to calculate the slit width in young's double slit diffraction/interference experiment? I have the wavelength, slit seperation, maxima/minima pattern, and screen to slit distance. the only equations i can find deal with slit width in single slit diffraction

ANSWER: 
It depends on the details you have of the pattern. If you just have positions of maxima and minima you can't infer anything about the slit width, only the spacing. However, if you have the intensities of the maxima, you can calculate the slit width (assuming both slits have the same width) since the diffraction pattern has the single-slit pattern modulating the double-slit pattern. See the picture at the right.


QUESTION: 
The first several statements are assumptions on my part. My question at the end will be based on the statements.
1. The resting mass of a proton is much greater than the resting mass of an electron.
2. The electron travels at, or very close to, the speed of light while it orbits the proton.
3. An objects' mass increases as it approaches the speed of light.
My question is this, Is the speed that the electron orbits the proton, determined by the difference in masses between the proton and electron? In other words, will the electron orbit just fast enough so that its' mass becomes equal to the protons' mass?

ANSWER: 
Statement 2 is blatantly false, so your question is moot. The speed of an electron in its orbit is much less than the speed of light, so relativity is a small correction. Also, the mass of a hydrogen atom is less than the mass of a proton plus the mass of an electron because of the binding energy; since it takes energy to pull the atom apart, it must have less mass than the sum of its parts.


QUESTION: 
What happens to water in a black hole? Since water cannot be compressed and a black hole compresses everything into a singularity the two things seem to be at odds.

ANSWER: 
"Springs obey Hooke's law." "Sliding friction is proportional to the normal force between the surfaces." "Water is incompressible." These are all false statements. Actually, they are approximately true statements under proper circumstances, within appropriate limits. But water's being compressible is not really the issue in answering your questions. Any matter, being sucked into a black hole, will be subject to enormous forces such that a water molecule will dissociate into its hydrogen and oxygen atoms, those atoms will become ionized, the nuclei will be torn asunder, and even the protons, neutrons, and electrons will lose their identities as they get compressed to a singularity.


QUESTION: 
What exists between two quarks?

ANSWER: 
The quanta of the field which binds the quarks, called gluons.


QUESTION: 
I was wondering about the ubiquitousness of Planck's constant in energy equations. What is this constant, not in physics terms, but why is it, why does it exist? Planck's length is said to be the smallest dimension in our spacetime reality. Why does this limit exist? What relationship does it have to the speed of light? What causes it? Does anybody ever look at questions like these in physics?

ANSWER: 
There can really be no answer to why certain things exist. That is more like philosophy than physics although physicists like philosophical questions sometimes. There are certain universal constants in nature, the values of which govern how the universe behaves. Other examples are the speed of light and the electron charge. We can ask what the charge of an electron is, but not why it is what it is. Planck's constant sets the scale for when quantum effects become important for energy. Regarding the Planck length, this is purely speculative and we do not know that space is discretized, that is that there is a smallest possible length. The Planck length is related to the Planck time, hypothesized to be the smallest possible increment of time, as the time it takes light to traverse the Planck length.


QUESTION: 
Is gravity a weaker force than it should be and if so why is this?

ANSWER: 
Who is to say how weak it should be? I can only tell you that gravity is the weakest force in nature. Also, it is the only force which cannot be understood in the context of quantum mechanics.


QUESTION: 
I understand that "centrifugal" force is ficticious. Centripetal force is, however, a real force - but doesn't Newton's third law state that for every action there is an equal and opposite reaction? And wouldn't that reaction, in this case, be a "centrifugal" force? And, if the answer is that Newton's laws do not apply in a non inertial frame of reference, how are we permitted to use Newton's second law, F = ma (= v^2/r in this case), to quantify centripetal force?

ANSWER: 
Anybody being introduced to Newton's laws often gets things confused. Let us state Newton's third law carefully. If body A exerts a force on body B, then body B exerts and equal and opposite force on body A. Consider the earth going around the sun. The centripetal force is the force which the sun exerts on the earth. This is the only force on the earth. Where is the so called "reaction force"? Just read the law carefully and you will see that the earth exerts a force on the sun which is equal and opposite to the force the sun exerts on the earth. The "reaction force" is never on the same body as the "action force". We are permitted to use F=ma because what this equation means is: the total force on a body is equal to the mass of that body times the acceleration of that body. The force the sun exerts on the earth is the only force on the earth.

Let's take a look at what it means for there to be a "ficticious force". Suppose that you are in a car which is accelerating. What are all the forces on you? There is your weight, the force which the seat exerts up on you (equal and opposite your weight but having nothing to do with Newton's third law because they are both on you), and the force which the seat back exerts forward on you. Nothing mysterious, the seat back accelerates you forward. But suppose you want to do physics using the car as your reference frame. Then you are not accelerating (relative to the car) but the sum of the forces on you is not equal to zero; Newton's first law is not true in this reference frame. But, you insist on doing Newtonian mechanics in this frame. To do this you say, "hey, there is a force pushing me back in my seat". It feels that way but there is no such force. This added force is called a ficticious force added to make Newton's laws to be true in this accelerating system. If you are in a system which is spinning, imagine some carnival ride, you feel like you are being smashed back against the outside but what is really happening is that the outside is pushing in on you to provide your acceleration and you interpret that as your being pushed back. This is just like the car example in that there is no force pushing you outward, it just feels that way. And if you want to do physics using the spinning frame as your reference and using Newton's laws, you have to add a centrifugal force even though there is no such thing.


QUESTION: 
Since energy has mass, does a wire with an electrical current have more mass than an identical wire with electrical current flowing through it? Does this also mean that its gravitational field (no matter how small) increases with a mass increase?

ANSWER: 
Where did you get the idea that "energy has mass"? Light has energy but does not have mass. The mass of a bound system like an atom or a nucleus is less than the sum of its parts. I have my own prejudices about how one should think about the mass of a moving object (see earlier answer). A segment of wire may be interpreted as having increased mass (both from the motion of the electrons and the increased temperature of the wire as a whole resulting from ohmic loss), so it would have increased gravitational field. It would be impossibly small to measure (average electron speed is incredibly small).


QUESTION: 
A poser which my drinking buddies cannot agree on. A train traveling the speed of a bullet A man, stood on top of the train fire's a gun in the in the direction from which the train has come from he pulls the trigger at station 'A' another man, standing on the platform at station 'A' would witness the bullet drop at his feet. True or false? {no atmospheric conditions, physical attributes of the gun, bullet or train are taken into account}

ANSWER: 
True. Read another recent answer for more detail.


QUESTION: 
Does a magnetic field generated by a magnet affect nearby objects at the speed of light, or is it instantaneous? In other words, if I had an electromagnet (that was unpowered and 'not' a magnet at that time) and an iron rod sitting next to it. If I then powered the electromagnet, would the magnetic field created need time to 'reach' the iron bar and affect it, or would the field affect the iron bar instantaneously? I would also ask the same question about gravity fields.

ANSWER: 
Magnetic and electric fields both propogate through empty space with the speed of light. Gravitational fields are believed to also propogate with the speed of light, but a definitive measurement has never been made.


QUESTION: 
What would be the result as regard to ground speed and distance from launch device and launch point at the time of impact to the horizontal surface to an object (lead ball ½” in dia) launched from a device with a muzzle velocity of 60 mph, with the launch device moving at 60 mph, at sea level, in normal still air. Launch the object from the device as it passes a ground fixed launch point above and parallel to the ground which is flat and level. Height above the horizontal surface is always the same at launch, with the elapsed time of one second from launch to impact. Below are the four variables which will be applied to the problem.
1. Launch the object in the direction of travel.
2. Launch the object in the opposite direction of travel.
3. Place the device and the object inside a container which is moving at 60 mph (removing the wind resistance of moving at the speed of 60 mph) (the launch device is fixed inside the moving container) then launch the object in the direction of travel.
4. Place the device and the object inside a container which is moving at 60 mph (removing the lack of wind resistance of moving at the speed of 60 mph) (the launch device is fixed inside the moving container) then launch the object opposite the direction of travel.
Apply this question to each variable; what is the approximate ground speed of the object at the time of launch and what is the approximate distance of travel of the object at the point of impact from the device and the launch point and what is the ground distance from the launch device at object impact. Ground distance = the distance the device travels minus or plus from the from the time of launch to the point of impact. (in one case the device is moving forward and will have moved a specific distance from the launch point closer to the point of impact lessening the distance from the device to the impact point.) The results need not be precise as to include the lift or lack of lift in upon the object in flight. I hope I have explained the problem sufficiently. My friend and I differ as to the result of this question and hope you can resolve it for us. Would there be a substantial difference in the equation if the muzzle velocity and the speed were increased or decreased if they both remained equal to each other? (Forward speed 1000 mph and muzzle velocity 1000 mph.)

ANSWER: 
This question seriously violates the site groundrule stipulating single, well-focused questions; maybe if I answer just this once you will send me a nice donation! I will only address air friction qualitatively because it is very difficult to do quantitatively. Ball 1 would initially move forward (relative to a stationary observer) with speed 120 mph. Ball 2 would drop straight to the ground. For scenario 3, it is not clear how you mean it to work; maybe we should just say there is a "tail wind" of 60 mph. It would behave pretty much like ball 1 but would go farther because air friction would have less effect on it; air friction would not be zero, though, because as soon as it starts falling it feels upward frictional force. For scenario 4, you do not need your box or a wind; it would already start at rest relative to the air. The effects which air friction would have depend on how long the ball is in the air and so, by your conditions, the altitude from which it is launched.
The air friction force is always opposite the direction of the velocity (relative to the air). So ball 2 would drop like a freely falling object landing with an approximate speed
√(2gh) if not in the air too long; if falling more than several seconds, it would acquire enough velocity for air friction to be noticable and therefore hit with less speed. Ball 1 would land with approximate horizontal speed 120 and vertical speed √(2gh) if not in the air too long; however, air friction would probably not be negligible for 120 mph and it would land with a horizontal speed quite a bit less than 120. If it were in the air for a long time, it would end up falling straight down. Outfielders instinctively know this since fly balls end up falling much more downwardly than you would predict neglecting air friction. If you want to get an idea of how difficult it can be to include air friction, see one of my earlier answers. Increasing the speed to 1000 mph would have a big effect on the air friction; air friction increases like the square of the speed for high-speed objects.


QUESTION: 
Are there different types of ultraviolet rays with regards to burning vs tanning? Or is this an innate biological process that is dependent on the amount of exposure and damage to proteins + pigments in the skin?

ANSWER: 
This is a pretty complex topic. I recommend the Wikepedia article on ultraviolet radiation for details.


QUESTION: 
is an electromagnetic wave ( light ) always connected to its source , tethered like or is it something that is shot out like a squirt from a water pistol?

ANSWER: 
No. Once it exits the source it can no longer be affected by the source and is "autonomous". Also, when the source is turned off, that which previously was emitted keeps on going.


QUESTION: 
How and why is the speed of light constant regardless of reference point?

ANSWER: 
See earlier answers.


QUESTION: 
When an antimatter particle interacts with a particle, I have heard two explanations of what happens:
1. They annihilate completely and their mass is instantly converted into energy.
2. They form a force-carrying boson.
Which one of these explanations is correct, or is there a way in which both can happen?

ANSWER: 
The general rule in particle physics is that anything which is not forbidden to happen (by conservation laws or selection rules) can happen. The different possibilities occur with different probabilities. Normally, the classic example of electron-positron annihilation results in annihilation resulting in two photons (or rarely, more than two photons) because the mass energy available is not large enough to create anything else. However, if the electron and positron collide at very high energies, all kinds of things can and do happen.


QUESTION: 
In the famous youngs double slit experiment it was concluded that matter alters via conscious observation....this happens via probability fields being flattened. I want to know what is defined as conscious observation in this context. I.e would the experiment result differently if the observer was sub conscious or an animal perhaps? Could a person with a perception disorder also alter the outcome of the experiment?

ANSWER: 
I do not know where you got the idea that the observation must be conscious
—one of those new age science popularizations? The double slit experiment, classically has no problem being understood. Waves passing through each slit interfere with each other. I think you are talking about having a quantum understanding where the light intensity is so small that photons pass through one at a time. (Similarly, understanding the diffraction if there are particles like electrons.) However, this is just the opposite of what you suggest—for the diffraction pattern to appear a measurement must not be made to determine which slit the particle passed through. In quantum physics, the process of measurement is believed to "collapse the wave function", that is to put a particle which was previously in a superposition of more than one possible state into the state we observe. This, however, has nothing to do with consciousness and could be done just fine by a robotic measuring device.


QUESTION: 
If you're on a plane and you pass me when I'm on the Earth, we each think each other's time slows down, but if you leave Earth and travel at significant speed, when you come back we both agree that more time has passed for me than for you, so how could you initially perceive time to be running slower for me?

ANSWER: 
What you describe is, essentially, what is called the twin paradox. You should read my earlier answer to see how the twin paradox may be explained. The answer just after this one is also germane to your question.


QUESTION: 
Assuming two objects are moving along a trajectories perpendicular to each other, away from the shared origin. At any given point, they, and the shared origin form the points of a right triangle. If they are light-generating/reflecting objects moving each at half the speed of light (for ease of math, but any speed will do), then as they move apart, the time it takes while standing on one body observing the light from the other seems to take longer than the time it takes to transmit the same amount of information. For example, one second's worth of light (information) emission will take, if my calculations are correct, more than one second to receive it all because of the new positions. This effect is proportional to the velocities of the objects. So, in laymens terms... if you stand on one object observing the other, wouldn't you see the other object in slow motion? It took you more than 1 second to see 1 second's worth of light/information.

ANSWER: 
There is no reason to consider the two moving as you have described. Viewed from the perspective of either, the other is receding at some constant speed easy to calculate from the geometry. It is often said that "moving clocks appear to run slow." This illustrates how difficult it is to get our minds around the basic ideas of special relativity. The correct statement is "moving clocks run slow". How clocks appear to run is a different matter and usually of no interest to a scientist because we are usually interested in how things are, not how they appear. My previous discussion of the twin paradox illustrates this: a clock moving away from you appears to run slow, a clock moving toward you appears to run fast, but a moving clock runs slow. The answer to your question is that something moving away from you would appear in slow motion as you suggest. Something coming toward you would appear to be in fast motion, but its clock still runs slow.


QUESTION: 
my question is regarding E=MC2 and the internet. after watching NASA TV last night i thought of this question whilst falling asleep. 'where does the internet/virtual 'stuff' fit into to the equation as it is not really there but is?' Of course all the electricity and other electronics being used to power the internet have an energy but i refer to the content itself? i.e this page is actually here now but where will it go later? can there be another 'dimension' to the theory?

ANSWER: 
Although I would not characterize your question as "just a load of nonsense", it is not really meaningful in the context of E=mc2. It is sort of like asking "how much does an idea weigh?" or "what color is history?" The notion of information does not fit with the notion of mass.


QUESTION: 
I'm trying to remember the name of a principle that I learned about in a classical mechanics class. I think it went something like this: A physical system will evolve over time in a path that minimizes the integral (with respect to time) of the Hamiltonian for that system (sum of potential and kinetic energy). I'm not sure if that's even right, but I'd appreciate any insight you could give me. I think it's related to Hamilton's Principle and Lagrange equations of motion somehow, and I thought it had it's own name, but I'm not really sure.

ANSWER: 
I don't think that is right. If you start with Hamilton's principle, the statement is the same as yours except it is the Lagrangian, not the Hamiltonian. They cannot both be right. If you write the Lagrangian in terms of the Hamiltonian and the generalized coordinates, Hamilton's principle leads to Hamilton's equations of motion. See any intermediate-level classical mechanics book.


QUESTION: 
I understand that sound waves travel at 331 m/s through the air. Now, if I were to stand 662 meters away from the source of the sound, would it be correct to say that it would take 2 seconds for the sound waves to reach my ears? Furthermore, is it also correct to say that I am hearing the sound from the object as it was 2 seconds ago, and not exactly in real-time? The reason I say "real-time" is because it relates to the main part of my question. A commercial airplane flies at roughly 30,000 feet. 30,000 feet converts to 9144 m. 9144 meters divided by 331 m/s = 27.6 seconds. Does this mean that we are hearing the airplane as it was 27 seconds ago? This is what I can't get my mind around.

ANSWER: 
I am not sure why you can't get your mind around this. After all, many stars we see are as they were millions of years ago. When I hear an airplane and look up to see it I often find that I look in the wrong place since, if my brain gets location information from the sound, the place I will look is where it was some time ago. This may not always be apparent for planes at very high altitude because the sound may be too faint to get accurate position information and the plane, relative to the whole sky, does not move too far in a half minute.


QUESTION: 
Would you be able to verify the reasoning in the four simple paragraphs presented below and find a fault in them?
1. It is a generally accepted fact that in a head-on collision between two balls of equal mass moving with the same but opposite speeds, the two balls will rebound with equal but slower speeds. (R. Feynman, Lectures on Physics, Vol. 1, p. 10.7.)
2. If the above is true, then the following also must be true: When one of the balls in the above example is at rest, this ball must rebound with a slower speed after a collision with the moving ball of equal mass, while the moving ball comes to a perfectly “dead” stop.
3. Because the ball initially moving must come to a stop after the collision, the slower speed of the ball initially at rest must be used in the calculation of the total final momentum, making it smaller than the total initial momentum. Hence, the total momentum cannot be conserved in this type of collision.
4. Proof: The above outcome is confirmed by the fact that the total final energy cannot be conserved in the above collision. However, the only way that energy will not be conserved is if the speed of the ball initially at rest is slower than the speed of the ball initially moving, as the ball initially moving comes to a perfectly “dead” stop after the collision. In other words, the same slower speed of the ball initially at rest must be used in the calculation of both the total final energy and the total final momentum. Because both energy and momentum are the function of the same factors (the same masses and the same speeds), neither energy nor momentum can be conserved in the motions of the two balls, contradicting the law of conservation of momentum. Thus, we have mathematical proof that momentum cannot be conserved in all collisions.

ANSWER: 
Statement #1 is far from true in general. This is true only for an inelastic collision, one in which energy is lost in the collision. Statement #2, the incoming ball being at rest after the collision, is true only for a perfectly elastic collision (in which case the ball originally at rest exits with the same speed as that of the incoming ball). Hence, since #2 is incorrect, your "proof" fails. Linear momentum is always conserved in an isolated system, i.e. a system which experiences no outside forces.

FOLLOWUP QUESTION: 
Thank you for responding to my query. However, I am not happy with your answer. I thought the conditions in my query were clear. However, on the second thought, it is better to stipulate them. Here they are: Conditions:
1. The two collisions described in paragraphs 1 and 2 are real collisions that could be performed in a laboratory. In these real-world collisions, energy is not conserved. Therefore, in order to avoid confusion, elastic collisions should not be considered or even mentioned in this case.
2. A made a survey among a dozen of reputable physicists, professors of physics and a well-known physics textbook whiter about the real collision in paragraph 2. The general consensus was that a glider initially moving on a frictionless air track in a laboratory will come to a perfect “dead” stop after colliding with a glider at rest of equal mass, as confirmed, according to them, by numerous already performed experiments. Therefore, the assumption in paragraph 2 is that the ball initially moving would come to the above-mentioned perfect ‘dead” stop. What would then happen to the ball initially at rest? You stated in your answer, that the ball initially moving would come to a stop only in a perfectly elastic collision, which do no exist in nature. This means that in a real collision, you assume that the ball initially moving will not come to a perfect "dead" stop. In what direction and at what speed will it move after the collision and what would happen to the ball initially at rest? Your assumption contradicts the result of my survey. Indeed, what exactly would happen in a real head-on collision where energy is not conserved. Suppose the two balls are 2 kg each, and one moves initially at 2 m/s, while the other ball is at rest. What will be the speeds of the two balls after the collision? Once again, let's forget elastic collisions.

ANSWER: 
Your conditions were crystal clear to me. All I said about condition #1 was that this is only true for an inelastic collision. I agree that real world, macroscopic collisions will always result in energy loss. Your dozen reputable physicists are either not competent or else they are telling you what they think you want to hear
—the stock answer for elastic collisions. It is only for elastic collisions between identical particles, one initially at rest, that the other is at rest after the collision. So how could those physicists be wrong? Quite simply because the apparatus they describe is designed to have a very low (not zero, since we agree that is not possible) energy loss when the gliders collide and to have minimum friction (not frictionless as you state). Hence the collision between gliders is so close to elastic that the velocity of the incoming glider is so small that it is not noticable or is stopped by the (nonzero) friction. I will present two situations to you to try to convince you that condition #2 is flat-out incorrect if energy is lost.

  • Consider a perfectly inelastic collision, e.g. two balls of putty. In the head on collision, all kinetic energy is lost and the balls end up at rest, stuck together. If only one is moving before the collision, is it at rest after the collision? I think you must agree that it cannot be.
  • My second example is a little more complicated. I will take your choice of 2 kg balls moving at a speed of 2 m/s toward each other. Then the energy before the collision is 2x(½x2x22)=8 J. Suppose that ¾ of the energy is lost in the collision. Then after the collision the energy is 2 J. Therefore, 2x(½x2xv2)=2 J so v=1 m/s; each ball leaves the collision with a speed of 1 m/s. To find out how this same collision looks if one ball is at rest, I will not assume momentum conservation because you would object since you think you have disproved it. I will watch the original collision but while running alongside one of the balls, ok? Before the collision I see one ball at rest and the other approaching with speed 4 m/s. After the collision I see the struck ball moving (in the same direction as the other ball came in) with a speed of 3 m/s and the incoming ball moving (in the same direction) with a speed of 1 m/s. (Note that the same amount of energy, 6J, is lost in both scenarios.)
  • If you redo the second example for 10% energy loss (0.8 J) you will find the speeds after the collision are about 3.9 m/s (for the struck one) and 0.1 m/s for the incoming one. For a 1% loss (0.08 J) they are about 3.99 m/s and 0.01 m/s.

QUESTION: 
Is it true that everything is energy?

ANSWER: 
I guess you could say that. Understanding that mass is a form of energy pretty much ices it, right?


QUESTION: 
A surface wave in water, such as a ripple in a pond, moves at less than a meter a second, yet the speed of sound in water is about 1.4 kilometers a second. Why aren't these speeds the same? How can they be so different?

ANSWER: 
The simple answer is simply that the surface waves are not sound waves. Light travels through water with a vastly larger speed also; why is it so different from the other two?


QUESTION: 
A few years ago it was not known whether neutrinos had mass. Then it was discovered that they can change types (electron - muon etc) and this was taken to imply that they MUST have mass. I don't understand this - why does the fact that they change types imply mass?

ANSWER: 
The answer requires a little quantum mechanics to understand. If an object (like a neutrino) is actually a superposition of two or more different particles, it has a sort of Jekyl/Hyde characteristic. Now, each "particle piece" has a wevelength which is determined by its momentum which, in turn, is determined by its mass. If the masses of the two particles are the same then the waves will just move along unchanged because they will be the same wavelengths. But if their masses are different, they will have slightly different wavelengths and that will cause "beats" just like the beats between nearly equal musical tones; in quantum mechanics, these beats are what the oscillations are. So, different "pieces" of the neutrino must have different masses which obviously means they all cannot be zero.


QUESTION: 
Does gravity or electromagnetic energy hold things and people on earth?

ANSWER: 
Gravity. The electromagnetic force has nothing to do with it.


QUESTION: 
Do you think that space is curved because all matter particle that ocupy space are spherical?

ANSWER: 
I know that is not the case because, if for no other reason, all objects are not spherical. The earth and the sun, the two most important objects for us as far as gravity is concerned, are both oblate (a larger radius at the equator than at the poles) and not spherical.


QUESTION: 
I need a simple explanation on kilograms - newton conversion formula to explain to my nephew who is 11 years old.

ANSWER: 
Technically, you cannot convert one to the other because they measure different things: a newton (N) is a unit of force and a kilogram (kg) is a unit of mass. Here is the rub, though: in contries where metric measures are used, the kg is used to measure weight even though weight, the force which the earth exerts on something, is not a mass. This, naturally, leads to confusion when we first are learning about mass and force. If you take a 1 kg mass to the moon, it would weigh less than it does on the earth. So, to make this clear we must carefully define force and weight as they relate to mass. A force of one newton is that force which, when applied to a 1 kg mass results in an acceleration of 1 m/s2. Now, an 11 year old does not usually understand what acceleration is other than the qualitative speeding up or slowing down. An object, starting from rest, with an acceleration of 1 m/s2 has a speed of 1 m/s after 1 s, a speed of 2 m/s after 2 s, etc; it is the rate at which speed changes. (I guess it is also helpful to know Newton's second law which says force is mass times acceleration, so a 3 N force acting on a 3 kg mass also results in an acceleration of 1 m/s.) Now, if you drop an object (regardless of its mass) near the surface of the earth it will be observed to have an acceleration of 9.8 m/s2 so, the force on it must equal its mass times 9.8. Therefore, e.g., a 10 kg mass will have a weight of 98 N. At the market, what is measured is the weight of the potatoes so when the scale reads 10 kg it means that the weight is 98 N. That same scale would not read 10 kg on the moon for the same 10 kg of potatoes.


QUESTION: 
According to Google the mass of an electron is 9.10938188 × 10 (power of) 31 kilograms. Does that mean that a charged capacitor has more mass than the same capacitor without a charge?

ANSWER: 
First the simple answer. The same number of electrons added to one plate of the capacitor are taken away from the other plate, so there is no net charge and the net mass is unchanged. Now, the trickier answer. Since it takes energy to charge a capacitor, the whole system has more energy than it started with. Therefore, according to E=mc2, the whole mass of the capacitor must have increased. However, since the energy stored is very small, this increase of mass would be impossible to observe.


QUESTION: 
As a beam of green light passes through a window pane it slows down inside the window pane glass because the index of refraction of the glass is higher then that of air. Is the color (frequency) of the light while it is inside the glass, therefore, shifted toward red?

ANSWER: 
You are on the right track, but not quite there. The color of light is determined by its wavelength,
λ, not its frequency, f. If the velocity of the light changes, as it does when it enters the glass, its frequency stays the same. But, there is a relation between velocity, v, the frequency, and the wavelength: λ=v/f . Since the frequency is unchanged, a little algebra leads to λglass=λair(vglass/vair). Since vglass<vair , the wavelength in glass is shorter, that is it is shifted toward the blue, not red.


QUESTION: 
I began thinking about Heisenberg's Uncertainty Principle. Is there any possibility it might be connected to the expansion of spacetime? Is it impossible to know both where a particle is and where it will be, because the constant against which it's measured (spacetime) is always changing?

ANSWER: 
No, this is not possible. Think of it in terms of wave particle duality. If you try to localize a wave (measure "where it is") you truncate it llike maybe with a camera shutter. However, a piece of a wave does not have specific frequency (related to its momentum) but a range of contributing frequencies. For a more detailed discussion of the uncertainty principle, see an earlier answer.


QUESTION: 
From what I understand about quantum entanglement, it allows for transference of information between two segregated segments of a photon instantaneously. Essentially giving the impression that the component parts are still connected somehow. Firstly, is this information actually travelling faster than light, or is something else causing the effect? Secondly (if it's the former), is causality being violated? Is the information being received before it was sent?

ANSWER: 
It is not "two segregated segments of a photon", it is two photons. The photons are in a single state. But, in quantum mechanics a single state is not clear cut like it is in classical physics. Suppose that the possible states of a photon are labeled A and B but that the two of them must contain equal amounts of A and B. So, classically, we would think that one must be A and the other B. Quantum mechanically, however, we could have each photon be 50% A and 50% B so we still have the same amount of A and B between the two photons as if each were pure. Now, when we look at one of the photons (that is make a measurement) we find it in either A or in B, that is a measurement "puts" the photon in the state we observe. This measurement instantenously "puts" the other photon in the other state, even if it is halfway across the galaxy. However, no information has really been transmitted to the other photon; there is no way that you could use this experiment to send a message to somebody halfway across the galaxy using this experiment. When you measure one you are actually putting the whole system into a definite state, not transmitting the information to the other half of the system.


QUESTION: 
It is generally accepted that the movement of a free electron generates a magnetic field. However, could not it be the possible that the Magnetic flux is already distributed in space and that the movement simply uncoils the flux? This is similar to the views of some Physicists that extra dimensions are coiled up in space.

ANSWER: 
No, not possible.


QUESTION: 
would frequencies such as 45 hertz, 450 hertz, etc. on up to 450 megahertz be harmonics or coefficients of one another?

ANSWER: 
I find the following dictionary definition of the noun harmonic: "A wave whose frequency is a whole-number multiple of that of another". Thus, the frequencies you list are harmonics of each other.


QUESTION: 
My question relates to gravity, and, more specifically, why there is gravity. I find it very difficult to put into words what I'd like to ask, so I'm going to give it my best shot and see what happens. It seems to me that the manipulation of space itself by the existence of matter is the reason behind gravity. We know that there is an incredible amount of space within an atom relative to the size of the nucleus, however that space appears to be "trapped" within the atom. By trapped, I mean we cannot access or enter it without the expenditure of seemingly large amounts of energy. This space had to come from somewhere, presumably the universe, and I'm wondering if by "trapping" it within an atom. is space itself is being "stretched" to accomodate the atom? If that "stretching" of space results in a directional "grain" or pull, could that be what gravity is? Has this been proven or disproven?

ANSWER: 
Regarding gravity, your speculations are roughly in line with the best current theory of gravity, general relativity. Here gravity is seen as being due to the warping of space-time in the vicinity of objects possessing mass. One of the best ways to appreciate this is to realize that gravity bends light even though light does not have mass. Your musings about atoms, however, do not have any merit. For a little more about general relativity, see my earlier discussion.


QUESTION: 
I'm familiar with electromagnetism and I know that visible light and similar radiation are part of the electromagnetic spectrum. Am I to understand that this spectrum is actually electric current, coupled with a sympathetic magnetic field? Is electromagnetic radiation part electricity?

ANSWER: 
See earlier answer.


QUESTION: 
many popular science books discuss traveling such that a spacecraft maintains an acceleration equal to 1 g for long distance space travel. how much time would elapse before the spacecraft would have a mass equivalent to that of the earth? how long before it would achieve a mass that could form a black hole? would that dash the hopes of those who dream of interstellar space travel?

ANSWER: 
The mass of the spacecraft in its own frame would not change, so no black hole creation is in the picture. Furthermore, the "increase of mass" in the earth frame is just an interpretation of the overall picture and not to be taken too literally in my view. What really happens is that if you want to write mometum as mass times velocity and you want momentum to be conserved in an isolated system, momentum must be redefined; you can interpret this as mass increase, but I do not; in my view, only rest mass (inertia of a particle at rest) is a useful quantity in special relativity. Furthermore, uniform acceleration (as observed from the earth frame) is not possible. Read my earlier posts on relativistic momentum and uniform acceleration.


QUESTION: 
But concerning light, If it falls out of the sky equally across my front yard, would a 10" round lens focusing light on a 10" round black piece of metal end up hotter than just a regular 10" round piece of black metal setting out in the sun? My theory would be that the one with the lens would be hotter. But why? the same amount of light is shining on both pieces. Unless focusing "unlocks" potential heat that would otherwise be masked in light in its regular state.

ANSWER: 
If you could keep the energy absorbed by the disk from leaving, there would be no difference in the final temperature. The center (for the focused situation) would get very hot and then heat would flow throughout the disk until the temperature was uniform everywhere. (I assume you will do the experiment by exposing the disk both ways for equal times.) However, all objects radiate energy to their environment and the rate of radiation is proportional to T4 where T is the absolute temperature. So, I suspect that the focused situation would end up less hot in the end since it would radiate away more energy.


QUESTION: 
I have heard of Dark Matter for a long time and i've been wondering what it does, what it can do, and how it can be applied to science.

ANSWER: 
I am probably not the best person to ask since I am a skeptic and do not adhere to the "party line". The fact is that there are many aspects of the the observable universe which suggest that there is far more matter than we observe. One of the best examples is the way in which stars revolve around the center of their galaxy. It should, in principle, be easy to look at all the objects we can see and calculate the orbital speeds of the stars but invariably we get predictions which do not agree with observations. So, astrophysicists say that there must be some new kind of matter which interacts only via the gravitational force with other matter. Since it does not interact at all via the electromagnetic force, we cannot see this stuff, just its effects on the motion of normal matter. My view, though, is that maybe we just do not really understand how gravity works over very large distances. (Our only real data are from our solar system which is tiny compared to the size of a galaxy). That is not to say that it does not exist, just that it does not necessarily exist until a direct observation can be made. Many astronomers and astrophysicists are looking for direct evidence for dark matter but none has been found yet; it is a hypothesis.


QUESTION: 
This isn't about astronomy but about the speed of light. A traveler orbits around the earth at 90% of the speed of light. She/he measures one on-board hour of travel, and also counts the number of orbits. Observers on earth will measure the length of the trip and also their count of orbits. The earth-bound observers will measure a much longer time than one hour. Exactly how long would be fun to know but is not important. My guess is that the traveler and the earth-bound observers will count the same number of orbits. I think this means that traveler and observers will come up with quite different estimates of the velocity of the trip. Is that right? I don't think there is actually a problem here-- it's more an example of the fundamentally irreconcilable observations that are made from different frames of reference.

ANSWER: 
You make the problem conceptually harder because of general relativistic effects which arise from both gravity and the nonconstant velocity of the moving observer. I will ignore these since I do not believe that is really where your interest lies. I will explain purely from a special relativity perspective. And, in relativity there are never "fundamentally irreconcilable observations that are made from different frames of reference" or else it would not be an acceptable theory of physics. Both observers will agree on the speed of the spacecraft. Say the earthbound observer (O) measures 1000 orbits in 10 seconds and the orbiting observer (O') measures 1000 orbits in 1 second (I am just making these up to illustrate). So O says the speed is v=1000C/10=100C where C is the orbit circumference. But, O' sees the orbital path zooming past him with speed v and so its length is contracted; he decuces his speed to be v'=1000C'/1=1000C' where C' is the circumference he sees. Since the speeds must be the same, O' evidently sees a much diminished circumference, C'=C/10. Their observations are completely reconcilable. (As I said, this is not exactly accurate for this situation. It would be if O' were moving in a straight line path with constant speed.)


QUESTION: 
I'm am having a little trouble understanding some of the concepts in the energies at work within an atom. I know that E=MC^2 tells one that maximum potential energy within a unit of mass. Thus I know the total energy potential of 1 kilogram would be about 90 petajoules (assuming I remember my formulas correctly) This is what I don't get I know there are 4 binding forces, gravity, electromagnetic, strong, and weak. If you added the sum of all binding energy within 1 kilogram of matter, would it equal 90 petajoules itself, or would it be a much higher or lower figure given that binding energy tells us how much is needed to break those bonds? Or am I just totally misunderstanding something?

ANSWER: 
There is always mass energy associated with binding forces, but you cannot trace mass energy to binding. In fact, a binding force in a bound system reduces the total mass of the system. For example, suppose that you have two particles bound together (for example, a proton and a neutron or two hydrogen atoms). If you pull them apart, you have to do work, right? Hence you increase the energy of the system and therefore increase the mass. A proton and a neutron bound in a deuteron weigh less than a proton plus a neutron. But, if you make mass M disappear, you will make energy appear in some other form to the tune of Mc2. For example, if you have an electron and a positron (the electron's antiparticle), they will annihilate each other and energy in the form of photons (light, basically) will appear and have exactly the energy of the masses times c2.


QUESTION: 
No stars are seen in the photos taken of the earth from the moon. Why is that? We see stars when we look at the moon from earth.

ANSWER: 
It is most likely due to the exposure setting of the camera. Since the earth is much brighter than the stars and the exposure would have been set so that the image of the earth would not have been overexposed. There is no reason that stars would not be there.


QUESTION: 
if you had two balls of opposite polarity i.e. a negative and a positive ball, could they come together and stay together if they were incapable of giving up any energy to the surrounding space. Could this occur in newtonian physics? Wouldn't these two balls bounce off each other and speed off in the opposite direction slowing until they stopped and then pulled back toward each other in a repetative fashion? Like a wave?

ANSWER: 
I am not sure what you are getting at here. I guess it would depend on the elastic properties of the balls. If they were very inelastic, like putty, they would stick together and that would be that. If they were perfectly elastic the balls would bounce back to their initial positions and the process would repeat forever as you suggest. Anything in between these extremes would result in bouncing with less and less amplitude until they finally stopped, in contact with each other. All this assumes that the charges stay on the balls unchanged.


QUESTION: 
Could you tell me if there is any evidence to suggest that there is infinite matter therefore infinite energy in the universe, i.e., is the universe infinitely large / complex?

ANSWER: 
Everything we know or can measure about the universe points to its being finite.


QUESTION: 
Recently, I have been working with magnetic repulsion; however, I am hard pressed to find any information regarding the repulsion force of like poles. I have found information allowing me to calculate the attraction force of a rated magnet to a specific material of a specific thickness. Can you tell me of a way to calculate the repulsive forces of two magnets with the same poles facing each other. How does one calculate the relation of attraction to repulsion? Is there a one to one correspondence?

ANSWER: 
Basically, the reason you have not found the information you need is that there is no such thing as a magnetic pole. Magnetic fields are caused by moving electric charges, not by analogous magnetiic charges. The most fundamental magnetic field looks just like the electric field caused by a positive and negative electric charges very close together and so it is natural to say that the field is caused by a positive (north) and negative (south) magnetic charge. It is actually possible to cook up a description of magnetism based on magnetic charges but it is all a fiction. I recommend that you approach magnetism from the traditional view that fields are due to electric currents and you will glean a more accurate understanding.


QUESTION: 
If you spin two gyroscopes against each other in opposite directions, why does the gyroscopic inertia cancel out? Is it possible to cancel out all that energy? does this mean energy is being destroyed?

ANSWER: 
I never heard of "gyroscopic inertia". I guess what you mean is that if an object has an angular momentum then a torque is required to change the direction of the angular momentum. On the other hand, a pair of oppositely spinning gyroscopes have zero angular momentum and so it is no harder to turn them than if they were not spinning at all. On the other hand, this does not mean that their energies have disappeared. They have exactly the same energy as if they were spinning in the same direction. The key here is that energy is a scalar and angular momentum is a vector so two kinetic energies cannot add to zero but two angular momenta can.


QUESTION: 
Earth's rotation around its own axis is about 700 mph. How is it that we have the 4 seasons, Summer season when we're facing Sun's direct sunlight, and in Winter, we're facing the sunlight indirectly? Doesn't spinning on its own axis cancels out the 4 seasons? I am assuming the Earth rotates from East to West.

ANSWER: 
The reason for seasons is that in winter the sun's rays strike the earth more obliquely because of the tilt of the axis relative to the plane in which we orbit the sun. The earth's rotation is about this axis and so does nothing to change the tilt. There is no way the spinning "cancels out" the seasons.


QUESTION: 
If there were no motion in the universe (I know not possible, however...) would two bodies near each other, with sufficient mass, start moving toward each other due to mutual gravitational attraction or must bodies already be in motion in order for gravity to be apparent?

ANSWER: 
This question violates the site groundrule against impossible situations, but I can answer it succinctly. Gravity has nothing to do with the motion of the bodies. If the universe consisted of two objects at rest separated by some distance of empty space, they would accelerate toward each other.


QUESTION: 
Do you think it would be possible to prevent being liquified into your seat if you were to accelerate to near light speed in milliseconds? i mean, suppose you had a large enough magnetic field and it were possible to control the rate of change of the magnetic field very precisely, could a such a field, in theory, prevent the degredation of the bonds between molecules thus keeping you intact as you accelerate almost instantly?

ANSWER: 
I fail to see how a changing magnetic field would help things. However, there is no getting around the fact that, for that kind of acceleration, astronomically large forces would be required on your body to accelerate it.


QUESTION: 
Can you provide a formula for the Earth's acceleration due to garvity and the speed of the Earth's rotation? Would like to know how g = 9.81 m./sec sq. would change if the Earth had 16-hour days instead of 24.

ANSWER: 
First, I must disabuse you of the notion that the acceleration due to gravity would change. What happens is that your apparent weight changes because you are accelerating in a circle. Your true weight is unchanged since weight is simply the force with which the earth's gravity attracts you. (The answer to your question depends on the latitude where you do the experiment; for simplicity, I will do it for the equator where the effect is largest.) From Newton's second law you may write N-mg=-m(2
πR/T)2/R where N is apparent weight, mg is true weight, R is the radius of the earth, T is the period of rotation. Solving this, I find N=mg(1-(2/T2)) if T is measured in hours. Therefore the effect on apparent weight is about 200/T2 %. This is about a 0.35% effect for T=24 hours, a 0.78% effect for T=16 hours. If the length of a day were less than about 1.4 hours, you would leave the surface.


QUESTION: 
The polarity of the earth is evidently in the midst of a shift from north to south. The nucleus of an atom has a positive charge. How does that shift affect that charge if at all.

ANSWER: 
The polarity of the earth is a measure of the direction of the magnetic field around the earth. The polarity of an electric charge is an entirely different thing (electricity, not magnetism) and the electric charge on something is entirely independent of the magnetic field in which that charge might find itself.


QUESTION: 
ok i know that light has no mass, but when you have a flash light and turn it on, light shines on wherever you point at it. my question is that light has to have mass. this is because light is energy and enery is mass because of what Einstein said (E=MC^2).this also means that that the flashlight has to losse some mass because light is leaving the object (flashlight).so i think im saying why does light have no mass? light is energy stored in a photon, but where does that energy come from if light has no mass?

ANSWER: 
As often happens with a famous formula, you misuse it. E=mc2 means that the energy of an object at rest is its rest mass times the speed of light squared, but light is never at rest so you cannot use this formula to find its energy. For more details on this, see an earlier answer. You are, however, right regarding the flashlight getting lighter. The energy which the photons carry away from the flashlight get their energy from chemical reactions in the batteries and these chemical reactions result in a loss of mass. The catch here is that chemistry is an extraordinarily inefficient source of energy and you could never hope to measure the change in mass because it would be so small. If you use a nuclear reactor to generate your photons you would ultimately be able to measure a reduction in mass.


QUESTION: 
Is their any difference between the total center of mass energy of the system of two nuclei if we interchange their target and projectile (the masses of both are different) by keeping their lab energy fixed and using lorentz transformations? if not then why?

ANSWER: 
If your suggestion works for relativistic (Lorentz transformation), it will work for nonrelativistic situations since the relativistic must reduce to the nonrelativistic in the limit of small velocities. So, let's try it: The lab energy of a particle of mass m (M) with speed v (V) is Tlab=
½mv2MV2. So, clearly, unequal masses with equal lab energies have unequal velocities. Now, the center of mass energy is given by Ecmμv2 or E'cmμV2 where μ=Mm/(M+m) is the reduced mass. Since the v≠V, it follows that EcmE'cm, so your hypothesis is incorrect.


QUESTION: 
Is time travel possiblle? or are wormholes tunnels through spacetime?

ANSWER: 
The laws of physics, as we currently understand them, allow time travel to the future but not the past. I have nothing to say about wormholes except that they are speculative.


QUESTION: 
Why does not light travel faster than approximately 3.0 x 10^8 ms^-1 when photons do not have mass? Wouldn't this mean that it does not require any energy for accelerating even faster? Then why is not the speed of light infinite?

ANSWER: 
You are attempting to apply Newtonian physics to photons, but that is what relativity has taught us
—that Newtonian physics is wrong if speeds are comparable to the speed of light. Light itself has the speed it does because of the theory of electromagnetism which predicts the speed of light to be a certain invariant number.


QUESTION: 
How far apart are protons/nuetrons from each other in the nucleus of an atom?

ANSWER: 
If you think of them as tiny spheres, their radii are about 10-15 m. Because the force which binds them is short-ranged, the little spheres are tightly packed and so the distance between their centers is about 2x10-15 m.



QUESTION: 
I was wondering something about my pressure cooker. Obviously it requires energy, converting water into steam to create pressure to cook the food at higher temperatures than "normal" air pressure allows. If the food were submerged in the water totally, would I just be boiling my dinner and not getting any effects of the pressure? I was told you can't compress water, so any food sitting in water is not under extra pressure. In a pressurized environment, can water heat past 215 degrees F?

ANSWER: 
The boiling point of water is determined by the pressure, is approximately 2120 F at atmospheric pressure. I believe that pressure cookers were invented because at high altitudes the pressure is lower and so the lowered boiling point makes it take a long time to, say, boil a potato. So a pressure cooker is tightly confined and when heated the pressure increases causing the water in the pot to be have temperatures higher than 2120 so things can cook more rapidly. You are incorrect in assuming that, because water is (almost) incompressible, it is "not under extra pressure". The pressure in both the water and the steam/air above it are increased to the same value when the cooker is heated. To the left is a phase diagram for water. Note that when the pressure is increased beyond 1 atmosphere the boiling point (the temperature at which liquid and vapor coesist) increases rapidly as the pressure is increased.


QUESTION: 
Much is made of the fact that the speed of light is independent of its source speed (a speeding rocket, etc.). However, isn't the same true for other waves such as sound? No matter how fast a train is moving, its sound waves still travel at sound speed. I realize there is a doppler shift, but this does not change the wave speed. So why is the big deal made about light speed being constant?

ANSWER: 
What much is made of is that the speed of light is a universal constant regardless of any relative velocity between the source and the observer. No matter how those two move, the wave speed is the same. Sound is not the same since it moves relative to a medium (the air, normally). If the source moves toward you, as you note, you will still measure the same speed of the waves. However, if you move toward the source, you will measure the waves moving with a velocity of your speed plus the speed of the sound in still air. The key is that there is no medium with respect to which light waves move; they may move through completely empty space, unlike sound which requires a medium.


QUESTION: 
Can you tell me why the number e is called the "natural" base? I know how to use it, it's just been bugging me for a while that i can't figure out why it is called "natural".

ANSWER: 
This is not really phyiscs. I find this from Wikepedia: "The natural logarithm can be defined for all positive real numbers x as the area under the curve y = 1/t from 1 to x. The simplicity of this definition, which is matched in many other formulas involving the natural logarithm, leads to the term 'natural.' "


QUESTION: 
I took several photos from a window seat on the right side of a 2 engine propeller plane. The camera is an iphone. Why did the camera capture the blades of the plane as shown .Go to : http://gallery.me.com/douglaskochel#100050

ANSWER: 
I am not really sure, but I have an idea. A film camera exposes the whole image at once whereas I presume that a digital camera reads its data row-by-row. So when one row is read and recorded the propeller will be in a different place then when later rows are read. Anybody reading this who has a more informed idea, I would be glad to see it.

ANSWER: 
Thanks to the reader who sent this in: "The iPhone is demonstrating the "rolling shutter" effect, see paper: http://arxiv.org/PS_cache/cs/pdf/0503/0503076v1.pdf" Scanning through the paper briefly, it seems that my answer was, essentially, correct.

ANSWER: 
Here are some more links:
http://www.youtube.com/watch?v=T055cp-JFUA
http://www.youtube.com/watch?v=Um3bGnSqLRY&feature=related
 


QUESTION: 
My friend and I are having a debate about a certain topic trying to come up with different theories of how the mechanics of this process works, yet with each theory we develop we get pulled deeper into the problem with more possibilities and factors affecting the issue. The question is, if an average car, with ideal conditions, (ideal tire pressure, gear ratios, weight, driving conditions, weather) is traveling at a rate of 60 mph, are the tires moving faster than than the car itself. I argued that the rpms of the tire are moving at a rate faster than the car, (obviously) but since the wheel is attached to the car and the car is moving at a rate of 60 mph, the wheel too (not counting rpms) is moving at a rate of 60 mph where rubber meets road regardless the size of the tire. I do not understand his theory, but he claims, that with rpm's aside the tire in and of itself is moving faster than the car. I told him if you took a snap shot of the car moving at 60 mph, everything about the car is moving at 60 mph. To be able to figure out the exact speed of the tire as a seperate entity, you would HAVE to factor in its rpm. Please help me win this battle!

ANSWER: 
Sorry, but you absolutely lose this battle! If the brakes are locked, every point on the wheel will be moving forward with a velocity equal to the velocity of the car. If the wheel is rolling, the point where "where rubber meets road" is momentarily at rest. The very top of the wheel is moving forward with a speed of twice the speed of the car. Points on the front half of the tire are moving both down and forward (with speeds and directions easily calculated) and those on the back half are moving both up and forward. You cannot talk about the speed the tires are moving since every point on the tire moves with a different speed. The best way to visualize it is that every point on the tire is rotating about the "where rubber meets road" point at any instant; the axel moves forward with speed equal to the speed of the car.


QUESTION: 
I have a problem, I need to know how many KeV does a X-ray machine produces, but I only know the voltage and amperage it has, is there a way to know this?

ANSWER: 
There is no way to know since the energy of the x-rays depends on the target material used, not the energy of the electron beam used to excite the atoms.


QUESTION: 
What determines the number of neutrons in an atom?

ANSWER: 
The number of protons in an atom of a given element is fixed (atomic number). However, the number of neutrons might vary and nuclei of the same element with different neutron numbers are called isotopes. There are often two or more stable isotopes for an element. The number of neutrons is determined by the nuclear structure, that is by the forces between neutrons and protons in a nucleus, and is too complicated to try to explain here. The lightest nuclei tend to have equal numbers of neutrons and protons but, as the atomic number increases beyond about 20 or so, there tend to be relatively more neutrons. For example, one stable lead isotope has 82 protons and 126 neutrons.


QUESTION: 
How do superconducting magnets work?ie I read that the magnetic flux lines are excluded from the material when it is a superconductor.But in superconducting magnets large magnetic field is produced.

ANSWER: 
In a conventional electromagnet there is resistance in the wires and so, as the current increases the power lost to heat increases like current squared. Since the magnetic field is proportional to current, the bigger field you want the more you have to worry about carrying the heat away from the wires. This limits how much current and therefore field you can have. If the wires are superconducting, there is no loss to this ohmic heating and you can have much higher currents and therefore much higher fields.


QUESTION: 
I have read that francium has a higher first ionization energy than cesium. Is this correct AND if it is, what is the explanation?

ANSWER: 
Yes, the ionization energy is slightly higher for francium, 380 vs. 376 kJ/mol. But what are your expectations and why? The simplest model would be a Bohr model where the alkali metal has a single s-electron outside a nobel gas core which has a net charge of one. This is a reasonable first guess, but obviously not perfect. The trend, as seen above, is a slow decrease with increasing atomic number, so that would be a reason to expect Fr to be lower. On the other hand, a full-blown calculation would be very complicated and the results would not be easy to understand. It would depend on the details of the nobel-gas core, among other things. One important difference between Fr and the other alkali metals is that it is the first to have a filled f-shell in its atomic structure. Relative to reasonable expectations, however, the ionization energies of the alkali metals are essentially equal.


QUESTION: 
How do we know that energy is not created nor destroyed? I understand E=MC squared but is there evidence that shows this to be true?

ANSWER: 
If you think that your two sentences are related, you do not understand, as you claim, E=mc2. This equation says that mass is just another form of energy and tells you how much energy a mass m contains. Your first sentence is a statement of conservation of energy: the total energy of an isolated system never changes. This law is partly axiomatic, that is it is true because of the way we define energy and isolated system. It is also very well verified by experiments.


QUESTION: 
Batteries and fossil fuels are our key accumulators. What do you know about research/possibilites in compact energy? Antimatter is extremely compact with an extremely high energy output. It's drawbacks are that it is virtually impossible with known technology to generate a useful amount and it's highly unstable.

ANSWER: 
Folks with brilliant ideas for sources of energy often do not take into account the energy cost to create it. A good example is ethanol as fuel which takes a large amount of energy to produce. Antimatter has to be created and the only way to do that is to supply the energy needed to make it. That energy will inevitably be greater than the energy you get back. This is not to mention that it is virtually impossible to contain any reasonable amount of antimatter for longer than a few microseconds (although one could imagine improving technology if there were an abundant supply of antimatter available). Use of antimatter is not in the foreseeable future.


QUESTION: 
If I had a rod one light second long, and began to rotate one end of the rod, would it take more than a second for the other end of the rod to begin rotating, regardless of the material used. Is the rotation like a wave therefore that passes through the rod at less than the speed of light?

ANSWER: 
Your question has been previously answered. That question involved translation instead of rotation, but the idea is basicall the same: The information would travel down the rod at the speed of sound in the rod (or slower).


QUESTION: 
If everything that could be known about electrons, protons and nutrons and the forces that affect them are known would it be possible to prdict all the attributes of the atoms that could result from the combinations of these three components.

ANSWER: 
You also have to know everything about the dynamics, that is how to calculate the results of the interactions. If so, you could predict everything which you could know about an atom. There are some things which, as we understand physics, are not knowable. For example, you cannot know where an electron is, with absolute precision, at a given time.


QUESTION: 
I am familiar with electromagnetism. Why do we say that colors are part of the 'electromagnetic spectrum'. What is electromagnetic about light rays and their different frequencies?

ANSWER: 
Light which your eye detects is electromagnetic waves and the color is determined by the frequency and wavelength of these waves. But there are lots of other electromagnetic waves which have different wavelengths and frequencies to which your eye is not sensitive, e.g. x-rays, radio, microwaves, gamma rays, infrared, ultraviolet, etc. For a description of the waves, see my earlier answer.


QUESTION: 
Does time existed before the Big bang?

ANSWER: 
See earlier answer.


QUESTION: 
you can sefely put your hand inside a hot oven for a short time, but even a momentary contact with the metal walls of the oven will cause a burn. explain

ANSWER: 
Heat flows from high temperature to low temperature. If your hand gets hot enough, the cells will be damaged (a burn). Air is a poor conductor of heat and so it cannot cause the temperature to increase rapidly enough to burn you. Metal is an excellent conductor of heat and raises the temperature of your hand rapidly.


QUESTION: 
What is a proton made of, and do these things also contribute to what makes electrons and neutrons?

ANSWER: 
A proton is made of three quarks bound by gluons, at least this is the current understanding. The neutron is also (but of different quarks). An electron is believed to be an elementary particle with no deeper constituents.


QUESTION: 
I am trying to learn how physical systems work at the quantum level and I am running into a problem  problem: I am trying to evaluate a simple problem which has what appear to be negative subscripts. I am trying to work with particle momentum (I am a major novice) and I keep seeing negative subscripts in my books!!! x-n In that form. Often the negative subscript is a P, which should be momentum.

ANSWER: 
I do not understand why a negative subscript should be a problem. A subscript is a notational device to label something. For example, suppose that xn=n2-4; then x2=x-2=0, x-7=45, x-4=12 etc.


QUESTION: 
Are the properties of light the same under water and in space? More specifically, would the double slit experiment yield the same results regardless of where we performed the experiment?

ANSWER: 
The speed of light in water is slower than the speed of light in vacuum. Since the frequency of the light stays the same, the wavelength must therefore be different. Therefore, the fringe spacing for a double-slit experiment would be different.


QUESTION: 
Is the fact that nothing can go faster than the speed of light a consequence of electromagnetic waves being the fastest method of information transmission that we are able to detect? For example, imagine a life form that could detect sound, but not light. Would that life form develop a theory of physics in which nothing could go faster than the speed of sound?

ANSWER: 
This fact is due to nothing but the laws of nature. It is the fact that the speed of light is a universal constant, the foundation of special relativity, which results in the well-known "speed limit".


QUESTION: 
I am writing a novel and wish to come up with relatively "realistic" attributes for something. A gargantuan land vehicle, possibly 10 miles long, 5 miles wide, and half a mile high, made of concrete, steel, and glass, like a city. I envision it like a stepped pyramid, on tens of thousands of 30 foot in diameter wheels. How much energy would it take, given a flat surface, to get such a thing moving, up to 35 miles per hour? How long to accelerate? Decelerate? If two such things collided, each going 35 miles per hour, what would happen? Would it be like an atomic bomb, or something less? How fast could such a thing decelerate or turn without stressing the materials so much that it disintegrated?

ANSWER: 
Here is a very rough, order-of-magnitude calculation. I calculated the mass of your vehicle using its size and the density of concrete. Then I calculated the kinetic energy it would have at 35 mph, about 1020 Joules. To get it up to this energy in one day would require a power input of about 1015 watts. The total power output of the entire world is about 1013 watts. These are very rough calculations, but keep in mind that very much of the power will be wasted overcoming friction, so factors of 10 or so more energy required might be in order. Clearly your ideas are not "
relatively 'realistic'". Two of these things have much more energy than an atomic bomb releases, obviously but much of it would probably be used just to break them apart.


QUESTION: 
Where do electrons get there infinite and constant supply of energy from, in order to move so rapidly around the nucleus?

ANSWER: 
First of all, electrons do not have infinite energy. Apart from that, I have recently answered your question. Just so you are sure, an atom has a certain amount of energy and you do not need to keep adding energy to keep it going any more than you need to add energy to the earth to keep it going around the sun.


QUESTION: 
why does CO2 leak very quickly from a bicycle tube, while N2 (rather than pesky messy air) leaks very slowly? I'm told "no easy answers", but maybe there is for the specific situation?

ANSWER: 
Actually, there is an easy answer. CO2 is a much smaller molecule than N2.

FOLLOWUP QUESTION: 
can you elaborate just a bit? for this 7th grader mentality, it would seem that N2 & O2 should be about the same size - aren't all electrons are in the same orbital shells? and then the addition of a C to the O2 should make it bigger? is it something to do with the double bonds on the CO2 that makes the molecule smaller?

ANSWER: 
You should not think of atoms as little balls of constant size. What really determines how large a molecule is is how strongly bound it is. Imagine atoms A and B make a molecule AB and C and D make a molecule CD and that all four atoms have about the same sizes. If A and B attract each other much more strongly than C and D (i.e. A and B are more tightly bound), then we can expect AB to be smaller than CD. I am not a chemist, but maybe your double bond idea is a way of saying this. I am a nuclear physicist and there is a similar effect in the following example: a deuteron, consisting of a proton and a neutron has a radius of about 2.1x10-15 m and an alpha particle, consisting of two protons and two neutrons, has a radius of about 1.6x10-15 m. The alpha particle is much more tightly bound than the deuteron.


QUESTION: 
I read the book, Angels & Demons, and a vital part of the plot line is based on a canister of antimatter, pure positrons, compressed into the center of the vacuum inside the canister by a strong magnetic field. I was wondering about this, and I have this question: Is it possible to, say, create a plasma out of argon, for instance (because it's easy, just use microwaves), and use a magnetic field to separate the electrons and nuclei, so that the end result would be pure electrons/nuclei on either end of a magnet? In other words, would you have a cloud of normal electrons at one end, or would they affect the magnet in some way? If you stopped bombarding the argon with microwaves so that it went back to the gas state, what in the world would happen if the nuclei and electrons were on opposite sides of a magnet? Or would all of this be impossible? Can you explain it in terms that a 7th grader would understand? I'm curious to know what a blob of electrons or nuclei would look like.

ANSWER: 
Magnetic confinement is notoriously tricky to achieve; that is one big reason why there has been limited progress toward a fusion reactor over the last 50 years. One major misconception you seem to have is that a charged particle is attracted to a magnet, positive charges to one end and negative charges to the other. In fact, any electric charge at rest experiences no force from a magnet. The charge must be moving to experience a force from a magnet. So, the idea that if you create a plasma that you can separate the positive and negative charges with a bar magneti is doomed to failure. By the way, I do not know how you use microwaves to ionize argon but I very much doubt that you separate it into electrons and nuclei; more likely that atoms lose one electron.


QUESTION: 
If total energy is constant, can you explain where the seeming inconsistency with gravitational potential energy at large distances. I guess what I mean is that at some great distance between two objects, at some great distance this potential energy will be essentially zero. Then if one of these particles drifts closer, such that it cannot escape the gravitational pull between them, then suddenly the potential energy would be quite significant. Where does this potential energy come from?

ANSWER: 
I do not really understand what you are asking. Let me outline, from the perspective of energy, what goes on with two objects separated by a large distance. Suppose they are both at rest. Then, as you suggest, the potential energy is very small (but not exactly zero). One of the things about potential energy is that the value is not important, you may set it equal to zero anywhere, but it is usually set equal to zero when the two are infinitely far apart. For our purposes here, I am going to set the potential energy equal to zero where the two objects are when they are at rest; therefore, these two particles have a total energy of zero. If there is nothing else around, this system will always have zero energy; this is conservation of energy. Since they are not infinitely separated, they exert forces on each other and start accelerating toward each other, so each acquires a kinetic energy. Because of energy conservation, they must therefore acquire a negative potential energy equal in magnitude to the (positive) kinetic energy. There is no inconsistency here. Your error was in not considering that, in "drifiting closer", kinetic energy changes also.


QUESTION: 
Since there is no atmosphere in space, is there such a thing as 'heat' in space?

ANSWER: 
Heat is a very special term which refers to energy transfer. If there is a perfect vacuum, there is no possibility for heat to be transferred by conduction or convection. However, heat can also be transferred by radiation so heat can be transferred through a vacuum. However, space is not a perfect vacuum and so it can have a temperature which is usually defined as being a measure of the average kinetic energy per molecule.


QUESTION: 
In isotope decays and nuclear reactions, many sources give the energy of the products (neutrons, alpha particles etc.) in MeV. I understand that MeV is a measurement of energy (and has a standard conversion factor to joules) but I don't understand how to calculate the speed/momentum of the out-going particle from this, or the recoil force on the reactants. I always had a hard time in high-school distinguishing between power, energy and work as they all seem like the same things to me. This is probably because they aren't as easy to visualise as distance, mass, velocity etc. For this problem I've tried converting everything down to the base units but I always seem to get some pesky distance measurement in there, which I don't understand what it has to do with anything. Any light you could shine on this problem would be most appreciated.

ANSWER: 
Let's first address the work/energy/power issue. Assuming that we know what energy is, work is whatever is done to change the energy of an otherwise closed system. (Actually, heat which is different from work can also change the energy of a system.) The amount of work done is equal to the change in energy, and so work and energy have the same units, joules. Power is something else; it is the rate at which work is done or energy changes. It therefore has the units of joules/second; one joule/second is called a watt. Now to your question. One electron volt (eV) is the same as 1.6x10-19 joules (so 1 MeV=1.6x10-13 joules). Usually when the energy of a decay product is given it is the kinetic energy. If this energy is small compared to the rest mass energy of the particle, you may approximate the kinetic energy to be
½mv2 and so, convert the energy to joules and solve for the speed v. Since the rest energy of a proton or a neutron is about 1 GeV=1000 MeV, this method would be fairly accurate for up to tens of MeV kinetic energies. For higher energies, the velocity would have to be calculated relativistically.


QUESTION: 
When scientist's talk about the uncertainty principle in quantum mechanics are the particle and wave aspects of an electron equivalent to its mass and energy respectively? In other words does a particle equate to an electron's mass and wave to its energy? Are these aspects in constant flux creating the uncertainty; i.e., perhaps coexisting in both mas and energy forms?

ANSWER: 
No. A particle has both mass and energy. A wave has energy and may also have mass. The wave-particle duality is related to the uncertainty principle, but not in the way you suggest.


QUESTION: 
What is the formula for displacement as a function of time for a particle undergoing constant acceleration? It's d=1/2 a*t2 at a Newtonian scale, but relativity kicks in as it gets closer to the speed of light. By "constant acceleration" I mean constant from an observer at "rest" relative to the accelerating particle (as would be the case of a particle falling towards a gravitational well) as opposed to "constant in the accelerating particle's frame of reference" (which would be the case for a rocket burning fuel). Here's the function for the case I DON'T need: http://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html.

ANSWER: 
Why is acceleration an important quantity in Newtonian physics? Because it is what is called an invariant quantity; any two observers in different inertial frames will measure the same acceleration of a third object. This simple fact is the basis for Newton's second law because, since the force is what causes the acceleration and therefore if there is some force on an object all inertial observers must measure the same acceleration. Acceleration has no such hallowed position in special relativity and, as you may know, Newton's laws of motion must be revised by redefining momentum. Acceleration is essentially a useless quantity in relativity. In fact, it is not possible to have uniform acceleration in the sense that you define it (that is why it is defined differently in Baez's discussion). Think about it: Suppose that the "uniform acceleration" were 1 m/s2 and your speed right now was 0.5 m/s below the speed of light; if acceleration stayed constant, your speed in one second from now would be 0.5 m/s above the speed of light, a forbidden situation.

FOLLOWUP QUESTION: 
If you could drop a rock down an infinitely deep well with a constant gravitational "pull," what formula would describe its velocity ("v") in terms of time falling ("t")? I know it starts as v = at (where "a" is the gravitational pull) and approaches v = c (where c is the speed of light), but what does it do in between?

ANSWER: 
Let us stay away from gravity since the definition of a uniform gravitational field is problematical. But, I think what you are interested in is what is the velocity if the applied force on a mass m is constant. I know that an object with zero net force has dp/dt=0 where p=mv/
√[1-(v/c)2] (again, see the link to relativistic momentum). Now, I am going to define a constant force F to be one for which the rate of change of momentum is constant, that is dp/dt=F where F is the constant. Now, we know that vFt/m for small time and v≈c for large time. It is very easy to integrate dp/dt=F to get momentum as a function of time, p=Ft. Putting in what p is in terms of v and solving for v, I find v/c=(Ft/(mc))/√[1+(Ft/(mc))2]. This function has the correct properties at small and large t and is shown in the graph at the right. This is the correct v(t) for a constant rate of change of momentum F. (For purists, I am talking about 3-momentum.)

NOTE ADDED LATER: 
Someone expressed interest in the position as a function of time for this problem. This is straightforward to do by integrating dx=vdt. Doing this I find x=(mc2/F)(
√[1+(Ft/(mc))2]-1). Note that this has the expected properties that for small time, x≈½(F/m)t2, and for large time, x≈ct. (I assumed x=0 at t=0.)

NOTE ADDED LATER: 
In a much more recent answer I have worked out the analogous problem for a constant weight F=mg/√[1-(v/c)2] which is a velocity-dependent force.


QUESTION: 
we can ride a bicycle with tyres fully inflated easier and faster than a tyre with less inflated . why is it so?

ANSWER: 
The reason is called "rolling friction" and is due to the fact that the tire is "squashed" when in contact with the ground but not "squashed" when not in contact with the ground. Since the wheel is rolling, the tire is constantly being compressed and uncompressed. Every time you compress the tire it costs energy; every time it uncompresses, you get energy back. But, the tire is not perfectly elastic and so you get back less energy than you put in. If the tire is inflated so that it is very hard, there is much less compression and therefore much less energy lost. A hard tire, however, provides a much less comfortable ride and so we make a compromise between comfort and effort.


QUESTION: 
if an enviornment were created that truly had 0 gravity would air particles still cause friction on an object to the extant of drasticly slowing it down, and if not in this environment would an object be able to accelorate faster than light?

ANSWER: 
Answer to your first question: gravity has nothing to do with air friction; however, if there were no gravity here on earth, our air would all dissipate into space. Answer to your second question: it is not air friction which imposes a limit of the speed of light on all speeds, it is the laws of nature. We can eliminate air friction (not by eliminating gravity) by simply eliminating air; even in a vacuum, no object may go faster than (or even as fast as) the speed of light.


QUESTION: 
Hi, I'm reading Hal Clement's Mission of Gravity set on an oblate Jovian sized planet. The planet spins at 8 minutes per rev to create such squashed sphere. The main special thing about the planet is that the gravity at the equator is about 2 g and the gravity at the poles is a massive few hundred g. Now, the question is, is Hal Clement's physics right? Would there really be a gravity disparity on a planet like this? And if so, what creates it?

ANSWER: 
The planet does not even have to be oblate for such an effect to take place. The reason is the rotation. If you stand on a pole, you are not rotating and so your apparent weight, the force the ground exerts up on you is equal to your true weight (the force the planet exerts on you). If, however, you are at the equator you are rotating around a circle of radius R with speed V and so you have an acceleration equal to V2/R pointed toward the center of the circle. The force causing this acceleration must be however much of your weight is needed. Thus, from Newton's second law, MV2/R=W-WA where W is your true weight and WA is your apparent weight. Hence, WA=W-MV2/R, much less than your true weight if there is a large rotational speed. Eventually, as you decreased the length of a "day", you would become "weightless". Be certain to note that this effect is one of dynamics, not of a gravity disparity. Incidentally, the true weight at the poles actually decreases as the planet becomes more oblate. This can be deduced from imagining a "maximally oblate" planet, a disk; because of symmetry, you would experience zero gravitational force at the center of a disk.


QUESTION: 
I have two cartridges, one has a known powder charge and travels at, say, 2800 fps. This "known" cartridge has a consistent point of impact at 100 meters that we will call its "zero". The other cartridge has an unknown powder charge and will thus travel either faster or slower with respect to the "known" cartridge, and should hit the target above of below the zero. Since we are dealing with ideal conditions, the wind speed would be zero, so the projectiles will strike in the same vertical axis.

ANSWER: 
I have a hunch that air resistance is not negligible for such high speed projectiles, but its inclusion here would make the solution too involved and require approximation or computation techniques too technical for this site. If you neglect air friction and assume the gun is aimed horizontally, the time it takes the bullet to traverse 100 m
≈328 ft with a speed 2800 ft/s is about 0.117 s. During this time it falls a distance ½x32x(0.117)2=0.219 ft. If the bullet has some different speed v, then the distance y it will fall is y=½x32x(328/v)2, so v=1312/√y. The reason that air friction is difficult to handle is that it depends on the velocity so it is no longer trivial to calculate the time it takes to go a given horizontal distance.


QUESTION: 
how can we make acceleration equal to zero for a body?

ANSWER: 
Arrange for it to have zero net force on it.


QUESTION: 
how would i calculate how high an object of a set weight would go when a set force is applied for an instant. for example how would i figure out how high a 5 pound ball would go if i were to hit it strait up with a bat applying a force of 500 pounds for an instant.

ANSWER: 
I am sorry, but "an instant" doesn't get it in physics. If it is 1/100 of a second the answer is much different than if it is 1/1000 of a second.


QUESTION: 
An electron is spinning around the nucleus with circa 2 million m/s. From where did they get this energy and in billions of years there are still so many atoms? Finally, why electrons are not falling in nucleus and there are no electrons in rest?

ANSWER: 
An atom certainly contains energy. But, where did it come from? That question is equivalent to the question "where did the universe come from" because the entire universe contains a certain amount of energy which never changes. It can be (and is) changed from one form to another but the total amount is always the same. This is conservation of energy. Why electrons, if not static, did not radiate all their energy away was one of the main puzzles of late 19th century physics. Bohr's model of the atom and quantum mechanics answered this puzzle. In essence, an atom has a minimum amount of energy it can have (called the ground state); it is against the laws of physics for the atom to have any less energy than this.


QUESTION: 
Is it possible that time never had a beginning?

ANSWER: 
It is generally thought that before the universe existed time would be a meaningless concept. On the other hand, time is not really fully understood (see the following answer).


QUESTION: 
I hope this will not be considered "off the wall." My friend was asking me the other day about time reversal symmetry and I asked her, "What is time?" She had no idea. I'm not sure that I really do either. I'm not asking in a "We see the effects of the wind" sort of way, but rather what actually causes time.

ANSWER: 
I have heard very smart theoretical physicists say that if we really understood time we might have a chance at developing a theory of quantum gravity, one of the main missing features of modern theoretical physics.


QUESTION: 
If mass is able to change/convert into energy (at speed C), then will energy such as light or other forms be able to change/convert back into mass?

ANSWER: 
What makes you think it converts at speed c? No mass can achieve a speed c. But a particle at rest can convert mass into energy. One example is simply the radioactive decay of a nucleus: lost mass is where the energy of the radiation comes from. And light can certainly convert into mass; the best known example is called pair production where a photon (with sufficiently high energy) turns into an electron-positron pair.


QUESTION: 
Has anyone tried to split the electron? If it's infeasible to do this could electrons be the smallest particle possible?

ANSWER: 
So, what does it mean, "smallest particle possible"? A neutrino has a much smaller mass. The "size" of a proton in a nucleus is considerably smaller then the size of an electron in an atom (since it is smeared out over the whole atom). As far as we know, an electron is a truly elementary particle, i.e. it has no known constituent components. That does not mean it is either indestructable or that it cannot participate in the creation of other particles.


QUESTION: 
In a long and enclosed compartment which is filled with water nearly to the top, would a water jet from a rapidly spinning motorboat propeller inside the compartment reach the opposite side with less or equal force than when it left the propeller blades. If less, how is conflict avoided with Newton's third law since the propeller is being propelled in one direction with no seeming force against the opposite side of the compartment to hold the compartment, motor, and propeller in place?

ANSWER: 
The boat is not propelled by pushing on the opposite wall of the container. If you were in a boat in the middle of the ocean you are not pushing on the distant shore. The propeller is pushing on the water and the water pushes back, Newton's 3rd law as you suggest. And, there is not that much of a "jet" and what there is quickly dissipates; the energy from this dissipation would show up as a slight increase in water temperature locally. The way a propeller works is much the same as the way a paddle works: the paddle pushes back on the water and the water exerts an equal and opposite force forward on the paddle (which results in the canoe accelerating forward).


QUESTION: 
I understand that current creates electric fields and induces magnetism - but there is no current in magnetic material like iron, and nickel. How are they, and magnets, magnetized with no current? What is the magnetic field? - What is actually happening between opposite polls on two magnets next to each other?

ANSWER: 
You are right, electric currents cause magnetic fields (a magnetic field can also be caused by a changing electric field). But now, think of the simplest model of an atom. What do you have? Electrons running around in little orbits, which are equivalent to tiny current loops. Also, look at the electrons themselves: they look sort of like spinning charges so each electron is like a tiny bar magnet. In most materials, all the microscopic currents average more or less to zero so the whole macroscopic thing does not have a magnetic field. In a few materials, noatbly iron, there is a tendency of all the outermost electrons to align with their neighbors in other atoms and the result is a very strong magnetic field. It is a huge number of tiny current loops all aligned in the same direction.


QUESTION: 
Suppose we have a long plank glued at one end to a wall at a ninety degree angle. If we place a weight on the very end of the plank, does that mean more force will be felt at the wall than if we had placed the weight on the plank closer to the wall (as in a lever)? What direction would the force be exerted in at the wall (straight down? with rotation?)

ANSWER: 
Talking about the plank "glued" to the wall makes this hard to talk about. In fact, for real planks you would be hard pressed to find a strong enough glue. It is perhaps best to take the simpler case of the plank being able to slide frictionlessly on the wall but to be attached by a hinge at its upper edge. This gets to the essentials since the hinge and the wall can exert the necessary forces on the plank to keep it in equilibrium. What you find is that the hinge exerts two forces on the plank, a vertical (up) force which equals the weight of the plank plus the other weight and a horizontal (into the wall) force that keeps the plank from falling (i.e. pivoting around its lower edge). The wall exerts a force horizontally away from the wall on the lower edge of the plank which ensures equilibrium in the horizontal direction, that is, it is equal and opposite the horizontal force exerted by the hinge. The closer you place the weight to the wall, the smaller will be the horizontal forces by the hinge and wall.


QUESTION: 
How does evidence for a gravity particle (such as a graviton or Higgs Boson) effect or integrate into Eistein's General Theory of Relativity, where gravity is seen not as a particle but the warping of space caused by matter?

ANSWER: 
Neither of these has been experimentally observed and so there is no evidence for either. There is no theoretical basis for a graviton either since there is no successful theory of quantum gravity. There is theoretical expectation that the Higgs boson exists, but it will never fit into general relativity because quantum physics and general relativity are not compatible. That is why a theory of quantum gravity is being sought.


QUESTION: 
from the different type of matter be it liquid, gas, solids which one has the losest electrons around the atoms, example copper has electrons that are easier to knock from there orbit then steel etc. what would have electrons that are easier to knock free from the atom ?

ANSWER: 
First of all, the atoms must be close enough to interact with their neighbors because the phenomena you refer to are the result of collective behavior, the realm of condensed matter physics, so gases are out. Then the behavior of electrons is dependent on things which are not determined by whether it is a solid or a liquid but by the atomic structrure of the molecules/atoms and their interactions with their neighbors. Further, it depends on temperature sensitively. The most important thing is that good conductors, like silver or copper (solids) or mercury (liquid) have the outermost electrons almost entirely free. In a good conductor there is approximately one electron per atom which is free to move around in the material. A quite good model of a good conductor is to think of there being a "gas" of free electrons bouncing around.


QUESTION: 
if one person is travelling on a train at a speed of 100 mph and another person is stood still and as the train passes the person stood still they both fire an equel powerd gun at the same time, would there be a difference in the bullets speed?

ANSWER: 
The person on the ground will fire a bullet with velocity equal to the muzzle velocity of the gun. The person on the ground will fire a bullet with a velocity, as seen by the person on the ground, of the muzzle velocity of the gun plus the velocity of the train. It is important that those two velocities be added as vectors. For example, if the muzzle velocity is 200 mph, the speed relative to the ground would be 300 mph (forward) if fired forward and 100 mph (backwards) if fired backward.


QUESTION: 
Why does light, if it is a result of the electromagnetic phenomenon seem to exhibit little, to none of the properties of electricity or magnetism (like charge for example)? I've asked about this before and have been told that it's explained in Maxwell's field equations, but I'm still in high school and won't be getting into that until university (I think). Are you able to explain it without resorting to more advanced mathematics like that?

ANSWER: 
Everything about light and how it interacts is electromagnetic! When it strikes your eye the electric fields in the light beam cause chemical reactions and electrical nerve impulses in your eye. When a radio wave (the same phenomenon as light, just not visible to our eyes) strike an antenna the electric fields in the wave causes electrons in the antenna to move around causing electric currents which can be amplified and decoded. All electromagnetic waves are produced by electromagnetic phenomena, whether the electrons in an atom jostling around or the electrons in your cell phone antenna sending out messages. To understand, at least qualitatively, Maxwell's discoveries is not really hard. It goes like this: there are four fundamental equations which describe all aspects of electricity and magnetism. These four equations, if manipulated mathematically, can be transformed into two equations which are recognized by all physicists and mathematicians as wave equations. The speed of the waves predicted by Maxwell's equations just happens to be 3 x 108 m/s, the speed of light! Until this discovery, it was not known what light was but its speed had been measured. If you want to read a little more detail about electromagnetic waves, see one of my earlier answers. You might also be interested in my earlier discussion of electric and magnetic fields which is essentially Maxwell's equations in words.


QUESTION: 
If, in the future, space ships can travel at speeds that approach the speed of light, iI.e., where time would slow down, etc,. Will it be possible for NASA to communicate with the ship? How will the space ship experience any messages coming from a place that is experiencing time at a different rate?

ANSWER: 
Yes, communication will be possible. But, if the speed is very large the received signals will be doppler shifted so the ship will have to tune to a frequency lower than that transmitted. Similarly, signals sent back to earth from the ship will be doppler shifted.


QUESTION: 
how do i talk about kinetic and potential energy of the pendulum?

ANSWER: 
First choose potential energy to be zero at the bottom. The potential energy is therefore mgy where y is the height of the pendulum bob above the bottom. The kinetic energy is
½mv2. Because the only force on the pendulum bob does no work, because it is always perpendicular to the motion, the total energy of the system never changes, that is it is conserved. Therefore Emv2+mgy. You need to determine E at one point. For example, if you start it at rest at a point where y=h, then E=mgh.


QUESTION: 
picture a roof made made metal. stick a magnet to that roof. The magnet has weight naturally. so by keeping itself stuck to the roof it needs to oppose the force of gravity acting on it. It does so by exerting another force of attraction in the opposite direction, this force needs to be stronger than that of its own weight if it is to remain stuck to the roof. This is where my question comes in: that force is work is it not. It is actively working to oppose its own weight. But its a permanent magnet and the amount of energy used to magnatise it compared to the amount it expends holding itself aloft is disproportional. I read that you do the work for the magnet when you pick it up to sick it to the roof and you'll need to do the same amount of work to pry it off again so in essence you have done the work for it and you have simply transferred it to a different potential state. But im sorry it does not make sense. There is a constant force opposing its attraction force to the roof. the force of gravity. This is in violation of newton, to apply a constant force you need a constant supply of energy. Where does the magnet derrive that energy from?

ANSWER: 
You have this entirely wrong. First of all, the force necessary to hold the magnet to the roof must be exactly equal to the weight of the magnet. Second, the force holding it to the roof does no work because it does not act over a distance; if you used the magnet to move a nail across the table it would do work on the nail. Finally, exerting a force does not require energy, only if the force does work.

FOLLOWUP QUESTION: 
I am satisfied to a point with the answer but it still doesnt make sense If you look at it like this: Take an ordinary object and attach it to a pully system. pull on the chord until the object meets the roof. now keep it there. since the object is not exerting its own force keeping it there you have to do that for it. in doing so over a period of time you will begin to tire as you are burning calories, expending enegry to keep that onbject aloft. the magnet stuck to the metal roof is only kept there by the force it is applying to the roof. you said that force is not greater than its weight but just equal and opposite to gravity. but what about an industrial magnet. you can easily support its weight on your hand, but put the magnet on your hand when resting on a metal plate it will crush your hand. I want to know how a magnet can apply a constant force and not "tire".

ANSWER: 
Your question now verges on biology rather than physics. I do recording for the blind and recently read a discussion regarding just what you are asking, viz. how can you say I am not doing work when I hold a box when I know energy is required to do so? The gist of the answer is that muscles exert a force by individual fibers of the muscle continually slipping and then recontracting, so for this special case the individual componenets of the total force are all contiually pulling over a distance and hence doing work. This is not the case for a mechanical system like your magnet; or say you simply tied the rope in your pully to something (not a muscle)
—the rope exerts the necessary force and does not use any energy to do so. I do not see the relevance of your "crush your hand" remarks. While your hand is being crushed the magnet is moving so work is being done; when it is all crushed, no more work is done.


QUESTION: 
If I secure 2 hemispheres in a vacuum (ie in space), then I bring it back to Earth at around sea level, what would be the strength of the hemishperes against being torn apart?

ANSWER: 
I presume you mean how much force must be applied to pull them apart. The geometry of the sphere makes a quantitative answer to your question difficult (too mathematical) but I can give you an idea how big the force would be. I will assume we have two "hemicubes", two halves of a hollow cube which we put together. Atmospheric pressure is about 105 N/m2, about 2100 lb/ft2. The forces pushing on the cube (or sphere) depend on its size; imagine a 1 ft cube. The force holding it together, due to the pressure on the two ends opposite the seam, would be 4200 pounds.


QUESTION: 
I am studying special relativity independently, focusing more on logic and thought experiments than math, and I have hit a wall. I think that an altered version of the twins paradox could work to explain my problem. Consider the twins paradox with two alterations:
1. Twin B's ship, taking off from the planet, is destined for another solar system. This particular solar system happens to have no motion relative to the origin planet's solar system---the systems do not move with respect to one another---they are in the same inertial reference frame.
2. Twin B's ship, instead of turning around and going back to its origin, decides to stay at its destination.
Now, because the solar systems they reside in are in the same inertial reference frame, Twin A and Twin B are back in the same frame. But which one is older? Or are they the same age? Without the acceleration and frame switch of a return trip resulting in asymmetry, how is the symmetry paradox of their time dilation resolved?

This question could also be taken as questioning how the results of the Hafele–Keating experiment are possible (how the contradiction of observed time dilation was resolved when a plane landed and came to a standstill on the Earth), or indeed simply how the each-frame-observing-the-other-slower paradox resolves itself in the minuscule time dilation I create when I walk a short distance and then stop and rejoin the Earth's inertial reference frame). If I turned around and walked back, then the frame switch of the twins paradox would solve things. But what if I don't?

ANSWER: 
You should carefully read my explanation of the twin paradox. You will note that there is absolutely no need to make arguments either relating to acceleration or asymmetry of the trip. All that is required is time dilation and length contraction (actually, only length contraction is required, time dilation follows). It should be clear that when the traveling twin arrives his clock has recorded 6 years and the earth twin's clock has recorded 10 years. If he now stops he is 4 years younger. Because of the time it takes information to reach the earth twin, he does not measure his twin's age until 18 years have passed on his (earth) clock. Thereafter he will observe his twin's clock to run at exactly the same rate as his. Similarly, the previously traveling twin will have to wait 8 more years after arrival to verify that the earth clock had 10 years elapse; during those years he will perceive both clocks running at the same rate.

Your second question is not really relevant because the experiment was somewhat more complicated because of general relativistic considerations, but it had nothing profound to do with the airplane stopping; all that did was place the two clocks in the same frame again so they could be compared. Every time you move your clock gets out of sync with "stationery" clocks. Why should that be paradoxical? If you walked from A to B and there were clocks synchronized at A and B, your clock would be slow when you arrived at B.

By the way, if you are trying to understand relativity nonmathematically, be sure to use the light clock to understand time dilation intuitively, if you already haven't.


QUESTION: 
does a thought have mass? if the brain is an electric grid, can't electrons that pass through it have mass and weight?

ANSWER: 
Is the thought the collection of electrical impulses which happen when you have it? Of course electrons have mass/weight regardless of what they are doing as do all the nerve cells involved. Incidentally, most electrical currents in neurons are ions, not electrons. To my mind, a thought is not a quantitative thing about which you can ask such questions.


QUESTION: 
If i managed to get near the speed of light and wanted to go one light year would it take me one year at that speed to get there (i.e. the clock i take with me) or would a clock on earth take one year?

ANSWER: 
I assume that the one light year is as measured by an earth-bound observer. Furthermore, neither clock will register one year since that is the time that it takes light to go and no spaceship can ever go that fast. Assuming that the speed is very close to the speed of light, the time for the spaceship to get there as measured by a clock on earth is very slightly longer than one year. The time elapsed on a clock on the clock on the speceship would be much less than one year. How do we reconcile this? Think of the distance from the earth to the star as a long tape measure; from the perspective of the spaceship, this is zipping by at a very high speed. However, moving lengths are shorter (called length contraction) and so you see a distance much shorter than a light year you must traverse so it takes much less than a year to do it. If your speed were 99% the speed of light, I reckon it would take about 51 days; the earth-bound clock would measure about 1 year, 4 days.


QUESTION: 
How can we say that the largest attainable speed is that of light.the light from a moving car with respect to an observer outside is that of its velocity plus that of car's,similarly a beta particle(speed 10^7 m/s) will cross the speed of light on a vehicle with velocity greater than 33 m/s.

ANSWER: 
You are assuming that the speed of the light v' is given by the speed of the car v plus the speed of your beta particle u, v'=(u+v). This is called the Galilean velocity addition formula and is incorrect if any of the velocities involved are not small compared to the speed of light c (which is, of course, your situation since u is not small compared to c. The correct addition formula is v'=(u+v)/[1+(uv/c2)]; read my earlier answer on velocity addition.


QUESTION: 
I've found in a book that the velocity of an electron in an atom is based on equation , v=((2.2*(Z^2)/(n^2)), Z=At: no:,n=shell((1,2,3,.............)for(K,L,M,........)).so if an atom with at: no:137 is found, will it K shell exist as it crosses the speed of light according to this equation.

ANSWER: 
This is a formula which should not be taken too seriously. It is a model-dependent formula which, I presume, is based on a Bohr-like model of the atom in which electrons move in well-defined orbits around an infinitely massive nuclues taken to be a point mass.


QUESTION: 
On what part of the electormagnetic spectrum does the force of magnetism exist? Does magnetism have a "wavelength"?

ANSWER: 
"
…the force of magnetism…" does not belong on the electromagnetic spectrum. The electromagnetic spectrum (light, radio, microwave, x-ray, infrared, etc.) is composed of electromagnetic waves with time varying fields, both electric and magnetic.


QUESTION: 
If you twirl a toy on a string at a constant speed how would you draw the velocity?

ANSWER: 
Perpendicular to the string, the direction in which the object is moving at that instant.


QUESTION: 
I have been reading up on some of Albert Einstein's theories and discoveries and encountered the topic of time dilation. I know that time slows down near the speed of light, but why is that? Would the hands on the clock physically move slower? Do you have to see time as a fabric rather than the arbitrary measurement between two events? Are there any "good" analogies to help explain this phenomenon?

ANSWER: 
The fact is that a moving clock actually runs more slowly. And, time is measured exactly like you expect: you measure the time interval between two different times at the same point in space. Similarly, length is defined as the distance between two different points in space measured at the same time. Fortunately, time dilation can be understood intuitively if you accept the postulate that the speed of light is a universal constant. Here is the way to do it: if I convince you that one clock is a perfectly good clock and you agree that it will keep the same time as any clock in the same reference frame (including mechanical clocks, biological clocks, etc.), then you understand. So, go to an earlier answer to read a description of a light clock.


QUESTION: 
How do I find out the (approximate) weight I am lifting if I am picking up only 1 end of an object (known total weight) and leaving the other end on the ground?

ANSWER: 
It all depends on how the weight of the object is distributed. For example, if the weight were uniformly distributed, like for a long board, you would be lifting half the weight. If most of the weight were at one end, say a sledge hammer, for example, you would be lifting almost all the weight if you lifted the heavy end and almost none of it if you lifted the handle.


QUESTION: 
Could there be or is there a way to decay or transform a gamma ray photon into another kind of photon on the electromagnetic spectrum, like UV rays or visible light? (i.e. 1 gamma photon transformed into two UV photons each with 1/2 the energy of the gamma photon, or something like that) I want to know because I have been interested in matter-antimatter destruction into gamma radiation, and wondered if you could make it into something solar panels could pick up safely and efficiently. If there isn't, do you know of a way to harness gamma power? Thank you so much for your time.

ANSWER: 
I have previously answered a similar question. Also, it is quite possible to convert a gamma ray into a bunch of low energy photons; this is what a scintillator does.


QUESTION: 
In reading about the "Ladder Paradox" in Special Relativity, I feel as though I'm getting part of it, but I also know that if a paradox remains, I'm not really getting it. (I assume you know all this, but I relate it both to talk about what I understand, and how it develops into the paradox I'm unable to resolve.) In the ladder paradox, a long ladder is passing through a garage at relativistic speeds. At rest, the ladder is too long to be completely within the garage. However, from the garage's point of view, there exists a time, as the ladder is passing through, when the ladder is compressed enough that doors on both sides of the garage can be shut, and the ladder is completely inside. Then both doors are opened again, and the ladder continues through. This would seem to lead to the case of the paradox, because from the ladder's point of view, it is the garage that is compressed in space, and so the ladder can never reside completely within the garage, and so the doors can't ever be shut. The apparent paradox is resolved by noting that simulteneity is viewed differently by the ladder and the garage, so what the garage views as the simultaneous closing of the doors, the ladder views as different times. Well enough. But what if we adjust the problem slightly? Say we set up a balanced tower within the garage. If door A opens first, the tower is knocked one way, if door B opens first, it knocks the tower the other way. It is relatively simple to set up the problem so that door A appears to open first from one point of view, while door B opens first from the other. But there's only one tower...which way does it fall? I suspect that I'm glossing over some details that need to be precisely laid out, and when everything is made very explicit, the paradox will suddenly disappear, but I'm not seeing exactly how. Can you provide any pointers?

ANSWER: 
I believe that what you are forgetting is that the tower is moving in the ladder's frame so the times for the information to travel from each door to the tower are different. In fact, the signals reach the tower simultaneously so there is no net effect to knock it over. Both observers agree on the simultaneity of the signals reaching the tower, but not of the doors closing.


QUESTION: 
When you heat up a cold limp balloon, and it gets bigger, where does the space that makes it bigger come from? I know it comes from the energy that was applied externally to the balloon, but internally, what is happening?

ANSWER: 
I don't think you want to ask where the space comes from, the space was already there and the balloon expanded into it. What happens is that when you heat a gas you add energy to it, that is you make all the molecules in the gas move faster. So if the balloon were rigid, the energy would all go into the kinetic energy of the gas; the result would be that the pressure of the gas would be larger because the molecules would exert more force on the walls when they collide with it. But, if the gas can expand, as in your example, the energy can also go into work being done by the gas. If the gas expands, it does work, that is it gives back some of the energy added. This is how an internal combustion engine works for example; energy is added to the fuel (via chemistry) and the gas both heats up and expands (by moving the piston) propelling the car.


QUESTION: 
Why do people put their hands up during a rollercoaster ride?

ANSWER: 
I do not think there is any physics involved here, just the "look, Ma, no hands" principle.


QUESTION: 
How does rocket moves in spaces(vacuum)? where newtons third law become fails. 'every action gives equal and opposite reaction' without air this law become fails. so, tell how rocket moves in spaces(vacuum)? which law apply on it.

ANSWER: 
Rocket fuel is being expelled from the rocket. Think of the following situation: a man is floating in empty space and he has a baseball. He throws the baseball. The baseball moves away from him and he also recoils, moves in the opposite direction than the baseball. Momentum conservation of the system is one way to look at this and this is equivalent to applying Newton's second law. The momentum is zero before the ball is thrown and must be zero afterwards. The ball has a momentum in one direction and the man has a momentum of equal magnitude but opposite direction in the other, so they add to zero. Because the ball has a much smaller mass than the man, it has a much bigger velocity. But, you can also look at the situation from the perspective of Newton's third law: to throw the ball the man exerts a force on it, so the ball exerts an equal and opposite force on the man.


QUESTION: 
OK there is a question , the answer of which I cannot seem to fathom and it has always bugged me. What is the nature of probablity? Let me state this question with examples to show you what I am getting at. As I myself am at a loss at how to express the exact nature of this question any other way. Lets say that you flip a quarter, nececcary assumptions being of course that you are flipping by hand and that by doing such you have perfectly random coin flipping ability. The first time you do you have a 50/50 chance of getting heads or tail. Now the more times you get the same result (lets say heads) will automatically make the next result more likely to be of the opposite result. Since it is unlikely to flip heads lets say 4 times in a row, it is even more unlikely to flip it 40 times in a row. So if you flip twice and get heads then it is less likely that you will get heads a third flip and even less so for a 4th, a 5th, etc. My question is that if I have just flipped three heads then what is the invisible force that makes the next flip more likely to be tails. what is its nature, and what negates it. For instance If I let a different person flip for me after the third heads result then will I be more likely to get a heads again since now it is a 50/50 chance. Or would the statistics be the same, since we would be flipping as a group. What about time. If I flipped three heads, and then waited 4 years before flipping again would I still be under the influence of the quarters that I flipped 4 years earlier? If not then why would 4 years be different than 4 seconds? or 4 milliseconds (If I could flip that fast). Is it the grouping of these events together that causes the probabability influence? What would happen if I decided to simply arbitrarily group random things into a structured pattern of my own making, then could I influence the outcome of future events. If not then why not? Another way to ask the question, could I make myself more likely to win at a coin flippiping contest if I flipped coins at home before the contest and waited until I flipped three or four heads in a row, then did nothing until the contest......would I be more likely to flip a tails, and If I chose tails before the contest then would I therefore be beating the odds? If not then why not?

ANSWER: 
This violates the groundrule requiring concise, well-focused questions! Furthermore it is not physics. Nevertheless, I will answer it because it is such a pervasive misconception. The chance that a coin will come heads up is 50/50 every time you toss it. Asking what it will be this time is not the same question as asking what is the probability of having 40 heads in a row. I have known very smart people who cannot get over the feeling that if you throw five heads in a row that a tails is somehow due to come up. If you cannot get this, don't gamble!


QUESTION: 
Is an atom an example of perpetual motion? If not, what happens to the components of an atom when it runs out of energy?

ANSWER: 
I guess you could say that, but you should be careful about thinking of the atom in terms of a simplistic model of little electrons orbiting around the nucleus. If an atom is in its lowest (ground) energy state, it can go no lower, that is, cannot lose any energy.


QUESTION: 
If you freeze an item with liquid nitrogen does it shatter or does it bounce or does it depend on the time that is frozen? Example: Meat vs. Fruit.

ANSWER: 
There is no answer to this question. If you bring the "frozen" object down to liquid nitrogen temperature, whether it bounced or shattered would be determined by how fast it was going when colliding, the nature of what it collided with, what its size and shape were, etc.


QUESTION: 
I am a physical science teacher, basic chemistry, one student asked me a question that threw me for a loop. We were talking about the speed of light and this question came up. What happens if all light just stopped?

ANSWER: 
Most light moves in empty space or through normal matter. It is a form of energy but, unlike many energy forms, it has no mass and so the only energy it has is its kinetic energy, the energy it has by virtue of its motion. Hence, if it were to stop it would violate a sacrosanct law of nature that energy is conserved. In empty space there is nothing to absorb this energy so it cannot stop or even slow down. In a material, the energy could be transferred to the atoms of the material and then the "stopping" is simply a disappearance of the light. Recently physicists have successfully stopped light (in a very ultracold cloud of atoms) and then been able to "restart" it, but this is done by storing the energy and information in the atoms and then cleverly retrieving them. But these techniques work only for very special wavelengths of light. Also, do not forget that there is a lot more than just light which is all the same "stuff"
—radio waves, microwaves, x-rays, ultraviolet light, infrared radiation, gamma rays, etc.—which basically behave the same as light. It is in the very nature of electromagnetic waves, the very laws of nature, that electromagnetic radiation travels in a vacuum with a constant speed and if that speed were suddenly zero it would mean that the laws of physics suddenly changed.


QUESTION: 
gravity is pulling down towards earth right? well lets say we dug a huge straight tunnel (hypothetically) through the earth to the other side. lets say we drop a car in the tunnel. when the car reaches the center of the earth what happens? does it keep going and accelerate into the atmosphere or what?

ANSWER: 
See an earlier answer to a similar question.


QUESTION: 
Why does it take more energy to raise colder water by a degree ? I'm guessing that its because thermal conductivity rates vary with temperature. eg
At 4 °C : the amount of energy required to warm one gram of air-free water from 3.5 °C to 4.5 °C at standard atmospheric pressure is about 4.204 J.
At 15 °C : the amount of energy required to warm one gram of air-free water from 14.5 °C to 15.5 °C at standard atmospheric pressure is about 4.1855 J.

ANSWER: 
Conductivity is not the reason. The reason is that the specific heat of a material is a function of temperature.


QUESTION: 
If I suspended a weight and let it swing down into bodies of different weights it would move the lighter one much faster and further than the heavy one. But if I dropped the same two bodies from the same height they would reach the ground at the same time. If gravity is a constant force, how does it accelerate all bodies at the same rate? It would appear that gravity exerts a force upon a body that is proportional to the mass of the body.

ANSWER: 
Your two experiments are not comparable. The reason all masses fall with the same acceleration under the influence of gravity alone is that the acceleration a is proportional the the force F divided by the mass m (that's Newton's second law) but also the force F is proportional to the mass m (which is part of Newton's universal law of gravitation). Hence the acceleration is independent of the mass. In your first experiment, the falling weight exerts approximately the same force on each target mass (assuming similar materials for all targets) so, again because of Newton's second law, the acceleration gets bigger as the mass gets smaller.

FOLLOWUP QUESTION: 
I think you are avoiding my real question which is, why do we not feel a gravitational force but do feel a mechanical force of the same strength?

ANSWER: 
The gravitational force is due to a field, that is every atom in your body experiences a force proportional to its mass and so the entire weight force is spread out over your whole body. When something like a jetback exerts a force on you, it is at a specific location where all the force is applied. It is wrong to say that you do not feel the gravitational force; if gravity suddenly turned off or suddenly increased by a factor of 10, you would "feel" it. However, if you mean you do not feel it when in free fall, that is right. In fact, there is a legend that Einstein once observed a man fall off a ladder and realized that there was no experiment he could do to distinguish whether he was in freefall in a gravitational field or if he was simply in empty space. This is usually restated as there is no experiment you can do to distinguish whether you are in a gravitational field or in an accelerating frame of reference with the same acceleration as the acceleration of gravity in the field. This is called the principal of equivalence and is the cornerstone of general realtivity.


QUESTION: 
If a life-bearing planet, say Earth, was to be destroyed somehow, until all that was left were rocks ranging from a few hundred metres to a few hundred kilometres across, would they clump together, or spin off into space? If the answer to that was clump together, would they: 1) Have a breathable atmosphere, that stuck around because of gravity, or would that be gone as well? 2) Have a gravity similar to the original planet, because they're all together, or would each rock have it's own gravity, unaffected by the others?

ANSWER: 
It would depend entirely on the cause of the destruction. If the initial explosion or whatever were big enough, the pieces would fly away with a large velocity and not reassemble, at least not completely. If they did reassemble after some time, there would be no atmosphere because the little pieces would not have been massive enough to hold their shares of the origninal atmosphere and it would escape before the reassembly took place, at least a large part of it. When it reassembled, the gravity would be the same as before the catastrophe.


QUESTION: 
This question is one that everyone is curious about including my science teacher!! I'm a girl in 7th Grade and in Physics we are now learning about Electrical Circuits. As muchy as I love science I am more curious than the rest of my classmates;with my teacher on How does an electron know if there is more than one bulb (or resistor) in a series circuit and know to give each bulb a certain amount of voltage? Its a question no one can answer! But I am always trying to find the answer. And I am so curious and really want to know! I have had some theories and so have my classmatesand teacher! If you do know the certain answer please inform me it will be a big help! If you don't acctually know the certain answer don't worry, it's not a major thing that can come in a test it's just extra! For my , the teachers and everyones benift of learning!!

ANSWER: 
No individual electron needs to "know" anything because when the switch is closed every conduction electron in the wire (which amounts to about one electron per atom) begins moving so it is the collective behavior of the electrons that determines what happens. In fact, if there are two light bulbs in series it is unlikely that an electron in bulb #1 ever gets into bulb #2 because the individual electrons move very, very slowly
—maybe something like one centimeter per hour! So, instead of thinking about individual electrons you should think about all the electrons as if they were a fluid. Then it becomes analogous to water flowing through pipes. Suppose you have two identical pipes (like light bulbs) and you can push a certain rate of water (like current) through one of them with a certain pressure difference (like voltage). Then when the two are in parallel, the given pressure will push through twice as much water as through one. But, in series, only half as much will flow because the pressure difference across one will be only half the pressure difference across them both.


QUESTION: 
I was wondering how long does it really take for Styrofoam to break down? I know it's on the internet, but there are mostly questions and different answers. I was hoping for the real answer. Does it ever break down? How long?

ANSWER: 
The reason you have not been able to find a definitive answer is that there is not one. The degradation depends on environmental conditions. The best answer I have found is at http://wiki.answers.com/Q/How_long_does_it_take_styrofoam_to_degrade.


QUESTION: 
What is longitudinal waves?

ANSWER: 
It is a wave in which the medium moves in the same direction as the wave. An example is a wave in a spring where the spring coils oscillate along the direction of the spring which is the direction in which the waves move. The other type of wave, transverse, is a wave where the medium moves perpendicular to the direction of the wave. An example of this is a water wave where the water goes up and down but the wave moves along the surface.


QUESTION: file:///C:/Documents%20and%20Settings/todd/Desktop/Relative_Humidity.jpg
I am discussing a heat question with a HS physics teacher. I have asked him what would be the expected temperature increase to a perfectly insulated room which was 10x10x30 feet and at -50dF temperature, after you brought into the room a 1x1x1 foot pressure cooker at 250dF and you gradually released the valve to allow all the water to escape into the room as steam. I have estimated it would increase the temp by about 1dF or less. He would say it is much much more. My view is that the steam would not be hot at all in the air and would only FEEL hot on your skin if it were to condense and release its latent heat. Also if the relative humidity of the room was at, say, 35% would you expect the steam to remain in the air or to condense? Assume there are no surfaces colder than the air in the room.

ANSWER: 
First of all, we must work in SI units since all data which physicists use are in terms of these. Your room has a volume of 85 m3 and the water has a volume of 0.028 m3. I can then approximate the masses as 102 kg of air and 28 kg of water. The starting temperatures are -460C for air and 1210C for water. The specific heats of air and water are 10-3 J/kg0C and 4.2x10-3 J/kg0C. I assume the mass of the container is negligible since you gave me no information. Also, the specific heats are really functions of temperature but I ignore that. Similarly, the latent heat of vaporization is a function of temperature, but I am going to first let the water and air come to equilibrium and then I will let the water evaporate (or, as you will see, try to evaporate). The only purpose of the pressure cooker, therefore, is to allow the water to be heated to a temperature greater than its boiling point. I now do the calorimetry and find the final temperature is about 440C=1100F, not far from halfway between the two. This is not unexpected because you have about 4 times as much air as water but the specific heat of the water is about 4 times smaller. Now I will let the water evaporate. But, it takes a lot of energy to evaporate the water, on the order of 2500 J/gram. To evaporate all the water would require so much energy that everything would be way colder than the air started! And the unevaporated water would freeze as you passed 00C which would further complicate things. But look at the graph to the right. What this tells you is that, around 400C, you can only have about 5 kg of water in our 102 kg of air, that is 100% humidity at that temperature. But even that cannot happen in your scenario because the evaporation of that much water would require everthing to cool down a lot and the much colder air (as shown by the graph) could hold much less water still. I think it is not worth trying to be any more exact: the temperature change will not be small and the amount of water vapor in the air will not be large.


QUESTION: 
Is heavy water toxic? Would it react with other chemicals and/or molecules differently than normal water?

ANSWER: 
Absolutely not. Chemistry is determined by the elements present, not the isotopes of those elements. I am not sure, however, about the possible biological implications if you replaced all the water in an organism with heavy water, but the problem would be a mechanical one due to weight differences rather than a chemical problem.


QUESTION: 
why the force that a wall exerts on a ladder leaning against it must equal the force that the ladder exerts on a wall?

ANSWER: 
Because of Newton's third law which states that if any object exerts a force on another the other must exert an equal and opposite force on the first. This is a law based on experimental observations.


QUESTION: 
I am a physics student and we were doing work with the newtons cradle toy in class. I understand the equations that explain why energy and momentum is stored in the collision systems. I was wondering why then it is not possible for ( in a 5 ball system) ball 1 to hit ball 2 and have ball 4, and 5 bounce off while ball 1 bounces back from where it was initially dropped. I know this doesn't happen but it would still conserve to same amount of energy and momentum wouldn,t it?

ANSWER: 
If the outgoing ball rose to the same height from which it came, then it exited the collision with the same energy it entered so there was no energy for balls 4 and 5 to get was there?


QUESTION: 
Suppose the Higgs Boson is found. Would it be safe to say, or at least consider, that the Higgs field "causes" gravity?

ANSWER: 
Not sure why you would say that. If you say that the Higgs imparts mass and mass causes gravitational attraction, then I guess you could say that. But gravity is actually "caused" by what happens to spacetime because of the presence of mass.


QUESTION: 
I have heard conflicting answers on this question, so I am trying you. Does current flow through a capacitor? The 2 answers I see are: 1) That ,yes electrons flow across the capacitor until the plates are charged. Once they are charged there is no flow. 2) That electrons never flow across the capacitor they only "bounce" electrons from the other side into the circuit. Is this is true then "Newton’s cradle" can be used as a visual example.

ANSWER: 
Answer number 1 is definitely wrong. If a potential difference exists across the plates of a capacitor electrons will flow from one plate and on to the other (not the same electrons) until some limit is reached which depends on only three things, the potential difference, the geometry of the capacitor, and the material which is between the plates. If any physical current leaks across the gap it is not a capacitor, strictly speaking.


QUESTION: 
Gunshot residue particles are composed of Pb, Ba, & Sb. They are usually ~spherical and sometimes solid, sometimes hallow. Typically they have 5 microns to submicron diameters. In my physics classes we always treated air resistance as negligible, but in this case it might not be. Question: How far can a 1 micron solid spherical lead particle travel if accelerated at the speed of sound from the muzzle of a firearm at standard temp and pressure. Assume horizontal trajectory from 1 meter above the ground.

ANSWER: 
First of all a proviso: this is too difficult a problem to solve accurately for many reasons. One of the main reasons is that air friction is a tricky business which depends on many factors like the size and shape of the object, its density, and how fast it is going.

How the drag force depends on velocity is tricky; a dimensionless quantity called the Reynolds number (Re) is usually used to characterize what the velocity dependence is. If Re<1 the force is mainly proportional to the speed v whereas if Re>1000 the force will be quadratic in v, that is proportional to v2. Taking a lead sphere with diameter 1 micron, I reckon that Re is about 20 at sound speed but drops quickly to about 10-5 as the object approaches its terminal speed which is about 0.3 mm/s. I have done some rough calculations where I assume that the frictional force is proportional to v even though a better calculation would have it proportional to a polynomial of the form av+bv2; since Re<<1000 at all times, this is probably a pretty good approximation.

Let's talk qualitatively what happens: The sphere begins with a speed of about 300 m/s horizontally. It rapidly loses its forward velocity before it has had a chance to drop under the influence of its weight and approaches what is called the terminal velocity; a parachutist does not get killed because he has a small enough terminal velocity. Then it drops vertically to the ground with the terminal velocity. So we need to estimate how far it goes before reaching the terminal velocity and that should give a rough answer to your question.

I have used a nice article about sedimentation as a guide in my rough calculations. The Wikepedia entry on drag was also helpful. Using Stokes' theorem I estimate that the terminal velocity is about 0.3 mm/s (small!) and assuming the drag force is proportional to v, F=γv I find that the characteristic time for the particle to lose its initial velocity is about 30 microseconds. This time is kind of like a half life for a decay of something and so it would be a reasonable approximation to say that the speed at this time would be the average speed over the flight. The speed at t=30 μs is about 100 m/s. If we now say the particle goes about 100 μs before it is falling vertically, then the distance it has gone is about (100 m/s)(100 x 10-6 s)=10-2 m=1 cm.

This is all very rough but it is the kind of calculation which scientists like to do to get an idea of what they are working with. I would be surprised if it were wrong by more than an order of magnitude, that is a meter would be the most I would expect the range to be.

Finally, let us examine a basic assumption in all this: the air itself must be still. If the air is moving it will carry the particles right along with it. So here is the rub: what is the likelihood that the air near the muzzle of the gun will be still? Probably a poor approximation to say that there is no "wind". Also, suppose that it gets to where it is dropping with a speed of 3x10-4 m/s; at this rate it will take about an hour to drop a meter! The tiniest of drafts will move the particles all over the place. You said you are a forensic scientist; I hope that you can see that the location of these particles after some crime is not going to provide much information forensically speaking! (All this last paragraph I thought about after having done the main calculations above. If I had thought this out first I probably would not have put the effort into researching and performing the calculations!)


QUESTION: 
We're having a debate of wether having a way of mapping the atomic or subatomic configuration of a single organic cell; that is know the position and structure of every atom in that cell, would give us the key to make animated organic objects by combinig the elements necessary in the already atomic configuration of the original cell? He says that because of the uncertaintiy principle that is impossible.

ANSWER: 
It is not the uncertainty priniciple which makes this impossible. Just look at the images where one can "see" individual atoms with an atomic-force microscope, for example. But the idea that you could, with the knowledge of location of all atoms, construct a living cell is flawed because the cell is not a static thing. Chemical reactions are constantly taking place and molecules are moving around in the cell. Even if you could take a "snapshot", think of the data problem: on the order of 1020 molecules are in a cell and there is no computer which could store and manipulate that amount of data, let alone a machine which could assemble that many parts in a finite time. For example, if you made a machine which could assemble a million molecules per second it would take about 30 million years to assemble 1020 molecules!


QUESTION: 
Can a one turn coil using a large diameter piece of copper produce the same magnetic field as a many turn coil made with thin wire? In other words, what role does the gage of the wire play in the strength of an induced magnetic field?

ANSWER: 
What matters is the total amount of current which flows. 100 turns carrying 1 ampere would produce the same field as 1 turn carrying 100 amperes. A thicker wire can carry more current than a thinner wire, so a coil with a given number of turns of thick wire has the potential for a higher field than for the same number of turns of thin wire.


QUESTION: 
Are electrons the smallest or can you go smaller? (divide the electron)

ANSWER: 
As far as we know, the electron has no structure.


QUESTION: 
I'm a piping superintendant for a mechanical contractor and I have a pressure testing related question. I must pressure test (w/ air) the system at 660 psi. The pipe is 4" I.D. with a total developed length of 3,900 '. I have calculated the total cu/ft of air required at atmospheric pressure to be 340.27 cu/ft. How do I determine the amount of air that will be required in cu/ft at a pressure of 660 psi?

ANSWER: 
660 PSI is about 44 times atmospheric pressure. So, providing the temperature stays constant, you will need about 44 times more air to fill the pipe at this pressure.


QUESTION: 
How would a graviton interact with another particle?

ANSWER: 
Except in imagination, there is no such thing as a graviton since no theory of quantum gravity has been successful and no graviton has ever been detected. Hence, there is no knowing how it would interact.


QUESTION: 
there are two footballs of exactly same dimensions. One is filled with ordinary air and the other with helium. If kicked with exactly same force and same angle, which one will travel the farthest?

ANSWER: 
Provided that the pressures are such that the elastic properties of the two balls are identical, there should be no difference. The mass could have a very small effect if air resistance is taken into account, but it would be very small. Mass has no effect on motion of an object in a gravitational field. A baseball and a bowling ball given identical initial velocities will follow identical trajectories. A tiny piece of debris and the space station, if both are the same distance from earth with equal velocities, haive identical orbits. Again, all that is assuming that air resistance can be neglected.


QUESTION: 
When scientist use the term "beam of protons" where do they get the protons from? Is the beam made only of pure protons or do they come also with electrons and other particles in the beam.?

ANSWER: 
They are generally produced in an ion source which ionizes hydrogen (there are several methods of doing this). The beam normally is tailored tocontain only protons.


QUESTION: 
How a high energy photon as a gamma ray can knock out a proton from a nucleus of a light atom?

ANSWER: 
A photon carries momentum and energy and interacts well with charged particles, so why would it not be able to knock it out given sufficient energy and moemntum?


QUESTION: 
Are the bonds in atoms closer in water or further away?

ANSWER: 
I presume you mean molecular bonds in dissolved molecules. There is no simple answer since different molecules behave differently in solution, many are not soluable. In the most extreme case, an ionic solution, the molecule does not retain its identity but dissociates into ions. I remember a classic "Puzzler" on Car Talk on NPR: How much salt is there in salt water? Answer: none! The salt in solution is Na+ and Cl-, separate ions.


QUESTION: 
I'm designing a springboard that will be used to launch objects. I know the mass of the objects, how far I want them to travel, and the angle that they will be released. I used the equations for projectile motion to find the initial velocity of the objects to reach the given distance, but I'm not sure how to relate this initial velocity to the spring force, and therefore the spring constant, to know which spring to buy. Also, I need to take into account the mass of the springboard platform because it will likely be heavier than the objects I'm launching.

ANSWER: 
I am afraid that this is far too complicated a question to work out in a concise answer on this site. It would be a lot simpler if you were to launch horizontally and maybe just discussing that will give you the idea of how to generalize the problem. You need to use energy conservation. If you compress the spring by an amount d, the potential energy will be
½kd2 where k is the spring constant. At the instant that the projectile leaves the spring the energy is all kinetic, ½(M+m)v2 where M is the mass of the projectile and m is the mass of the platform. So you simply conserve energy (assuming no losses to friction) and solve for v. Of course, you can vary v by varying d, so you cannot determine k from v; any k will do if you compress it the right amount. It is more complicated but not impossible for the more general case. The thing that makes it harder is that potential energy will also include gravitational potential energy mgh because the height of the object varies during the launch time; also, the location where the projectile leaves the platform will not be when the spring is unstretched. If you use a spring with a very large spring constant k, you can probably ignore the gravitational contributions. So what I would do is get a quite stiff spring and experiment with compression to get the right range.


QUESTION: 
I have a little device that illustrates some optics principle or other, but I don't understand how it operates. It's a small spool (once held thread), one end of which has its opening covered with Al foil (so as to keep out light). A sharp pin was used to create a single, small hole in the foil. The other spool end is open, but a small, iron nail has been poked thru the end (so it's perpendicular to the long axis of the spool). When the spool is held close to the eye so that the nail end is closest to the eye and the other end directed toward a source of sufficient light, and then the nail moved so that it crosses the line of vision, the nail shadow appears to enter the field from the oppostie direction from which it actually is moving. So, my question is: What's going on here? I assume there's a camera obscura connection, but that doesn't help me.

QUERY: 
Sorry, I am not getting the picture. How close is the nail to the eye. What do you mean by the shadow?

FOLLOWUP: 
One would usually hold the device within a couple of cm from the eye, so as not to confuse the issue; nevertheless, the phenomenon remains observable on out to maybe 15 cm. But as you move the device farther from the eye, you must know to allow the eye to focus farther out, at the spools farther end light source (pinhole), as opposed to its closer end where the nail is moved back and forth across the opening. At several cm out, it takes a young person's young eyes to focus on the actual nail. And by shadow, I mean a black image (as opposed to the silver-colored nail) that forms on the retina.

ANSWER: 
This is an interesting device. I made one so that I could be sure I knew what was happening (I used a pin instead of a nail). This is essentially a pinhole camera except we are not using it to make an image in the usual way. Now, all the light coming through the pinhole is coming radially away from the hole and the nail, if it is below the center line, for example, blocks light which came from above the center line when entering the hole. The brain now interprets this lack of light as meaning that a dark object was placed above the center line and that is what you see. This is the same reason that the image, if we use this as a pinhole camera, is inverted
—light from above goes through the hole to below, from the left to the right, etc. I found it works best if the nail end is almost against the eye; and you do not want to focus on the nail, the "image" will appear to be far away. I put image in quotes because it is not really an image. It is very much like a virtual object formimg a real image (for optics afficionados).


QUESTION: 
Let's say there is a fixed point charge of charge q. A charged mass also of charge q is placed a distance away from the fixed point charge. With respect only to time what would be the acceleration function of the charged mass?

ANSWER: 
The answer to this question is extremely difficult to work out analytically. If you had asked me for acceleration or velocity as a function of distance away it would have been trivial. What I can tell you is the inverse relationship, that is the time as a function of the position:

Here, x is the distance away, a=x at time equal zero (at which time v=0), and b=
√[m/(2kq2)] where m is the mass and k is the force constant. Knowing t(x) you could, in principle, calculate x(t). But, to get a(t) you need to calculate dv/dt where v=dx/dt. But dx/dt=(dt/dx)-1 (provided you stay away from x=0), so you could get your answer but it would be very messy! (I am sure this is way more than you wanted to know!)


QUESTION: 
I have a question regarding cars and speed bumps. My father keeps telling me that it's better if a car goes straight at a speed bump thus crossing it with both front and then back wheels simultaneously while I on the other hand consider that it's better a car cross the speed bump with only one side, either left or right wheels going over the speed bump. He gives me an invalid example that this is like a human body trying to do push-ups with one arm; this can't possibly be even close as the body with all it's muscles can concentrate it's weight on a certain point and change it's center of mass while a car is a rigid body with unchanging center of mass.

ANSWER: 
Let's first examine the physics. I will consider just the front wheels where more of the weight is because of the engine. The same arguments I could make are equally valid for the rear wheels (except arguments relevant to steering). When a wheel encounters a speed bump the wheel must accelerate upwards so, by Newton's second law there must be an upward force on the wheel; the greater your speed, the greater the acceleration, and therefore the greater the force on your wheel. That is a little primer on how speed bumps work. So, when you go over with one wheel your total car experiences half the force as when you go over with two wheels (assuming the same speeds). In that sense, you are right; and your father's pushup example is wrong since only half the car's weight is lifted by the one wheel whereas all the weight must be lifted by the single arm. On the other hand, in addition to the upward push by the force, a torque about the opposite wheel is caused which may cause there to be a loss of control of steering. Also, asymmetric forces like this on the suspension system can cause alignment problems; for example, wheels can be knocked out of alignment by hitting a pothole at too high a speed. So although your father's reasoning is wrong, there is less force on the car by going over only half the bump, I still favor his speed bump technique over yours because of the wear and tear issues and safety issues.


QUESTION: 
Does Lorentz force helps electromotors to work?

ANSWER: 
It is the entire reason why electric motors work.


QUESTION: 
Why don't temperature and energy have the same unit, if temperature is the mean kinetic energy of matter?

ANSWER: 
The concept of temperature was defined well before it was appreciated what it meant at a microscopic level, so you could say that it is a historical accident.


QUESTION: 
what will happen when we put a positron in an electrostatic field? does positron behave like an electron or as proton? if we put an electron and positron together. do they attract each other or repel? i doubt that it will show the behavior as proton , in my opinion it will act as an electron in electrostatic field but as a proton in magnetic field. am i right? i have developed a new theory about all universe and particles. my new theory satisfies all the mysteries of universe and particles. it also unified all the four forces in a simple way. but the only problem about my theory is above question. according to my theory the positron will act as an electron in electrostatic field but as a proton in magnetic field. an electron and a positron will repel each other. if it is so, my theory will change all the previous ideas and theory and become a new theory. so please confirm about positron behavior

ANSWER: 
Electrically, a positron behaves like a proton with an electron mass, that is it has positive charge. An electron and positron attract each other, in fact there exists a bound state of one positron and one electron called positronium. It is very short lived because the two eventually annihilate. A positron also acts like a positively charged particle in a magnetic field. These are not results of some theory, they are cold, hard experimental facts. Looks like your theory needs some revision.


QUESTION: 
What is charge really? I don't understand it. I know charge means an attraction, but what does that mean? What makes one thing attract to another?

ANSWER: 
I have answered this question twice (1 2) before.


QUESTION: 
Today on NPR's "Cartalk," someone called in a physics question. I would like to have a definite answer (very easy for you I'm sure). Here it is: A lady's car is stuck in the mud. She of course is alone with no phone and is a physicist. She ties a rope to her car bumper and a nearby tree. She then finds the mid-point of the rope and pushes with max effort which she estimates to be 300 Newtons. The car just begins to budge with the rope at about a 5 degree angle. With what force is the rope pulling on the car? Ray, co-host of "Cartalk," said to find the sine of 5 degrees and then multiply by 300. Then he changed it to cosine of 5 degrees and multiply by 300. If any of these is right, I don't understand why. I've done vector problems before but they were simple Pythagorean probs (like, two ropes pull on an object at 90 degrees to each other--find the the single vector--easy).

ANSWER: 
One of my favorite shows! Neither of the answers is right which is surprising since Tom and Ray are both are MIT grads. Here is how you do the problem: See the drawing (sorry for the quality) on the left. The point where she is pulling is in equilibrium, so the vector sum of the three shown vectors (her 300 lb pull and the tensions in the two halves of the rope) must equal zero. The components perpendicular to her pull must add to zero, so the tension (T) in each side of the rope is the same. This comes from T1 cos 50-T2 cos 50=0, so T1=T2=T. Similarly, the components parallel to her pull must sum to zero, so 300-T sin 50-T sin 50=0. So, T=300/2sin 50=1721 lb.

 

 

 

 

 

 

 

 


QUESTION: 
In an not animated object are the atoms of the object moving inside the object or just the electrons orbitng each atom.?

ANSWER: 
The atoms themselves move. Think of all the atoms being connected to their neighbors by little springs and all vibrating around.


QUESTION: 
What equation do we use to measure the length of an atom? or the size of a proton. According to some books the size of a proton is 1x10 (-15) meters.

ANSWER: 
First, appreciate that these objects do not have well-defined sizes like, say, a baseball. They have "fuzzy" edges. The size of a proton is on the order of 10-15 m=1 fm (femptometer) as you suggest; This comes from measurements of nuclei which are several fm across. The size of atorms is typically on the order of an Angstrom (
Å, 10-10 m); this is determined by measuring typical spacings between atoms in a crystal. It can also be deduced from Avagadro's number and the density of something.


QUESTION: 
If objects traveling relative to each other "Age" differently (so to speak) then roughly how old would the surface of the earth be relative to its center as a result of eons of rotation?

ANSWER: 
I will ignore any effects from general relativity (gravitational effects) and ingnore the fact that there is an acceleration involved. That is, I will just calculate the special relativity time dilation. If you are at the center of the earth, you will see clocks at the surface of the earth running slower. The slowest will be at the equator where the speed of the surface is greatest, about v=465 m/s (around 1000 mi/hr). This is really tiny compared to the speed of light, c=3x108 m/s. The fraction is about 1.6x10-6=v/c=
β. I work the corresponding fractional time difference to be about ΔT/T=1.3x10-12 and, over the entire lifetime of the earth, T=4.5 billion years, the difference in the clocks would be about 2 days!


QUESTION: 
If gamma radiation are photons with a higher energy that we can't see. Is it possible decrease the energy of a photon thus converting it into a visible photon? if that is possible where the extra energy would go?

ANSWER: 
A photon may decrease its energy (increase its wavelength) by colliding with something. Compton scattering of photons (scattering from electrons) reduces the energy of the photon and the lost energy is carried off by the electron. It would, however, require a huge number of collisions to bring a gamma ray down to visible energies.


QUESTION: 
How is it possible to gather the energy carried away by neutrons in a fission or fusion reaction? and what kind of energy does those neutrons carry?

ANSWER: 
They carry kinetic energy and rest mass energy. You could gather their kinetic energy by slowing them down.


QUESTION: 
Why particle with mass like neutrons have properties like wavelength? i thought only the photon had that property of both particle and wave.

ANSWER: 
Well, you thought wrong! Because that is what nature is like. Google electron diffraction to see examples of wave properties of particles.


QUESTION: 
I have been told that the strong force becomes repulsive at small distances. Is this the case and can you explain why or why not?

ANSWER: 
It is certainly true. I cannot explain why since that is not really the goal of physics; we don't, for example, ask for an explanation of why the electron is negative, it just is. The veracity of the repulsive short-range force is easy to understand. If it were not so, the nucleus (held together by the strong interaction) would collapse. This is called saturation of nuclear forces.


QUESTION: 
Can xrays. microwaves and radiowaves only be made by man? Before we created them therefore did they not exist?

ANSWER: 
All are made naturally in nature.


QUESTION: 
Why do you balance when biking but don't when you stop?

ANSWER: 
See an earlier answer.


QUESTION: 
My question is if gravity is a distortion of spacetime, why are physicists seeking a particle called a "graviton" that causes gravity? I was not sure this is not an astrophysical question, but I thought you could steer me in the right direction!

ANSWER: 
The theory of gravity (general relativity) is extraordinarily successful so, as you say, it is curious why we don't "just leave it alone". The problem is that most of the rest of nature is quite well understood using quantum physics and it is highly appealing to have one theory which covers all of nature. Hence we seek a theory of quantum gravity. Such a theory would have a gravitational field which is quantized, and thus there would be a quantum of the field called a graviton. This would be analogous to the quantization of the electromagnetic field (by Feynman and others) and the field quantum a photon.


QUESTION: 
Assume that we measure the temperature of a gas while staying motionless to it's center of mass. Will we get different results (measure a higher temperature) if we measure it while moving at a high speed relative to its c.o.m. ?

ANSWER: 
Actually, the prevailing view has been that the temperature of a moving gas decreases,
T'=T√(1-v2/c2); this is known as the Planck-Einstein transformation for temperature. However, recent research has argued that this is incorrect and, essentially, that temperature is not a useful concept in special relativity, that is there is no simple transformation for T. I answered a similar question some time ago.


QUESTION: 
What is the difference between speed and velocity? Are they the same or are they different?

ANSWER: 
In everyday talk, these two terms are used interchangeably. In physics, however, they have different meanings. Velocity is a vector quantity, that is to specify a velocity you must give both its magnitude and its direction. For example: the car has a velocity of 50 mph (magnitude) on a level road in a northward direction (direction). Speed means the magnitude of the velocity vector. So, for the previous example: the car has a speed of 50 mph.


QUESTION: 
I've read conflicting material on the issue of length contraction. Is length contraction an actual physical phenomenon or is it just an observed phenomenon?

ANSWER: 
This is a very good question. Even many physics textbooks say that the length appears shorter. The simple fact is that moving lengths are actually shorter than if they are not moving. The appearance, how something looks, depends on the point of view of the observer and the direction of relative motion of the object; moving objects can appear shorter, longer, or the same, depending on these things. What we do is make a measurement based on a reasonable definition of length to find out what the length really is. We define length as the difference between the positions of the ends of the object, these positions having been measured at the same time using our clock. On this basis we find that moving sticks are really shorter. (Incidentally, this "shrinkage" only happens along the direction of motion; if a stick is moving with a velocity perpenticular to its length, the length is unchanged.)


QUESTION: 
Is the sum total gravity of separate mass objects less then than the united gravity of those same mass objects when they are placed together? Has there been confirmed experiments documenting these results?

ANSWER: 
I do not know what you mean by "the sum total gravity". A gravitational field is proportional to the mass of the source and inversely proportional to the square of the distance from the source. So, if you have ten equal point masses, each will have a field of equal strength a certain distance away and if you put them all together into a single point mass, the field that same distance away will be ten times stronger. Another way of saying it is that superposition is valid for gravitational fields; if several masses each contribute a field at a particular point in space, the net field is the (vector) sum of the individual fields.


QUESTION: 
Say there are two points in vacuum, A and B, being separated by 2 light-seconds. In point A I place a solenoid, and begin to give current so its core will be magnetized. Then, an iron in point B will "feel" the magnet influence just 2 seconds after it. If only I replace the vacuum between two points with glass (that has higher refractive index), then will B "feel" the magnet influence in more than 2 seconds?

ANSWER: 
Yes. A magnetic field will spread through empty space at the speed of light. That can be slowed down by a medium.


QUESTION: 
It is my understanding that the more heat applied to an object, the shorter the wavelength of its EM radiation. Hence the transition from red hot, blue, then white. Will an object ever demonstrate other wavelengths if given enough temperature. For example could an object be heated to the point that it would eventually give of violet? (and in kelvin terms what temp would be required?) Or after white-hot does it leave the visual spectrum passing into ultra-violet, gamma-ray etc?

ANSWER: 
An object does not emit a single wavelength but a spectrum of all wavelengths. The wavelength with maximum intensity is what is determined by the temperature. The wavelength of the most intense radiation is given by 2,898,000/T nm where T is the absolute temperature (measured in kelvins). If T is such that the maximum is a little below the visible spectrum, the object looks red; if far below, the radiation is infrared (allowing night-vision detectors to work); if essentially centered on the visual spectrum, it will look white; if well below the visual spectrum, it will look blue. Going much farther will result in the peak being in the UV and eventually in x-rays and
γ-rays. An interesting thing is that it will not look violet after blue since the eye is much more sensitive to blue than violet; this is why the sky is blue and not violet (see earlier answer). You can play around for yourself with this aplet. Incidentally, this discussion assumes blackbody radiation which is a pretty good approximation of the spectra of glowing objects, minus some details which are unimportant in understanding the basic principles.


QUESTION: 
My question is, in effect, what really *is* thermal energy? From what I've been able to ascertain, atoms and molecules jiggle, the more they jiggle, the more thermal energy they have. If I have a cup of hot tea sitting on a table in the cool morning air, the jiggling atoms and molecules in the tea will bang into the relatively less jiggling atoms and molecules of the air, and impart the thermal energy into them, until thermal equilibrium is reached between the tea and the air. The original question I have asked of other people is: if I transported my teacup into deep space, where there may be 1 atom per cubic meter of space, would the tea "cool" as it does in the cool morning air? They say yes. If thermal energy is jiggling atoms and molecules, and there are no atoms and molecules in space to impart the thermal energy to, how does the tea "cool"? Would the teacup cool if placed in a vacuum chamber here on earth? If a rotating body in space such as the earth with nothing such as other solar system bodies to slow it's spin, would ostensibly spin at the same rate forever, why would not the tea also maintain it's thermal energy?

ANSWER: 
Energy takes many forms, one of which is kinetic energy, the energy something has by virtue of its motion. A car driving down the road has kinetic energy which means that, to get it moving you must give it that energy. Thermal energy is, essentially, kinetic energy of the atoms in the object. The temperature of something is a measure of how much of this kind of energy it has (per atom, on average). In your example the hot teacup is losing its kinetic energy by increasing the temperature of its cooler environment; this is called cooling by conduction. (Convection, currents of air and tea, also plays an important role.) If you isolate the teacup with a vacuum (which is what a thermos bottle does) you take away the possibility that conduction can cool the tea. However, as you know, you cannot keep tea in a thermos hot forever. The reason is that there is another contributor to cooling which is radiative cooling. All objects emit electromagnetic radiation and your teacup is radiating infrared radiation which is invisible to our eyes but nonetheless carries energy. Eventually all objects come to thermal equilibrium with their environments one way or another.


QUESTION: 
I am trying to convince my uncle that his idea for a potential invention will not work as he thinks it will. He wants to build an electric vehicle (that performs as current tractor trailers do) that never has to be plugged in. To accomplish this he plans on harvesting the air resistance standard travel applies on the vehicle through something like a wind generator or turbine and solar cells over the cargo area as well as the standard breaking energy reclamation. I've told him that his idea might cut down the number of stops, but he will still have to stop thanks to energy lost in the form of heat. He still won't listen. Is there anything else I could do to show him where he is in error? A model truck in ideal conditions being shown to loose energy would be awesome, but I don't expect it.

ANSWER: 
In principle he is right in the following sense: if you can take more energy from your environment than you lose via friction (heat, as you state), you can keep going forever. So the solar cells could, if large enough and efficient enough, suffice (during the day). However, the idea that he could harvest energy from air friction using a fan of some sort is completely wrong. The fan would certainly take more energy from the vehicle than the energy it would store in the battery. Given the efficiencies of current solar cells, a completely solar vehicle is not practicable with current technology.


QUESTION: 
My son and I got into an argument about the arm speed of baseball pitchers. He tried to tell me that in order for a pitcher to throw a ball 100 mph, that the release point of said pitcher's arm must be moving at least that speed (a little faster, in fact, to account for the weight of the ball and the resistance the ball would face before reaching the plate). I said it was preposterous to think that the pitcher's arm, at any point, could move that fast. Who is right and why?

ANSWER: 
Your son is right. If your hand were going slower than the ball it would not be in contact with the ball. When the ball is released it will never go any faster than it is at that time in the horizontal direction, so how could it have gotten going 100 mph if not by your hand?


QUESTION: 
why does the TV cathode tube needs to be inside of a vacuum? And can a light photon collide with an electron that is going from the cathode tube to the screen of the tv? If so, what type of scattering will that be? And will the electron still make it to the screen?

ANSWER: 
Two reasons. First, the electron beam would collide with air on its way to the screen and be lost. Second, very high voltages (kilovolts) are required and would result in arcing if there were gas in the tube. A photon could collide with an electron but energy and momentum considerations would cause almost no effect on the electron's path. The scattering would be called Compton scattering.


QUESTION: 
what is the solution for v (velocity) in the Lorentz contraction equation to make the contracted length the plankh length? I am not a student but I do have an interest in relativity. I'm attempting to determine the velocity a mass would have to achieve to become a black hole.

ANSWER: 
That, of course, depends on the length of the object moving. Suppose that it is a proton of size ~10-15 m. Taking the Planck length to be ~10-35 m, I find
β»1-.5x10-40 where β is the ratio of v to the speed of light. The general solution is β=√[1-(10-35/L)2]»1-½(10-35/L)2 for a length L.


QUESTION: 
Why atoms always moves???

ANSWER: 
Not sure what you are asking. Maybe use a gas as an example. The temperature of a gas is a measure of the average kinetic energy per molecule, so if the atoms were not moving we would be at absolute zero temperature. This is impossible. Why? Because of the Heisenberg uncertainty principle.


QUESTION: 
Energy changes into mass ,right???? so can mass change into energy???(E=mc^2)

ANSWER: 
Yes. A couple of examples are given in an earlier answer.


QUESTION: 
From a Van der Graff generator you can get a spark to jump through the air. Would the spark still jump in a perfect vacuum? Or is matter required?

ANSWER: 
The spark is the breakdown of the air molecules, so, no, the spark would not jump in vacuum.


QUESTION: 
how is the motion of the sun????does it rotate around it self ...???

ANSWER: 
Yes, the sun does rotate about an axis.


QUESTION: 
how can i produce a strong magnetic field around the human body which can oppose the bullet of gun and other iron thing.

ANSWER: 
Bullets are never iron (at least almost never). But the field required would be far too large to be practicable to create.


QUESTION: 
I am trying to turn on a triple AAA battery digital clock with potatoes. I used zinc screws and pennies. It will not turn on. How ever when I use the tester i get at least 2 Volts in the potatoes circuit but when i connect the clock i get zero volts.

ANSWER: 
An electrical device, like your clock, requires a power source which is able to provide the necessary current at the desired voltage. Evidently your clock requires more electric current than the potatoes can provide.


QUESTION: 
I am trying to get to grips with duality. Using a photon as an example, when a photon is traveling from say the Sun to the Earth: Does it have to be considered as both a wave and a particle at the the same time or does it change from a wave to a particle and back to a wave depending upon the circumstances. I.e is a photon a wave as it travels, it runs into a mirror which causes it to change to a particle and as it is reflected from the mirror carries on its journey as a wave.

QUESTION: 
A question regarding wave-particle duality. Light can be described either as a particle or a wave propagating in an EM field. The same can be said for matter, say an electron. What type of wave corresponds to an electron? What medium does that wave propagate through?

ANSWER: 
Both questioners should read earlier answers concerning duality (1, 2, 3). The answer to the first question is that light is both a particle and a wave until you make a measurement at which time it becomes what you determine it to be. The examples you give are neither definitive: either a wave or a photon may be reflected from a mirror and either a wave or a photon may move "on its journey". The answer to the second question is that the wave is what is called a probability wave and requires no medium to propagate through.


QUESTION: 
if light doesn't require a medium it can travel through any substance....but is it possible????

ANSWER: 
When light encounters a substance, it changes its speed (slows down) and loses energy because of its interactions with primarily the electrons in the material. The properties of the substance determine what will happen in detail. For example, a metal will allow light to penetrate only a very small distance in whereas glass will absorb relatively little light as it passes through.


QUESTION: 
I got interested in Compton Effect, but realized that everything I read has to do with a static electron. I am looking for a book that talks about the Compton effect with moving electron. Question: I see that they always talk about Compton effect with X-rays, is it not possible to have the effect with lower frequencies, like visual frequencies (380–750 nm)?

ANSWER: 
I do not know why your interest is in moving electrons. But, since we are dealing with photons the electron velocity needs to be relativistic (not small compared to the speed of light) for there to be any significant difference from considering the electron to be at rest. Electron speeds in atoms are nonrelativistic so just doing the Compton effect on normal matter does not require worrying about the electron speed. Furthermore, in an atom the distribution of velocities is random and so the net effect would be to broaden the energies of the scattered photons since the velocities would all average out. If you are interested in Compton scattering from a very high energy electron beam from an accelerator, then you would have to work out the kinematics for a moving electron. Regarding the second question, whether you could see Compton scattering of visible light, I would say it is nearly impossible. The reason is that the change in wavelength in Compton scattering is of the order of 10-12 m, that is 10-3 nm, so the shift is negligible. The shorter the wavelength the bigger the fractional effect; gamma rays are even better than x-rays.


QUESTION: 
I am working on a group school project and we hit a snag. Building a simple roller coaster, with a steel ball and a track, we are tying to obtain the velocity of the ball at the bottom of loop which enters the loop at pi on a unit circle after falling a certain height. the ball is falling a certain height which is unknown to us yet because it depends on the radius of the loop we are using. The ball cannot exceed 10 g in the loop and we are looking to optimize the radius to obtain the greatest velocity for a ramp that will put the ball in projectile motion at the end, so we want the perfect height to place the ball at 10 g just at the bottom of the loop. If the ball enters the loop from a position higher than the loop on the left, it would be just past pi on the unit circle so that the drop of our ball is just less than vertical. the ball is falling a certain height which is unknown to us yet because it depends on the radius of the loop we are using. The ball cannot exceed 10 g in the loop and we are looking to optimize the radius to obtain the greatest velocity for a ramp that will put the ball in projectile motion at the end, so we want the perfect height to place the ball at 10 g just at the bottom of the loop. If the ball enters the loop from a position higher than the loop on the left, it would be just past pi on the unit circle so that the drop of our ball is just less than vertical.the ball is falling a certain height which is unknown to us yet because it depends on the radius of the loop we are using. The ball cannot exceed 10 g in the loop and we are looking to optimize the radius to obtain the greatest velocity for a ramp that will put the ball in projectile motion at the end, so we want the perfect height to place the ball at 10 g just at the bottom of the loop. If the ball enters the loop from a position higher than the loop on the left, it would be just past pi on the unit circle so that the drop of our ball is just less than vertical. i guess i should have said nine oclock is where the ball enters the loop and it is rolling down a ramp at a near vertical inclination. Yes the acceleration at the bottom of the loop should be 980 m/sec^2. So we only want to see how the velocity changes over the 1/4 circle the ball will roll to the bottom so that we can get as close to 10g at the bottom as possible.

ANSWER: 
(The question was the result of several messages back and forth.) At your level, there is no way you can reasonably calculate frictional losses. If the ball rolls mostly and if the radius of the ball is much smaller than the radius of the hoop, frictional losses will not be really big. In my discussion I will ignore energy the ball has due to its rotation and I will ignore friction. So the model is a point mass m sliding frictionlessly. The general idea is to first drop the ball from a height h above the bottom of the track of radius R and find the speed of the ball at the bottom; we do this from energy conservation. Next, calculate the acceleration from v2/R and equate it to 10g. Finally, solve for h.

Energy conservation: mgh=½mv2 so v=√(2gh). Acceleration: a=v2/R=2gh/R=10g. Solving, h=5R. Incidentally, note that the force which the track exerts on the ball is 11 times (not 10 times) the weight of the ball because it must also support the ball's weight mg.


QUESTION: 
Hello I used to have a feynman book that had this scenario and I forgot how he explained it I have since lost the book and was wondeering if you could explain it. I have a spaceship movin at 180,000 kms inside of that spaceship i have another spaceship moving at 180,000kms. To the observer on hte ground the second spaceship is moving at 360,000kms. That exceeds the speed of light please expain what would happen.

MY FIRST RESPONSE: 
Your recollection is wrong. I am sure Feynman never said the speed of the second space ship exceeds the speed of light because it doesn't.

FOLLOWUP: 
ya he may have never said this but can you explain what would happen in that scenario?

ANSWER: 
The equation which describes what is called "velocity addition" in relativity is v'=(u+v)/[1+(uv/c2)] where u is the speed of the first ship, v the speed of the second ship, c the speed of light (300,000 km/s), and v' is the speed of the second ship seen by the first ship. Note that if u and v are both very small compared to the speed of light, then the quantity (uv/c2) is very close to zero so that v'
(u+v), which is what you expect to be correct, is approximately true. However, in the example you cited the speeds are not small compared to c (they are 60% of c). If you do the arithmetic you will find that v'=265,000 km/s.


QUESTION: 
what is light? what is the medium of light?

ANSWER: 
I discussed electromagnetic waves in an earlier answer. Light does not require a medium for its transmission; it can travel through a perfect vacuum.


QUESTION: 
Is conservation of momentum proved practically if so what is the experiment?

ANSWER: 
Actually, momentum conservation is more a definition than something you verify. According to Newton's second law, the force equals the time rate of change of something called momentum. If the momentum of a system is not changing, it has, by definition, zero net force acting on it. In classical mechanics, the momentum turns out to be mass times velocity. In relativity, it is more complicated but still conserved for an isolated (no external forces) system.


QUESTION: 
Im taking this geology class and its primarily focused on Oceanography. Anyways were learning about waves and and how the gravitational pull from the moon creates them. Well I always wondered why this force can pull a huge mass of water for example the ocean and create waves and not do the same for a simple lake or pond or even a glass of water which is less dense?

ANSWER: 
Basically because the force exerted on an object is proportional to its mass. So, for example, the force the moon exerts on a glass of water is negligible. Understanding the details of the tides is much more complicated than this, but this is the basic idea of why a small body of water is unaffected.


QUESTION: 
I got into a conversation with my wife about this and we both realized we didn't know the answer. Let's say I have a box which is opaque. Inside it is mounted a lightbulb with an on/off switch mounted on the outside. I take the box to a dark room and flip the switch to turn on the lightbulb. After a minute, I turn off the lightbulb and then open the box. Unless I'm very mistaken, I don't see a big flash of light...but why not? What happened to all the light? The lightbulb burned for 60 seconds so...what happened to all the light it generated? Why wasn't it still whizzing around inside the box?

ANSWER: 
Quite simply, all the light is absorbed by the walls of the box. The energy carried by the absorbed light will show up as a slight increase of temperature of the box. This happens incredibly quickly. In an earlier answer with a box with "perfect" mirrors you will find some quanitative details.


QUESTION: 
what is the real meaning of momentum & how we can relate it with photons which has no mass at rest and not defined mass while moving with speed of light?

ANSWER: 
In classical physics, momentum  is mass times velocity, p=mv. One of the most important features of momentum is that the total momentum of an isolated system never changes; this is called conservation of momentum. However, in the theory of special relativity, if you choose momentum to have the same definition, you find that momentum conservation is lost. Conservation is such a powerful concept that we choose to redefine momentum. The details of all this are given in an earlier answer. The result is that if a particle of mass m has an energy E, then its momentum p is p=
√{(E/c)2-m2c2} where c is the speed of light. So, you see, it is not necessary for a particle to have mass to have momentum—it need only have energy.


QUESTION: 
I saw the movie "GalaxyQuest" and it made me wonder. How far do radio or television waves really go? If I was on a planet around our nearest star (about 4.3 LY away), could I really pick up radio or television signals from Earth? If no, why not - after all, they're just beams of energy, so wouldn't they continue through space forever?

ANSWER: 
The problem is that the radio waves spread out as they travel and the energy gets spread over an ever-increasing area. The intensity of the signal falls off roughly like 1/R2 where R is the distance to the transmitter so the signals get incredibly weak; this means you need an incredibly large antenna to receive any informaiton from the waves. Some time ago I did a very rough calculation of the size of antenna you would need.


QUESTION: 
If a photovoltaic cell is energised by artificial light, can the cell output (KW) exceed the energy consumed by the light source?

ANSWER: 
No. Three reasons:

  • Photovoltaic cells are not 100% efficient.
  • No light source is 100% efficient; for example a light bulb converts most of its energy to heat, not light.
  • Energy conservation forbids that you can get more energy out of a closed system than you put in.

QUESTION: 
how does the absorption of beta radiation by air depend on the distance travelled through air?...without using magnetic or electric fields?

ANSWER: 
   
The image “http://trshare.triumf.ca/~safety/EHS/rpt/rpt_2/beta_r.gif” cannot be displayed, because it contains errors.


QUESTION: 
Is there another element that fire can burn in besides oxygen? I'm having debate with friend. I argue that a race on a planet without oxygen could not develop technology since there is no way for it to begin because there is no fire. My friend doesn't agree with this, he believes other elements could be used. I know something of chemistry and I think he's dreaming.

ANSWER: 
The problem here is semantic, I believe. Burn is a qualitative term usually meaning a chemical reaction where oxygen combines with something else resulting in energy being released. But, lots of chemical reactions are exothermic, that is release energy, and so I could imagine that if fire as we know it could not happen, another chemical reaction could act in its place.


QUESTION: 
Can you explain how and why radiation escapes from black holes. I think this is the so called Hawking radiation. I understand basics of quantum mechanics, and good understanding of physics.

ANSWER: 
To understand the process in detail is highly technical. You can get a rough idea of how it works by considering the idea of virtual pair production. In empty space a particle-antiparticle pair may come into existence; because of the uncertainty principle, this "energy from nothing" is ok as long as it lasts a sufficiently short time. If such a pair of particles comes into existence near a black hole and one of the particles happens to get captured by the black hole and the other doesn't, the net result is the loss of mass of the black hole.


QUESTION: 
i did an experiment today which involved setting up an electric circuit with a 4.5V power supply pack, one 47ohm resistor, a digital ameter and a volt meter in parallel. then i connected some other 47ohm resistors in various combinations.
one on its own
two in series
three in series
two in parallel
three in parallel
two in parallel and
one in series
i then took readings for the current and voltages for all these combinations the thing im puzzled with is the relationship between the power dissipated (P=VI) and the resistance. the graph formed is an 'n-shaped' one. why does the power first increase as the resistance increases but then decreases again. is it due to the fact some of the combinations were in parallel and the others in series? i found that all the resistors in series, with the higher resistance and lower current, were the ones with the least power dissipated and the resistors in parallel were the ones which increasingly more dissipated the power as more were added in parallel. can you please just help me with the understanding behind why this happens?

ANSWER: 
I assume that you measure the voltage across the whole network in each case and the net current through the whole network. You should have learned how to calculate the effective resistance for any combination of series and parallel resistors. I am not going to tell you how to do this problem, but the important feature you need to know is that identical resistors in series have a larger resistance than each individual resistor and identical resistors in parallel have a smaller resistance than each individual resistor. Since, in every case, the voltage is 4.5 V, the bigger the current the bigger the power.


QUESTION: 
Consider two hollow spheres of equal size anchored under water. One is airtight. The other has an opening at its lowest point, but no water enters because the air pressure inside is equal to the water pressure at the opening. Will the open sphere have slightly less buoyancy because the pressurized air inside is more dense, or more buoyancy because the air pressure acting on all the upper surfaces exceeds the water pressure acting on the outside of those surfaces?

ANSWER: 
The buoyant force is determined only by the amount of fluid displaced (equal to the weight of that displaced fluid), so the buoyant force on each will be the same. The weight of the pressurized sphere will be slightly greater so its net upward force (buoyant force minus weight) will be smaller.


QUESTION: 
I have a point and a plate across which voltage is generated. I see the equipotential lines get closer and closer together near the point. Does this mean that the voltage at the point would be higher than expected, if there were only two plates?

ANSWER: 
Equipotentials becoming closer together means the electric field is increasing. The "voltage" at the point depends on the potential difference which you have set between the point and the plate, for example by attaching a battery or power supply between them. So, if you replaced the point by another plate but did not change the battery, the "voltage" at the second plate would still be the same as for the point. The reason I put voltage in quotes is because the actual value depends on where you choose zero voltage to be; the only meaningful quantity is the potential difference between two points.


QUESTION: 
is it possible for an object at an altitude of 1300ft to fall that distance to the ground while doing lots of work all the way down, and have any period of observable freefall?

ANSWER: 
First of all, by definition of free fall the only force which acts on the falling object is gravity, the force the earth exerts on the object. By Newton's third law the object exerts an equal an opposite force on the earth so the earth, technically, accelerates up to meet the object. However, since the mass of the earth is so huge you would be hard-pressed to observe the work done on the earth! So, apart from my hair splitting, no work is done by a freely falling object. If there is air friction, work is done on the air and that heats it up a bit (but that is not free fall). If it is attached to a rope over a pulley and the other end of the rope is attached to a smaller mass, the smaller mass will gain energy so you could say the falling mass is doing work on it (but that is not free fall). I guess I do not really know what you are asking.


QUESTION: 
does dark energy exist within the realms of particle physics..as the distances are as vast in the cosmos.

ANSWER: 
Nobody knows what dark energy is. It is a phrase invented to "explain" the observations that very distant galaxies appear to be receding with an accelerating rate. If it is actually something (as opposed to "sweeping under the rug" something we do not understand), then if it exists on large scales it would also exist on small scales, but it could very well be negligible.


QUESTION: 
Is the Heisenberg Uncertainty Principle related to the "Measurement Problem" i.e. the indeterminancy of, say, an electron spin before it is observed (measured)?

ANSWER: 
I am not sure what you mean by the "Measurement Problem". The uncertainty principle states that there are pairs of observables in nature which you cannot simultaneously know to arbitrary precision. For example, you cannot know precisely both the momentum and position of a particle. This is often illustrated by an example wherein if you try to measure the position of a particle, say by looking at it with reflected light, you change its momentum. (You may think interchangeably of momentum and speed.) In that sense, the uncertainty principle is related to measurement. The case which you cite, however, is of different origin. In the case of electron spin, you do not know whether it is "up" or "down" and the electron is actually in a superposition of both states such that if you make a measurement (where you actually, by measuring, put it in a particular state) you have a 50% chance of finding it in either. It is a different kind of uncertainty.


QUESTION: 
is sound and light made of the same material

ANSWER: 
Neither are "made of" a material. Both are waves. In the case of sound, it is the propogation of pressure pulses through a material medium (like air or water) so I guess you could say sound waves in air are "made of" air. Light is a whole other kettle of fish. It is a wave which can propogate through completely empty space. It is "made of" electric and magnetic fields. See my earlier answer on EM waves.


QUESTION: 
How do you determine how much work gravity is done if your given the weight in kg, the length of the distance, and the incline ( if any) ? - I am unfarmiliar with the steps

ANSWER: 
Whenever an object of mass m moves, the work done by gravity is -mg
Δy where Δy is the change of vertical position. Here, Δy is positive if the object goes up, negative if it goes down and g=9.8 m/s2.


QUESTION: 
If a stream of water is shot into a container of water from overhead, does all of the force from the stream eventually reach the bottom of the container and thereby push the container down or is the force dispersed throughout the container in the motion of the water?

ANSWER: 
This is a difficult question to answer because it really does not make sense to talk about force as something which eventually reaches somewhere. Let's talk briefly about what does happen. If the container were empty, we would agree that the stream of water would result in a force on the bottom. The reason is that the momentum (mass times velocity) of the water is changed; it goes in with a big speed and ends up with no speed. This happens in a very short time for the empty container. Now we come to Newton's second law which says that the rate of change of momentum equals the force, that is, the bottom of the container must exert a force on the water to change its momentum. Finally, because of Newton's third law, if the container exerts a force on the water, the water exerts an equal and opposite force on the container. Now, suppose the container is full. The incoming stream is again stopped but now it takes a very much longer time (which is maybe what you mean by "dispersed throughout"). Longer time means smaller rate of change of momentum and so the force felt by the bottom of the container will be smaller.


QUESTION: 
My question relates to space time, where as it's seen as a blanket or the surface of a trampoline. If space time is flexible and every planet, star, etc is resting on it then wouldn't it have to be a continually flat surface? I'm having trouble reconsiling the up, down and in between of space with this theory. If you could tell me where I'm missing something I would greatly appreciate it.

ANSWER: 
I think the key to your confusion is that everything is not "resting on it" but rather in motion. If you simply put the earth and the sun both at rest, and watched them you would find that they would crash into each other. So, returning to the "trampoline" model, imagine a bowling ball and a BB on a trampoline. When released the BB would roll over to the bowling ball because of the warping of the surface.


QUESTION: 
What type of field would result from producing a magnetic monopole? So, if somehow divergence wasn't zero for the magnetic curl and vice versa for the electric charge, what would one see?

ANSWER: 
If there were magnetic monopoles there would be no drastic changes in nature. It would be possible to see magnetic fields which did not eventually close on themselves, like the electric fields of localized charge distributions. I do not understand your second question.


QUESTION: 
The unit of force is newton,right.Now our weight refers to the force we exert on the surface of the earth.So our weight should be measured in newtons.But we measure it in kilograms,the unit of mass.Where did the confusion occur?

ANSWER: 
Technically you are right, a kilogram is not a proper measure for weight. However, in everyday life they may be used interchangeably because we do all our weighing at the surface of the earth and weight is proportional to the mass of an object. The fact that we do this makes the distinction between weight and mass confusing to students when introduced to elementary physics where the distinction is important. To make things even more confusing, the English system of units uses pounds to measure weight and a pound is a proper measure for forces, not masses. By the way, our weight is the force which the earth exerts on us, not the force we exert on the ground.


QUESTION: 
When the Big Rip happens 50 billion years from now, when even subatomic particles will get ripped apart, where will all the "stuff" go? If matter cannot be destroyed, and there is so much matter in our universe, how will it all just go away? Where will it go??

ANSWER: 
First of all, the big rip is highly speculative and relies on dark energy which nobody has any idea what it is. Secondly, it is wrong that matter cannot be destroyed. It can be converted into energy. Finally, the big rip, as I understand it, results in an infinite universe in finite time and so a finite amount of total energy spread over an infinite universe would be unobservable.


QUESTION: 
Is the clock that is traveling in the spaceship away from the clock on the ground moving slower because of mechanical reasons? How can we confirm that it is time that is speeding up and not the mechanics of the clock's lack of interference from gravity that "speeding up" the clock?

ANSWER: 
Quite simply, the moving clock runs slow because time in the moving frame time runs more slowly. I have no idea what your second sentence is asking. To get an intuitive feeling for why moving clocks run slow, read my earlier answer explaining the light clock. If one clock in the spaceship runs slow, then all clocks in the spaceship must also run slow.


QUESTION: 
My grade four class are studying weight. They want to know at how many kilometres above earth their weight would change (or the effect of gravity become less)? Also, how high above earth would you go to reach weightlessness?

ANSWER: 
Well, technically, the weight changes as soon as you change your altitude at all. Perhaps we should ask how far away we should go for it to change appreciably? And, of course, "appreciably" is qualitative, so let's say 10%. Also, it is easier to visualize, I think, if we use the earth's radius as a length measurement rather than a kilometer. The important thing is that the weight, the force with which the earth pulls on us, is inversely proportional to the square of our distance from the center of the earth. (I know, that is too technical for 4th graders, but I want to convey the details to you and you can water it down for them.) What that means is that if you go one earth radius above the earth's surface, then your weight would only be
¼ what it is on the earth's surface (doubling the distance means ¼ the weight). I did a quick calculation and find that when you go to an altitude of about 5% the earth's radius your weight would be about 10% less; this is an altitude of about 320 km (since the earth's radius is about 6400 km). You would never have zero weight, technically. You could ask how far away you would have to go for your weight to go to, say, 1% of your weight on earth; that would be about 10 earth radii or (64,000-6400) km=57,600 km in altitude. You often hear of astronauts being "weightless", but that is a misnomer. They feel weightless because they are actually in free fall, but their weight is very close to their real weight since they are not really very high (about 300 km).


QUESTION: 
If there is so much space within atoms, why can't I pass my hand thru a table or wall?

ANSWER: 
I have answered this question before.


QUESTION: 
I've read often that one of the main dangers in space travel (such as NASA's planned mission to Mars) is shielding astronauts from cosmic rays. Wouldn't a simple Faraday Cage around the spaceship stop cosmic rays?

ANSWER: 
A faraday cage keeps electric fields out, not electrically charged particles.


QUESTION: 
I saw a program recently that said, as I understand it, that the mass of an object, such as the Earth, warps spacetime, which creates the gravity that keeps me standing on the Earth. That that the Earth is not pulling me down but that warped space is pushing me down. is that true? Does that mean that there is no "gravitational pull"? Isn't there a search for the "graviton"?

ANSWER: 
You are attracted toward another mass like the earth because of the "warping" of space time, but I would not describe it as warped space pushing on you. The theory which predicts this is called general relativity. Yes, there is a search for a graviton, but it is more of a theoretical search rather than an experimental search because there is no successful theory of quantum gravity.


QUESTION: 
My question is about entanglement. Suppose we have two photons that are entangled, travelling in opposite directions. Experiment shows that if we change the quantum state of one photon the other entangled photon will instantly change, regardless of how far apart they are. Now, suppose one of the photons hits a photovoltaic cell and is absorbed. How will this affect the quantum state of the other entangled photon? Will, like the first photon, the second photon cease to exist or somehow turn into an electron-positron pair?

ANSWER: 
The disappearance of one photon does not mean the other will disappear. However, absorption of the first is equivalent to making a measurement. That is, when it interacts to be absorbed it will be put into a specific state which will cause the other entangled photon to be put into the appropriate other state.


QUESTION: 
A Creationist has argued that evolution is absurd because it violates the principle of entropy by creating greater order from an explosion (i.e. the Big Bang). The Evolutionist replies that life, and the development into more differentiated species, requires considerable energy, thereby increasing entropy of the system. A Layperson onlooker is confused because the Creationist also happens to have a PhD in a relevant science from a prominent Ivy League school where Einstein taught. Could you settle this great debate and posssibly help the Layperson understand the basic Physics?

ANSWER: 
The second law of thermodynamics (entropy argument) refers only to isolated systems. Any part of the system (life forms on earth, for example) may evolve to more ordered systems under the influence of other influences (energy from the sun, for example).


QUESTION: 
Could you please let me know when acceleration due to gravity is negative? Is it negative always? or is it negative when the particle is going up towards space, or is it negative when it is free falling?

ANSWER: 
Acceleration is a vector quantity. Its sign is determined by the coordinate system you choose. If you choose up to be your positive axis direction, then the acceleration (which is down) is always negative. If you choose a coordinate system with the positive direction down, the acceleration is always positive. We generally use the constant g to denote the magnitude of the acceleration due to gravity, so that number is always positive, g=9.8 m/s2.


QUESTION: 
What is the nature of the force of gravity? Is it a particle or a wave? Might it not just as likely be a force of repulsion caused by the mysterious dark matter/dark energy?

ANSWER: 
See my earlier answers to questions about gravity. How could it be a force of repulsion when it is clearly attractive?


QUESTION: 
Say one were to weigh several thousand LED bulbs and establish an average weight per bulb. Then they were to keep half the bulbs as a control, and illuminate the other half of the bulbs for a period of time. As the bulb are hermetically sealed, if one were to then weigh the bulbs that had been illuminated and they were found to be lighter than the control group, would this prove that light is a particle? If both groups still weighed the same, would it prove light is a wave? I recently heard the particle vs wave question raised on a physics television program and tried to think of a way to determine it.

ANSWER: 
Since a photon (the "particle of light") has no mass, your experiment would not prove anything. The thing to understand is that it is not a question of which it is, but that it is both. Any experiment you can devise to look for a particle will find a particle and any experiment you can devise to find a wave will find a wave. This is what we refer to in physics as duality.


QUESTION: 
We can measure distances of galaxies by the red shift of their light? That speed =the amount of red shift and that faster speeds =more distance? More distance also =further into the past. So further in the past galaxies were moving away from each other faster? I thought I heard recently that the universe was thought to be speeding up due to the influence of dark matter. How then was it faster in the past?

ANSWER: 
The important feature is that more distant galaxies move away from us faster than closer ones. What you say about more distant being earlier time is true but not really relevant. It is thought that dark energy, not dark matter, is the origin of the acceleration observed for distant galaxies.


QUESTION: 
I am attempting to write a mathematical model of heat flow in the walls of a house. A problem has arisen with thermal time constant. I suspect i have a definition wrong somewhere. Heat flow rate [q] through a slab of area [A] and thickness [l] and with temperature difference across the slab of [T deg K] is: proportional to [A] inversly proportional to [l] proportional to [T] i.e. q=k.A.T/l Watt [W] =Joules/sec =J/s so k has the dimensions W.l/A/T =(W.l)/(A.T) = W/(m.K) and k is thermal conductivity. The reciprocal is thermal resistivity [1/k] and thermal resistance is [1/k].thickness[l] so reisitance has dimensions (m.K)/(W).(m) = m^2K/W Now for thermal capacity = specific heat x mass. Heat [Q] = specific heat x mass x temp. difference Q = s.h. x kg x K K is deg Kelvin Thermal Capacity s.h. x mass Dimensions of thermal capacity are J/[kg.K].[kg] = [J]/[K] Thermal Time Constant = Resistance x Capacity Time = [m^2].[K]/[W].[J]/[K] = [m^2].[s] My time constant appears to be multiplied by area. Can you see the error?

ANSWER: 
Seems to me that resistance should be resistivity times length divided by area.


QUESTION: 
I'm doing a report for my physics class on gravity and I was wondering if the force of gravity from the sun and the size of the planets determines the distance that the planets are from the sun.

ANSWER: 
The size of the planets has nothing to do with it. The orbital period is independent of the mass. For example, any object, regardless of its mass, which orbits the earth near the earth's surface (like the shuttle) has a period of about an hour and a half. The distances of the planets from the sun is purely accidental. The distance does determine the period of the orbit (the length of a "year" on that planet). If the orbit is circular (which is approximately true for all planets), then the square of the period is proportional to the cube of the distance to the sun; this is Kepler's third law. For example, if a planet is a distance twice as far from the sun as the earth, then its period T may be found from (1/2)3=(1/T)2 , so T=
√8=2.83 years. To learn about Kepler's laws, see the Wikepedia entry.


QUESTION: 
OK! I am in some what of a debate with my colleague. I have just proven to him that whatever no matter the weight of 2 objects they both fall at the same speed and both land at the same time. Unsatisfied by the results we came across (a full 3tr milk bottle falling at the same rate as an empty 750ml bottle) he still thinks something doesn't make sense. So could you please explain what happens if. Now! if 2 catapults were primed to go off at the same force with the above weights ( a full 3tr milk bottle and an empty 750ml bottle) would they travel the same distance??

ANSWER: 
First you should read my earlier answer explaining why all objects fall with the same acceleration (neglecting air resistance). Now, for your question. Since each catapult exerts the same force, the less massive object (empty bottle) will have a greater acceleration over the time of launch and will therefore leave with a greater speed and therefore go farther. For a more massive object to move the same as a less massive object it must have a larger force and the catapults deliver equal force.


QUESTION: 
Can you describe the energy conversion that occurs when a tennis ball is dropped from waist height to the ground? Maybe you can do better than my teacher.

ANSWER: 
When an object falls, its own weight (the force the earth exerts on it) does work. It is shown in any elementary physics text that this work increases the kinetic energy of the object by an amount equal to the work done by the weight. The work done by the weight is weight (mg) times distance (h), mgh. We could simply stop there and never talk about potential energy. However, potential energy makes many problems very easy to solve. Essentially, think of your lifting the tennis ball a distance h. How much work did you do? You did mgh of work. So we now say that mhg joules of energy are stored in the tennis ball by virtue of its position (h above the ground). This is what potential energy is; note that it presents a way to internalize the work done by a force (weight in this case) so you never have to calculate the work done by that force again. So, the ball starts with all potential energy, converts it to kinetic energy as it falls, and ends with all kinetic energy.


QUESTION: 
Why is a metal door knob colder to touch then a wooden door knob?

ANSWER: 
When your hand comes in contact with something, the more rapidly it conducts heat away from your hand the colder it feels. Metal is a much better heat conductor than wood.


QUESTION: 
In a monoatomic gas, the mean velocities of the atoms is directly related to its temperature . When the gas is compressed by a very slow moving piston, the temperature raises. Within the kinetic theory of gases, what is the physical mechanim that makes the atoms increase their velocities?

ANSWER: 
I assume we are talking about a gas which is insulated from its environment, that is no heat flows in or out. When a gas is compressed, work must be done on it. The work must increase the energy of the gas and the only way to do that (for a monotonic ideal gas) is to increase the kinetic energy of the molecules. If you want a mechanism, think of a piston moving and colliding with the molecules and thereby increasing their kinetic energies.


QUESTION: 
I'm trying to understand relativity, and I've done a pretty good job. The one thing I don't understand is the Twin Paradox. Let's say one twin gets on a shuttle to a random star 10 light-years away, leaves, and comes back, going very close to the speed of light. Of course, the twin that left is younger than the twin that didn't. If a light-year is how far light can travel in 1 Earth year, then it would take someone going at about the same speed 1 Earth year to go the same distance. So, if he traveled a total of 20 light-years around the speed of light, wouldn't 20 years have passed for both him and Earth?

ANSWER: 
Please read my earlier answer. It is fairly straightforward to understand the twin paradox.


QUESTION: 
The Doppler Effect in light is simple to understand in wave theory... but under Richard Feynmans' Quantum Electrodynamics, how is the Doppler Effect explained?

ANSWER: 
In QED we deal with photons instead of waves. But a photon has an energy determined by the frequency of the associated wave, E=hf. Now, if you view this photon from a different reference frame (moving with respect to the first), the photon will have a different energy as determined by the special theory of relativity. Hence it will have a different frequency and, of course, that frequency is the doppler shifted frequency. [Incidentally, I do not find the doppler effect for electromagnetic waves to be "
simple to understand". It also must be done with special relativity and for EM waves it does not matter whether it is the source or the observer who is moving. For sound waves it does matter which is the one moving because there is a medium (air) which is not the case for light.]


QUESTION: 
Can You please tell me cause of Magnetic Force ? I know that it is becauz of Opposite charges but am not satisfied wid this cause.I dont come to know how can a non contact force occur. Therefore I want to know it in detail. Please tell.

ANSWER: 
A magnetic force is caused by a magnetic field. A magnetic field is caused by an electric current. A magnetic force is felt by a magnetic current. Although you "know" that it is caused by opposite charges, it is not
—there is no such thing as magnetic charge. Electric forces are caused by electric charges which create electric fields. The answer to how a force can act at a distance is far too complicated to include here. It is a question which has been puzzling thinkers since ancient Greece. Newton's law of gravitation met resistance because it simply postulated action at a distance without providing a mechanism. Nowadays we think of gravity as happening because mass distorts the space around it (general relativity). In classical electromagnetism it is the field which fills the space around electric charges and currents which allows for forces. In quantum electromagnetism the field is quantized and we think of photons as being the "messengers" of the force.


QUESTION: 
I would like to understand why time slows down at high speeds. Any information i find on this topic seems to complicated to get my head around, even my physics tutor said its too complicated and i should just assume 'it just does'. Could you explain it to me in a way i may be able to understand?

ANSWER: 
It is not hard to understand thanks to a clever idea by Feynman
—the light clock. Imagine a stick with a flashbulb and a light detector at one end and a mirror at the other end. A flash of light goes up to the mirror, bounces back and hits the detector. The detector is connected to a little electrical circuit and a speaker which says "tick" whenever it detects a flash of light and also tells the flashbulb to send out another flash of light. So, the clock goes "tick, tick, tick, ..." Now, this is a perfectly good clock, right? What you now do is make this clock go past you very fast (velocity being perpendicular to the stick). Now the light has to travel farther than when it was at rest, right? But, since the special theory of relativity asserts that the speed of light is the same as seen by all observers, the clicks are less frequent as seen by you because the flashes have farther to travel. So, the moving clock runs slow. You can play with a nice simulation of a light clock.


QUESTION: 
i am doing global challenge award with friends and we have a concept of a hybrid solar heating and lighting system. i was just wondering, is there anyway to extract heat/infared from sunlight?

ANSWER: 
Of course. Just hold your hand in the sunlight and it gets warm, right? There are already power stations which focus the sunlight on a fluid to heat the fluid to drive a turbine to generate electricity. Also, there are units you can place on your roof which are painted black to absorb a lot of light and heat water in pipes which snake around inside providing hot water for your house. Also, a greenhouse is just a big box which gets its heat from absorbed sunlight. And, passive solar heating just heats a house the same way a greenhouse works.


QUESTION: 
I'm trying to understand the Planck length. It is supposed to be the smallest unit of measure and is based on fundamental physical constants. Is it a flexible unit of measure--subject to length contraction? I've heard of DSR, and it really makes me wonder how our way of measuring (d/t) things such as the speed of light is not scale invariant...which would make me wonder if light actually moves any more than gravity does. Would the amount of energy contained in a sphere (Planck length/2)^3 make that sphere larger or smaller, regardless of the fact that it remains the smallest unit measurable? If so, I can't help but visualize the universe as a bag of marbles--each marble being a Planck sphere...most of which are less massive than the Planck particle. The problem with this visualization is that a bag of marbles has a bunch of empty (non-marble) space in it. Also, the marbles in the bag we call the universe would all be of different sizes yet would still be perceived to be equal. I know I'm supposed to ask a single question...so it would be this: Considering the thoughts and questions above, what is the significance of the Planck length?

ANSWER: 
The notion that space and/or time might be discrete, not continuous, is speculative and, in a nutshell, nobody knows. See an earlier answer.


QUESTION: 
If Rayleigh scattering is inversely proportional to the fourth power of the wavelength, then why isn't the sky violet, or at least indigo?

ANSWER: 
Actually, the sky is purple! Your eye, an imperfect instrument, is more sensitive to blue than to purple.


QUESTION: 
On a recent episode of Myth Busters I saw a demonstration of smething that I could not explain or understand. They basically took two phone books and wove the pages from each book into one another creating one larger book. At the end of each book steel plates were drilled and then steel cables were attached . It took two military tanks and 8,000 lbs of force to pull the two books apart! How do you explain that? There is no appreciable normal force applied to the books so what force was causing the resistance? I am somewhat baffled by this and when someone first told me about it I didn't even believe it until I saw a video clip of it. can you explain it?

ANSWER: 
As a physicist, I do not find this astonishing at all. There is just about no limit to how big friction can be. For example, if you press the palms of your hands together as hard as you can, you will be unable to slide them. Try this experiment: rip out one page of the phone book and put it in the middle sticking out some. You do not need to press very hard on the phone book to not be able to pull the page out, it will tear first. To understand this quantitatively is pretty hard because the maximum friction force betwen two surfaces is propotional to how hard they are pressed together (called the normal force) and I do not how to determine what the normal force might be. It appears that the books in the experiment are taped shut to keep them from flopping open, but this tape would put a modest normal force on all the pages. Suppose that each surrface contributes a frictional force of 1 lb (right order of magnitude from the one page experiment) and there are 1000 pages per book and therefore 2000 surfaces, then it would take 2000 lb, again the right order of magnitues.


QUESTION: 
I’m wondering whether or not a closed system like a terrarium with one plant seed in it would weigh more after the seed grew into a plant, my mind tells me that it can’t because it’s a closed system, but then again there is a plant where there was not one before. Can we actually measure the weight of the "light" that has been converted into the plant?

ANSWER: 
Energy must be added (in the form of light) to allow the plant to grow. (Most seeds will germinate in the dark using the energy stored in the seed, but this will quickly be exhausted.) Light provides the energy for the chemistry which happens when the plant grows. The actual material (atoms) used to make the plant can only come from material already in the terrarium
—seed, soil, water. The energy from the light gets converted into mass via E=mc2, but that amount of mass is extraordinarily tiny, way smaller than you would be able to measure.


QUESTION: 
I am having an argument with someone who insists that an untetherd balloon does NOT travel at the speed of the wind because of some wooly notion the movement relative to the Earths surface (even when not on it) requires an external force.

ANSWER: 
There are all kinds of hair-splitting arguments you could make about the preciseness of the balloon's being at rest relative to the air, but that does not seem to be the heart of your question. For your purposes, a balloon moves with the wind. This is why you cannot steer a balloon if it does not have any propulsion other than the wind. I remember reading a book when I was a boy in which they dragged a rope (touching the ground) to slow the balloon down so that it could be steered. Movement relative to the earth's surface requires a force only if you move through the air, but not if you and the air are moving together.


QUESTION: 
What is the largest proof of dark matter so far? (My friend and I are doing a science fair project on the topic).

ANSWER: 
You should be aware that you are asking a physicist, not an astronomer, so my view may not be all that mainstream. What is incontrovertible is that gravity acts much more strongly in and between galaxies than you can understand by looking at all the normal matter that you can detect. So it has become very widely accepted that there must be some kind of exotic matter in the universe which interacts only through gravity. And this is certainly possible. There are several ongoing experiments trying to detect this dark matter. There is, however, absolutely no direct evidence for this dark matter. There is only indirect evidence from the anomalous gravitational behaviors of galaxies.

Here is my personal perspective: It is generally thought that we understand gravity very well, that is, the theory of gravity, called general relativity, has been extremely successful in describing a wide variety of phenomena. But what if general relativity is not exactly correct for very large distances (much larger than our solar system)? In other words, what if we do not understand gravity as well as we think we do? In that case, the funny way gravity appears to behave might be perfectly normal and not require "inventing" a new kind of matter at all.


QUESTION: 
If a mass distorts space-time in a similar fashion to a weight placed on a stretched sheet of thin rubber - then does a mass experience some kind of resisting force to the distortion of space-time? [Clarification: Assuming space-time wants to be flat and smooth, if a body of mass warps space time - then does space-time want to resist this warping somehow? If so does it generate some kind of detectable force?]

ANSWER: 
You should not take the rubber membrane model too seriously. And, of course there is some kind of detectable force resulting from the curvature of spacetime
—it is the gravitational field which results from the curvature caused by the presence of the mass. However, if you are suggesting that somehow the mass causing the curvature will feel a force from the curvature it generates, then this will not happen because it would look like an object exerting a force on itself.


QUESTION: 
What would happen if we took an ultra-strong tube that was, for example, 400 miles long, and say, 1 foot (or 1 yard) wide, and extended it into space? Presume that we were able to prevent the tube from falling back to the earth, and presume that we were able to prevent it from being destoyed. Wouldn't the vacuum of space pull air out of the atmosphere?

ANSWER: 
The air in your tube would not be pulled into space for the same reason that the whole atmosphere is not pulled into space: the earth's gravity holds it to the earth. Incidentally, a vacuum does not pull something, rather pressure at the other end pushes it. It might seem a semantic point to you, but thinking of a vacuum as nothing, then nothing cannot do something.


QUESTION: 
I'm trying to get information regarding gravitational effects from objects moving at high sublight velocities. DVICE.com did an article about a duel between 2 spacecraft from 2 different science fiction series, but one of the arguments a reader of the article gave made me wonder. If I made a gun that fired (for the sake of argument) a mass of steel weighing 4 tons on earth at an object and missed it, but was within about 300 feet or less, would there be damage to the object due to gravitation forces? Or is this guy, who claims to be a professor, just full of excited molecules of atmospheric gases?

ANSWER: 
According to my expert on relativity, the answer is that the gravitational force from a passing object will increase as its speed increases close to the speed of light. The reason is that the warping of space-time (which causes gravity) increases with increasing energy density and a rapidly moving mass has a larger energy density than a slower one. A simpler way of saying that is that the mass appears to have very large mass when it is moving near the speed of light.


QUESTION: 
I am doing a science fair project on a clay boat and trying to find out which design floats better and I need to interview you for the project. Here's my questions:
1. Does the height of an object affect its buoyancy?
2. Are solid objects less buoyant than hollow objects?
3. What does the surface area of an object have to do with how well it floats?
4. What is water displacement and how does it work?
Here's My Hypothesis: If I mold 2 clay boats out of the same amount of clay, one with a smaller surface area and one with a larger, then the boat with the smaller surface area will float because the amount of area touching the water will be less.

ANSWER: 
You need a new hypothesis, because this one is not correct. What you need to do is read up on Archimedes' principle which says that when an object is placed in a fluid it experiences an upward force called the buoyant force which is equal the weight of the fluid which is displaced by the object. This is why boats can float and why helium balloons go up. For a boat to float you need to be sure that the water that will be displaced has a weight equal to the weight of the boat, that is all. So, you can see why a hollow object will float
—because it is bigger than a solid object of the same material so it displaces more water. Surface area is not a good guage of how well something will float; imagine, for example a sheet of lead 1 meter square and 1 millimeter thick. It has lots of surface area but will sink like a stone.


QUESTION: 
If you fold 3 sheets of paper -- each 8 x 11 inches. One is folded as an accordion. The second is folded as a triangular prism. The third is folded as a rectangular prism. If you stand these papers up (tall), place a piece of cardboard atop of each, and on that place a 500 gram weight, why does the accordian folded paper support the weight while the others do not? This is a Science experiment that I want to do with my class who is studying structures. Therefore, could you please explain it in simple terms --childlike language? ie. weight dispersal is difficult for them to understand without a clear explanation.

ANSWER: 
Try this: You have a very heavy table you wish to lift. You get ten children and place them all around the table and they can lift it easily. Now, you ask three children to do the same job and they cannot hold the table up at all.


QUESTION: 
What is the future of physics? I recently saw an episode of "Nova" on PBS. The plot involved furoughs at Fermilab. IMO the TV show made a pretty good argument that anything Fermilab can do, CERN can do better. So why fund Fermilab? When it comes to particle physic, is physics reaching the point where the knowledge gained is no longer cost effective? For example, the atomic bomb was designed on a chalk board. Now it takes tens of millions of dollars to find the Higgs Boson, and there is nothing to suggest finding the Higgs will yield anything nearly as significant as an atomic bomb. Has physics research reached the point where the law of diminishing returns comes into play?

ANSWER: 
Nobody has a crystal ball, let alone me. You romanticize the atomic bomb. It was a gigantic undertaking requiring enormous (for the times) investment. Maybe ideas were put on a blackboard, but it was certainly not "designed" there. Nobody can predict what the "returns" on investment in science will be. But, in a world where we cannot have everything we want, it is often hard to choose. You might also read my earlier comments regarding the "philosophical" arguments to be made.


QUESTION: 
I teach 6th grade science and need some help on force/work/motion. Our textbook says that when I lift a book I am doing work as the force is applied in the direction of the motion. However, if I then carry that book across the room I am doing no work as the force continues to be vertical but the motion is horizontal. In terms of the definition of work this makes sense to me. However, I know my students will protest. How can I carry the book across the room and do no work on the book. It will seem counterintuitive to them. Can you help me explain this to 12 year olds? Am I doing work on my body, not the book? Isn't work being done to move the book across the room?

ANSWER: 
The first thing you need to do is talk with your students about how science works. It is very important, particularly in physics, that concepts be well defined. And even though, sometimes, that definition may not jibe with every day usage, there is usually a pretty good reason why it has been defined the way it is. In the case of work done by a force on an object as the object moves some distance, the definition, as you know, is the distance times the component of the force along the distance. If this were not the definition, then the whole concept of energy, so enormously important to the whole structure of physics, would not be useful at all. So, in carrying the book across the room with constant speed, indeed no work is done.

I am not trying to beg the question here and now I will address the very valid concern you have. Let us just imagine an even simpler case, just holding the book in your hand with your arm stretched out from your body. Are you doing any work? What is needed to do work? Energy, right? Does it take any energy to hold the book there? It certainly does. Where does the energy come from? Essentially from last night's dinner and the oxygen you have recently breathed. Your muscles are doing work and the energy to do this work comes from chemical reactions in you body. I read about just this thing recently and it is very interesting. It turns out that all the little fibers in your muscles are constantly slipping and pulling back, so there really is a force being applied in the direction of the displacement. But the work is not being done on the book! The book remains at rest (or, if you are walking across the room, moving with constant speed), so it is not the recipient of this work. Remember, you can always identify work being done by energy changing, and the book moving at constant speed horizontally has constant energy.


QUESTION: 
Below is a link that shows a blond Russian lady Hammer thrower  which includes slow moton video of when her hammer hits the ground.!!! LINK In the video of the Russian lady... when she released the hammer... an observer up in the stands woud have seen the hammer rotatin CCW... but the slow motion part of the video (which is shown after her throw) shows that the hammer is rotatiin CW jus befor it hits the ground... what causes the CCW release of the hammer to change to a CW rotaton befor it hits the ground.???

ANSWER: 
I have pondered this and think I have an idea what is going on. The hammer is essentially a very heavy point mass connected by a relatively light string to a relatively light handle. Because the weight is so much heavier than the handle, the hammer will rotate around a point very close to its heavy end. The hammer will experience air friction as it moves through the air, with both the weight and the handle having forces on them. But the only appreciable torque about the center of mass will be due to the handle because the weight is so close to the center of mass. When the handle is, because of the rotation, moving forward it experiences a larger air resistance than when it is moving backward (because the wind speed it sees is larger), so the effect of wind resistance is to slow down the rotation whichever way it is rotating. If the rotation is slow enough, it might spend a relatively long time in the forward direction, long enough to actually reverse direction. (If it went on forever without hitting the ground, it would continue reversing directions until finally rotation stopped with the handle trailing the weight.) There is another video I found in which the rotation appears to me to reverse again near the end and finish almost not rotating.


QUESTION: 
I'm an English Ph.D. student writing about the figure of the microcosm in the modern novel, and am interested in the parallel with contemporary physics and the conception of the atom as a "little solar system." My question is: how much did the evidence from the experiments done by Rutherford, et. al. make this theory necessary and how much was it just a convenient metaphor? Would other pictures of the atom have been equally plausible, or was this pretty much as accurate a representation as possible?

ANSWER: 
As I have said in many earlier answers, the Borh model of the atom, electrons running around in well-defined circular orbits like planets around the sun is, at best, a very rough approximation of atomic structure. Historically, such a model was ruled out because an electron moving in a circle radiates its energy away and it would very quickly spiral into the center of the atom. However, Rutherford's experiments showed unambiguously that nearly all the mass of an atom and all its positive electric charge resided in a volume extraordinarily tiny relative to the size of the atom. Putting the electrons outside then became imperative and Bohr invented his model of the atom by postulating certain rules for orbits which would be special in that electrons in those orbits would not radiate away energy. His rules also had to, and did, explain the spectrum of the hydrogen atom, those colors of light which are emitted from hydrogen. But, this is clearly ad hoc in many ways as new ideas often are. Within just a few years, though, partly motivated by the success of this model, a new branch of physics, quantum mechanics, was developed which provides an accurate picture of both how we should think about atomic structure and why certain atomic states do not radiate away their energy.


QUESTION: 
According to Wikipedia, the universe is about 93 billion light years in diameter. If the earth were the size of a proton, how big would the universe be on this scale?

ANSWER: 
A light year is about 1016 m, so the size of the universe would be about 1027 m. The diameter of the earth is about 107 m and the size of a proton is about 10-15 m. Put it all together and get reduced size of the universe=1027(10-15/107)=100,000 m.


QUESTION: 
If two objects (a car and a truck) have the same momentum, which would be harder to stop?

ANSWER: 
If they have equal momenta then the force required to stop each in a given time would be the same. If you want to stop them in the same distance, the one with larger mass (truck) would require a greater force.


QUESTION: 
I'm trying to understand the nature of the electric force. I appreciate that it is mediated by exchanging photons between charged objects. Since like charges repel and unlike charges attract, there must be something fundamentally different between photons emitted by positive charges and those emitted by negative charges.

ANSWER: 
For a discussion of photon exchange, see an earlier answer.


QUESTION: 
What causes certain materials to be transparent?

ANSWER: 
This is not simple. When light enters a medium, it will be absorbed only if there are electronic levels in the atoms and molecules which can receive the energy of the light. Otherwise it will pass through. Wikepedia has a pretty good discussion of transparency.


QUESTION: 
Are atomic spectra the foundation of what we perceive as color? I am confused because it seems that atomic spectra are associated with excited states. What, atomically, is going on when we look around and see colors? Why does something not in an excited state, like the paint on a wall, emit a wavelength between 400 and 700nm?

ANSWER: 
For the most part, colors you normally perceive are due to white light falling on something and some of it being absorbed while some of it is reflected. The chemical properties of the object determine what colors are absorbed and hence what color it appears to be. Most of the white light which illuminates our world comes from something very hot which is radiating in a way that there are not specific lines but rather a continuum of wavelengths. Notable exceptions are neon lights which get their colors from atomic spectra, lasers, and sodium vapor lamps often used in street lights (the orange color of this light is atomic in origin).


QUESTION: 
Would a fire extinguisher offer enough thrust to enable the average person to go airborne on the moon?

ANSWER: 
Well, that depends on the fire extinguisher since I presume they have different thrusts. I found a web site in which a guy on a cart using a fire extinguisher as a rocket had an acceleration of about 1/5 m/s2. Assuming no friction, this would be a thrust of about 1/50 of his weight, about 2%. (His weight is his mass times the acceleration of gravity which is about 10 m/s2.) His weight on the moon would be about 17% his weight on earth. Therefore the fire extinguisher would not enable him to become airborne.


QUESTION: 
Foot Lb measurement for torque. If a rotating spindle of a horizontal wind mill (resembles a water mill) can lift 50LBs of bricks one foot (by the rope wrapping around the spindle as it turns without stalling the windmill) is this equivalent to 50 ft lbs? If this does not equate to 50ft lbs, how would you correctly measure the foot lbs on this ? -My goal is to demonstrate thru physical action how this is accurately measured.

ANSWER: 
I assume that the weight is being lifted at a constant speed. You are not measuring torque in this way. If the weight is lifted 1 foot, then you have done 50 ft-lb of work on it, increased its potential energy by 50 ft-lb. The torque being delivered is 50 lb times the radius of the axel around which the rope is wrapped. It is sort of coincidental that torque and energy are measured in the same units.


QUESTION: 
In a compressed liquid, why isn't the pressure lower when more objects are in the liquid than when the container of liquid is empty? Pressure = force / area, so as more total area(the area of the objects in the container of liquid)is present the pressure must decrease. What am I not considering?

ANSWER: 
Let's take an analogous situation. There are two lakes, each 20 m deep; one has an area of 1 acre and the other 10 acres. Is the pressure at the bottom of the larger lake 10 times larger? Of course not. The total force on the ground is 10 times larger. We usually think of liquids as being incompressible which is a very good approximation as long as the pressure is not huge. Now, if you put an object into a lake the surface of the lake does not significantly rise and so the pressure everywhere stays the same but the total force on everything increases due to the force the liquid exerts on the object (called the buoyant force). However, if the object is not very small compared to the total volume of the fluid, the surface will rise and the pressure will increase because the pressure in a liquid is proportional to the depth in the liquid. Suppose now that you have a gas in a closed container. If you now insert an object into the gas the pressure will increase but this is because the volume of the gas has been decreased and the pressure is inversely proportional to the volume, not because of more area.


QUESTION: 
I am a 6th grade math/(Earth)science teacher in California. We were discussing how lowering pressure can result in rock turning back into magma under certain conditions inside the Earth. To simplify things, I explained that lower pressure allows the atoms to move further apart to the point where it turns into the liquid state. Conversely, a lot of pressure can make things solid, when they might otherwise be liquid because the atoms are "squeezed" together to make it solid. Here is the next question I got from a student. He asked if water could be made solid (apart from freezing) by simply applying enough pressure. My degree is in engineering, so I know enough to be "dangerous", but I would like to get the right answer. I'm guessing the bi-polar nature of the water molecule may play a part and the fact that water in the solid state is a less dense crystal- but what is the physics in this situation? Is it theoretically possible to have "solid water" without freezing?

ANSWER: 
To the right is a phase diagram for water. You will note that no matter what the temperature and pressure are, if you keep the temperature of liquid water constant and increase the pressure, the liquid will never solidify. In fact, the reverse will happen: if you have ice just below the freezing point and increase the pressure sufficiently, it will melt.

ADDED ANSWER: 
It has been pointed out to me that my answer is too restrictive. If one considers much higher pressures there are several other phases of ice. Some of these indeed form at very high pressures and temperatures above 00 C.


QUESTION: 
Since light itself cannot leave a black hole because of the intense gravitational field, does light have mass?

ANSWER: 
Light does not have mass. Something does not have to have mass to be affected by gravity and it has been shown that light is bent in the viciinty of a massive object like the sun. This is the same reason why a black hole can "hold on" to light just like something with mass. See my earlier discussions of gravity bending lightgeneral relativity (the theory of gravity) and photon mass.


QUESTION: 
Why in an series circuit potential difference divides? Please dont give answer related to equation given by ohm. I wanna ask that why current remains same in series circuit. See Current = Charge/time. Charge in circuit will remain same and resistance will try to resist current therefore it will take more time to pass through resistance and hence current is changed because time has increased. according to me this should happen but it doesnt happen . why??

ANSWER: 
If the electrons slow down then they "bunch up", get closer together, and they do so such that the number of electrons per unit time crossing a point in the circuit stays the same. If the current in a resistor were less than the current in the wire going into it, charge would accumulate at the point where they were connected.

FOLLOWUP QUESTION: 
I got u but i still have one doubt. See this i ve understood why current remains same but this i ve not got how it remains same. I mean like we know that if it would not happen then wat would be outcome but this cannot be thought by wire or charge or cell. There must be some reason for how this potential divides. I think u have got my ques.???

ANSWER: 
I think that "if the current in a resistor were less than the current in the wire going into it, charge would accumulate at the point where they were connected" (Kirchoff's current rule) says it all, but try this. Current through a wire is like water flow through a pipe. Suppose that a pipe carries 5 gallons per minute. Now suppose the pipe had a section where it got three times larger; in this larger region the water would slow down considerably, but there would still be 5 gallons per minute flowing through the pipe no matter where you looked at it.


QUESTION: 
say you have a tube submerged vertically in a lake. this tube is like a well, with say, 5 gallons of water at the bottom. the top is open and is above the surface of the lake. the bottom of the tube is closed, not letting water in. I want to pump the 5 gallons of water out of the bottom of the tube. i can either pump it to the top of the tube and back into the lake from the top, or i can have a valve system at the bottom of the tube that pumps it through the wall at the bottom of the tube. i am sure it requires the same energy in both cases in an ideal situation. but what if i want to pump out the same 5 gallons of water at 40 feet or 60 feet depth. i would think the required energy would increase in direct proportion with depth, as the pressure increases in direct proportion to depth. what is the equation for the work required for pumping out the water and the most straight forward unit -maybe watts?

ANSWER: 
The work needed to lift an object with weight W to a height h is Wh. Five gallons of water weighs 40 lb, so the work required to lift it to 40 or 60 feet is 1600 and 2400 ft-lb, respectively. You asked for the answer in watts, but a watt is not a unit of energy, it is a unit of power; power is the rate of using energy. (Notice some time that the units used for energy consumed on your electric bill is the kilowatt-hour, since power times time equals energy.) A ft-lb is about 1.36 joules and a joule is a watt-s, so 1600 ft-lb=67.4 w-s and 2400 ft-lb=101 w-s. This assumes the pump is perfectly efficient which will not be the case, of course. 


QUESTION: 
I had a ring magnet made out of a typical magentic alloy. I dropped the thing and broke it. I naively expected the pieces of the ring, when reassembled in the correct order, to stick together. Instead, they all repel one another. What's going on here?

ANSWER: 
Usually ring magnets are magnetized such that their field is similar to a bar magnet on the axis of the ring. That is, one flat face is a north pole and the opposite face is the south pole. If it breaks and you try to put it back together, you are trying to put north to north, south to south.


QUESTION: 
Why can diffuse light not be concentrated to improve the performance of a solar collector on cloudy days? What about the way insect eyes work?

ANSWER: 
Because lenses or mirrors which do this depend on light rays coming in parallel which is not the case for diffuse light. Insect eyes are only a bunch of narrow tubes and I would expect, although some light might get to the collector that might not otherwise, other light that would have gotten there would not; hence I would think an "insect eye" would actually decrease intensity.


QUESTION: 
Waves from ocean are matter colliding...well what exactly are sound waves and electromagnetic waves made of? If its made out of particles then why does it behave like a wave?

ANSWER: 
Sound waves are oscillating air molecules. Electromagnetic waves are time varying electric and magnetic fields; see and earlier answer.


QUESTION: 
Does adding mass to a toy car affect the force of gravity acting on the car, the acceleration of the car, or both as it rolls down a ramp? I want to do a lab in my middle school classroom where students investigate Newton's 2nd Law of motion by rolling a toy car down a ramp and measure the distance it rolls. They will use washers to increase the mass of the toy car. How would increasing the mass affect the distance the object rolls down the ramp? I found this lab on the Internet (without an answer key) and need to be sure that I'm right about what the outcome should be.

ANSWER: 
Let's first talk about what the results should be according to elementary physics. The force of gravity (called the weight) is proportional to the mass. If you increase the mass by a factor of 2, the force impelling it down the plane increases by a factor of 2. But, what does a force do? It causes an acceleration in the direction of the force. But the acceleration (from Newton's second law) is inversely proportional to the mass of an object; this is called inertia, mass is the property of an object which measures how resistant it is to an acceleration if you exert a force on it. So, if you increase the mass by a certain factor you increase the force by that factor and the net result is that the acceleration is unchanged! This should be the lesson learned by doing this experiment. By the way, this is the same idea as the old story of Galileo dropping a marble and a cannonball from the leaning tower and finding them to hit the ground at the same time. However, in the real world, there are other forces besides gravity, mainly frictional forces, acting on the car and these can confuse the issue when trying to do an experiment like you describe. As you will see when you look over earlier answers to just this question, sometimes the heavier car wins and sometimes the lighter one wins; many details can make this experiment a mess! If I were you, I would carefully do the experiment beforehand to make sure that it will teach what you want it to!


QUESTION: 
consider a space rocket in motion has ke=1/2 mv2. when the rocket accelerates to 2x its velocity, we would get 4x ke. would it really take 4x the amount of fuel to increase the velocity 2x? where is conservation in this example?

ANSWER: 
First, talking about fuel is a little tricky because fuel has mass and you have a more complicated rocket if its mass is changing. So let's talk about a rocket whose total mass is much greater than the mass of the fuel or else imagine an external agent pushing. Now, what is conserved in an isolated system? The two important things are total energy and total linear momentum. There is no such thing as conservation of velocity. To change linear momentum which is mv, the mass times the velocity, you must exert a force F over a time t and the change in momentum is Ft. Ft is called the impulse. If you double the impulse, you double the change in velocity if mass does not change. So push twice as hard or twice as long and change the velocity twice as much. Now, if you want to change the kinetic energy of the rocket, you must also push on it. But now the conservation principle says that the change in the kinetic energy is proportional to the work done one it. Work is Fx where F is the force which acts for some distance x. So, if you double the amount of work you double the change in kinetic energy, so you double the change in v2 if mass is unchanged. If you start from rest and do an amount of work W and end up with speed v, then if you do 2W of work you will end up with only (
√2)v velocity. Burning twice the fuel will give twice the work (energy) so the kinetic energy increases by a factor of two and the velocity increases by a factor of √2.


QUESTION: 
I understand potential and kinetic energy for a roller coaster but I don't know about it in a basketball free throw or a diver diving from a platform or a diving board. Is the potential energy greatest when the diver is on the board or when the diver reaches the top of the dive and is about to descend? Where in the process of a free throw is potential energy the greatest?

ANSWER: 
The roller coaster and the basketball are analogous because there is kinetic energy and gravitational potential energy only in each case. Since the gravitational potential energy (near the earth's surface) is simply mgy, the greater y is the greater potential energy is; therefore the kinetic energy is smallest (neglecting frictional effects) at the top of the path. The diving board, however, is a different situation because the diving board is like a spring, that is when it is bent just before the dive it too has potential energy; so, for example, as the diver starts up from the position of maximum flex, the gravitational potential energy will be increaseing and the spring potential energy will be decreasing. But, if you start your problem at the exact instant the diver leaves the board, it is just the same as the basketball problem and the diver has maximum potential energy at the top of his trajectory.


QUESTION: 
Is a juggler, while juggling three weights or any number really, lighter at all times than she would be if she merely carried the weight about her person? If so then by how much, when and why? If not then what does happen to their weight while they juggle at the various times they are and are not in contact with the juggled objects?

ANSWER: 
First, I am a stickler for the use of the word weight. The juggler's weight is the force by which the earth attracts him and so it is always the same unless he overeats or goes on a diet. But, the apparent weight (what would be read by a scale he is standing on) depends what is going on with the balls. If all the balls are in the air at some time, his apparent weight will be his actual weight. If he is simply holding one ball, the scale will read his weight plus the ball's weight. If he is in the process of juggling one of the balls, he is exerting an upward force which will be larger than the weight of the ball (Newton's second law); but, because of Newton's third law, we can conclude that the ball exerts an equal and opposite force on him; and so the force read by the scale will be larger than the weight of the ball plus juggler. An interesting and related problem is that of what the apparent weight of an hourglass is, discussed in an earlier answer.


QUESTION: 
is it possible for a ball to rebound to a height greater than the height at which it is dropped?

ANSWER: 
Not unless energy is added to it (for example, have a little bit of plastic explosive stuck to it) or it is thrown down instead of being dropped. This is a classic conservation of energy problem.


QUESTION: 
Let's say that there are two different light sources that are both exactly 1 million light years in distance from me. One has a direct path to me, but the other one passes near a very massive object, which causes the light to "bend". Will the two reach me at the same time, or has the one that appears to bend traveled farther and therefore arrives later?

ANSWER: 
Whichever light ray travels the farthest will arrive latest. The speed of light is constant.


QUESTION: 
If I understand it, the laws of physics state that nothing may exceed the speed of light, is this correct? I have never fully understood how this is resolved from the expression E=MC2 is there something that I might be able to read that would explain it in very simple terms?

ANSWER: 
I am not sure what is bothering you. If it is that c2 is bigger than c but nothing can be bigger than c, read this earlier answer. If it is that you have trouble accepting that all observers see the same speed for light, then read this earlier answer.


QUESTION: 
If I drop 2 items of different weights off a 20 story building will they hit the ground at the same time? If this were in a vacuum what would be the effect?

ANSWER: 
If air resistance is negligible, they will hit simultaneously. However, from that height air resistance will likely not be negligible. However, you cannot predict which will hit first without more information, in particular their masses and shapes. Two examples: a feather (small mass) will hit after a bowling ball (large mass) but a marble (small mass) will hit before a parachutist (large mass). You may be interested in a lengthy discussion of terminal velocity and free fall in an earlier answer.


QUESTION: 
Is there any nuclear reaction (fission/fusion) that turns Iron into gold ?

ANSWER: 
No.


QUESTION: 
Energy is the capacity for doing work. Atom can be converted to energy. If I convert a single atom to energy in deep space where there is nothing else in the proximity for the energy to do work on, what happens to the energy?

ANSWER: 
It is really not possible to convert an isolated atom completely to energy. But suppose that we could by having it encounter an "antiatom" which would be an atom made with the antiparticles of the original atom. Then the energy would appear as photons. Energy in an isolated system is always conserved and does not have to appear as mechanical work.


QUESTION: 
Suppose all the particles in Rutherford's experiment went straight through the gold foil with absolutely no deflections What would this imply about the structure of the atom?

ANSWER: 
It would mean that the alpha particles had no interactions which would mean that there was no charge density in the foil. Atoms are electrically neutral but there is a charge density, viz. electrons and protons are spacially separated. If there were no charge density, the material would be perfectly homogeneous.


QUESTION: 
I was wondering about speed of light and the effect of “red shift” and “blue shift”. As I understand it red-shift and blue-shift is the same thing as the Doppler Effect on sound waves. My question is this: If light is a “constant” how can there be a “shift” of any kind? This is probably a silly question…..but it puzzles me.

ANSWER: 
The thing which is constant about light is its speed. But, if the observer or the source is moving, there will be a Doppler shift of the frequency and wavelength.


QUESTION: 
Is Teleportation of a living being possible without contradicting Quantum Mechanics in theory ? I know Quantum Mechanics won't differ between living and non-living matter, but, to my knowledge, Teleportation is the act of getting data of an object, destroying it and then reproducing it. We won't observe the new object as a new one but we'll think of it, as the title of the experiment "Teleportation" suggests, to be the same.

ANSWER: 
The answer to this question is far too involved for this site. I recommend a fun book "The Physics of Star Trek" by Lawrence Krauss published by HarperPerennial which devotes a whole chapter to "beam me up, Scotty".


QUESTION: 
I am trying to find out what the fastest wavelength is that we can mesure with a meter of some sort? I was looking on-line and from what I could find it seems that a photon seems to have the sortest waveleangh? one other question with a waveleanth of .000000000000000000000000002 cm What would this be in frequency do you have any ref for this?

ANSWER: 
Your question has no answer because there is no limit to how short the wavelength of electromagnetic radiation can be. Furthermore, "fastest wavelength" has no meaning because fastest implies a velocity. All light travels with speed c=3x108 m/s. Visible light has wavelengths on the order of 6000 nm=6x10-6 m. Gamma rays are what very short wavelength photons are called and one might make a 1 GeV photon at a modern accelerator. For reference, visible light is about 0.2 eV. (eV is an electron-volt, a unit of energy). The frequency of a wave is determined by the universal equation c=f
λ where c is the speed, f the frequency, and λ the wavelength. The wavelength of a 1 GeV gamma ray is about 3x10-16 m; the corresponding frequency is about 1024 Hz.


QUESTION: 
If a 50lb cannonball is travelling at 1000ft/second then how much kinetic energy does it produce when it hits a target?

ANSWER: 
You do not want to say "how much kinetic energy does it produce" because it has kinetic energy which it then loses when it hits the target. Kinetic energy is something an object has by virtue of its motion. When the cannonball hits the target and stops, this energy is converted into something else, for example heat, the target being torn apart, etc. So the question should be how much kinetic energy does it have just before hitting the target. Scientists do not like to work in English units, so I will convert 50 lb=22.7 kg=m and 1000 ft/s=305 m/s=v. Kinetic energy is
½mv2=1,056,000 Joules. This is about one megawatt-second, or 0.28 kilowatt-hour, or, if you prefer pure English units, about 738,000 foot-lb=0.37 horsepower-hour.


QUESTION: 
have structure simular to a shelf bracket, a vertical bar 24" fastened at top and bottom to wall. and at top a horizonal bar 36" extending outward. If a 250 LB weight was applied to the outter end of 36" bar, what would be the amount of force trying to pull out the fastner from the wall at the top of 24" bar? also provide formula.

ANSWER: 
You need to sum torques around the bottom of the 24" bar and set to zero. The torque due to the 250 lb weight will be 3x250=750 ft lb. The torque due to the fastener in the wall will be -2xF where F is the force the fastener exerts (horizontally) on the wall. So, 750-2F=0, and F=375 lb. This assumes the bracket itself has negligible weight.


QUESTION: 
If you were floating in a totally bare, totally empty space station that has no windows and may rotate to simulate gravity, would you notice the difference when the space station begins to rotate? Without any sort of benchmark, it would seem to me that you would simply seem to float idle, with the space station spins around you.

ANSWER: 
You are correct. However, if there is air inside it will eventually move with the station and eventually drag you with it.


QUESTION: 
Uncertainty principle: The principle it is based on the impossibility of measure the position and the momentum of an electron at the same time by using light as method of measurement. Why a technical limitation is taken as a fundamental principle? What if we can develop , in the future, a way of measure momentum and position without interfering on the state of the moving electron? I am stock on the first chapter of my quantum physics book because of this.

ANSWER: 
Your statement is not an accurate statement of the uncertainty principle. It is not specific to an electron, not specific to how one attempts to measure momentum and position. The uncertainty principle is that you may not know (via a measurement) simultaneously both the momentum and position of an object with arbitrary accuracy. It is a statement about the nature of position and momentum, not about how you measure them.


QUESTION: 
Why does a balloon first clings to a wall after rubbing then eventually fall down>

ANSWER: 
When you rub it, it becomes electrically charged. This charge then induces the opposite charge on the surface of the wall to which the balloon subsequently sticks because of the attractive electrical force. However, the charge on the baloon slowly leaks away and the balloon falls.


QUESTION: 
I have a question about light and its use for the purposes of creating environmentally friendly energy. Most of the approaches to create electricity I’ve seen involve using lights ability to heat things in order to produce the electricity. This is seen in solar panels and power plants using arrays of mirrors. Would we not produce more electricity if we focused on harnessing the kinetic energy of light (“light pressure”) instead of its heating properties?

ANSWER: 
Usually a solar panel refers to arrays of devices which convert light directly to electricity. You may be thinking of the flat boxes you see sometimes on a roof which heat water. The arrays of mirrors heat a a fluid (molten salt is usually used) which is then used to heat water to drive turbines to generate electricity. The advantage of using heat is that it is easier and cheaper to store than electricity is. Photovoltaic cells are, at least for now, more practical for individual homes rather than large power plants.


QUESTION: 
Say I hit something, I assume that there is a measurement that can be taken. eg Force That force can be converted to a weight? Yes or NO
Scenario: I have 2 items: Item 1 = a marble, weight = 1kg Item 2 = a bowling bowl, weight = 10 kg If I drop both items at the same time from a Distance of 10m
a) which item will hit the ground first??
b) what force (weight) will they hit the ground with when they impact the ground. Will it be 1kg and 10kg respectfully or will the items weigh more than there original stationary weight??
Obviously if you drop a 10kg Bowling ball from 1 m onto example "a car", then you will only ding the car slightly. But if you drop the ball off a 10m building that ball will make a larger impact Y or N. how much does it weigh on impact

ANSWER: 
You have a number of misconceptions and misunderstandings. I will attempt to explain. An object's weight is the force which the earth exerts on it and is determined by its mass (in kilograms) and the acceleration due to gravity (9.8 m/s2 in the vicinity of the earth's surface which is, I presume, where you are interested in it). The weight of an object is simply the mass times the acceleration of gravity, so the weight of a 10 kg object, for example, is 98 Newtons (kg m/s2). A Newton is about 0.225 pounds. The weight of an object is always the same. Now, if a mass is moving it can be stopped by a force. However, there is no unique force required because it all depends on how long the force acts. Consider one of your objects hitting the ground and stopping. If the ground is very hard, then the force will act over a short time and the force will be very large. However, if the ground is soft, the force will act over a longer time and the magnitude of the force will be smaller. The reason for all this is that the force required is equal to the mass times the acceleration (Newton's second law) and a longer time implies a smaller acceleration. The secret to not getting hurt by an impact is to prolong the duration of the impact. That is how airbags in cars work, it is why a bungee cord works, it is why you cushion the landing area for the high jump, etc. Thus, if you drop a marble and a bowling ball on your head and they take the about the same time to come to rest, your head will have to exert an upward force which is larger on the bowling ball because it has a larger mass but the same acceleration as the marble; because of Newton's third law, if you exert a force up on the object, the object exerts an equal and opposite force on your head. Ouch!


QUESTION: 
I saw the following equation on the back of a 2008 graduating MIT student. What does it say? [8,3] = ih where: both the 8 and the 3 have a ^ above them. i is in italics, so I assume imaginary unit h is written as h-bar, Plank's constant. My son pointed this out to me a year ago and we still haven't found an explanation. I'm pretty certain the student was being clever and we would love to learn what it means.

ANSWER: 
This is a commutation relation, something which plays an important role in quantum mechanics. The two objects in the brackets are operators (that is what the ^ signifies), not normal numbers, because what a commutator means is [A,B]=AB-BA and if A and B are numbers, [A,B]=0. The best known commutator is that of momentum with position, [x,p]=i
ħ. When two operators do not commute, the corresponding variables are said to be conjugate variables which can not be simultaneously known with perfect precision, also known as the Heisenberg uncertainty principle. However, I cannot figure out what the clever reference to 8 and 3 might be.


QUESTION: 
When light is incident on a medium with the angle of incidence being 90 degrees, does the speed of light change?

ANSWER: 
I presume you mean the light comes in normal to the surface. Yes, the speed of light in the other medium does not depend on how the light got there.


QUESTION: 
Which of all the discovered particles can be used in the quantum double slit experiment? I've heard of it done with photons and electrons, but no others. Will it work with the others?

ANSWER: 
Any particle will exhibit diffraction effects. The reason you see explicit examples for photons and electrons is that it is possible to make a double slit of appropriate dimensions for these. The slit spacing must be comparable to the wavelength of the particle. If you observe the diffraction from things other than a simple double slit, then we see diffraction of protons, neutrons, alpha particles, etc., etc., etc. when we observe scattering from nuclei which are sufficiently small to be comparable to the particle wavelengths.


QUESTION: 
I am curious to know whether the wind chill factor affects only people (and animals & other living tissue) or inanimate materials as well. For instance, I know that in the Artic and Anartic regions, certain tools need to be made of special steels or alloys because normal steel hammers, etc. will shatter like glass upon impact. Back in the U.S., however, if the ambient temperature is +5ºF, but the wind chill is -25ºF, are other materials (those with non-living cells) affected?

ANSWER: 
My understanding is that wind chill is meaningful only for exposed human skin. See the Wikepedia article on Wind Chill.


QUESTION: 
Is there a significant difference in the collision between a 1000 lb car travelling 50 mph hitting a stationary 10,000 lb truck, and a 10,000 lb truck travelling 50 mph hitting a stationary 1000 lb car? I think both collisions are equivalent (I assumed an inelastic collision, and therefore there was the same loss of kinetic energy in both collisions), but all my friends think I'm crazy (or stupid, or both).

ANSWER: 
For purposes of comparison, let's say that the collision is perfectly inelastic, that is the two stick together. And also, that, for each case, the time duration of the collision is the same. Now, what do you mean by equivalent? If that means that both collisions look the same, that is certainly not the case since if the car hits the truck the two will end up going about 4.5 mi/hr after the collision but if the truck hits the car, the two will end up going 45.5 mi/hr. (I figured these out using simple momentum conservation.) But, if you are sitting in the car, in both cases your velocity changed by 45.5 mi/hr over the time of the collision and so your acceleration, and thus the force which you felt, was identical in both cases.


QUESTION: 
If you have a tube with no air in it, making the the tube a vacuum. And stuck one end in space and the other end in the earth's ocean. Would it suck all the water out into space? If not what would happen?

ANSWER: 
The vacuum does not "suck" the water; rather the atomospheric pressure pushes it. If you had a very long evacuated tube and stuck it in the ocean, the water would rise only to about 34 feet. That is the highest that atmospheric pressure (about 15 lb/square inch) can lift water.


QUESTION: 
I read in my science textbook that some gas particles in the air vibrate and move at the speed of 500 meters per second. My question is "Will the gas particles moving in the air affect the flight of a bullet traveling at the same speed? Will it affect the flight the most if the bullet is traveling at the speed of the motion of the gas particles, slower than the speed of the particles, or if the bullet is traveling faster than the speed of the particles?

ANSWER: 
No single atom will have any noticeable effect on the bullet because the mass of an atom is on the order of 10-25 kg whereas the mass of a bullet is on the order of 10-2 kg. The net effect of many collisions is the origin of air resistance of an object moving through the gas.


QUESTION: 
If you had a plane travelling at the speed of sound and fired a bullet from a gun capable of firing a bullet at the speed of sound backwards out of it, would the bullet drop vertically downwards because its speed is now zero?

ANSWER: 
Yes (neglecting air resistance).


QUESTION: 
Modern mammography units have multiple x-ray target and filter combinations. Mo target with Mo filter and W target with Mo filter are two examples. I know the construction of an x--ray tube target (anode). How does a mammography unit have multiple targets (anodes)?

ANSWER: 
This would be just a simple engineering problem where one target could be moved away and another moved into position inside the tube. I did not know much about mammography but found an excellent overview at http://www.sprawls.org/resources/MAMMO/module.htm#1


QUESTION: 
My question is very simple, but think as I might I cannot figure it out! Why is the constant c in the famous equation E=mc2, squared? If c is the speed of light which cannot be surpassed, then why is it squared? Why not cubed or otherwise exponated?(sp. word?) Why is it not to the first power as the entity itself? I have always wondered this, maybe you could help me out.

ANSWER: 
The question boils down to what we call dimensional analysis in science. Every meaningful quantity in physics has dimensions which are either mass (M), length (L) or time (T), or some combination of the three. For example, a velocity, which is what c is, must have dimensions of L/T (like miles/hour). Now, energy had dimensions of ML2/T2, and mass, of course, has dimensions of M. The only way you can put M and L/T together to get ML2/T2, is M(L/T)2 and hence mc2 and not mc or mc3 or any other combination. Another example is kinetic energy of a particle of mass m moving with speed v,
½mv2. There are ways to make energy from quantities other than mass and velocity. For example energy may be written as a force times a distance (this kind of energy is often called work) and force has dimensions ML/T2, and distance is, of course, L, so together they are ML2/T2. It's the old you can't compare apples and oranges thing.

Incidentally, you can square the speed of light. The square is not a velocity and does not violate the fact that nothing can go faster. The speed of light is the largest possible velocity, not the largest possible number.


QUESTION: 
If it takes 8 minutes for a photon from the Sun to reach the Earth, what would happen if the Sun were to instantly vanish? Would the Earth continue on it's orbit for 8 minutes, or instantly travel off it's orbit like a rotating object at the end of a string whose string had been cut?

ANSWER: 
You are assuming that if the gravitational force is not propogated instantaneously, it will propogate at the speed of light. So the question is really what is the propogation speed? See my earlier answer to a similar question.


QUESTION: 
If a bucket tied to a rope is revolving vertically in an counter clockwise direction, what would be the Free Body Diagram of "water" in the bucket at the top position? I am guessing there would be two forces: gravitational pointing downward and contact force with the bucket pointing in the direction opposite to the motion of the bucket so that would be left if the bucket is going counter clockwise vertically. Are there any more forces on water inside the bucket at the top position and am I right about the direction of the two forces.

ANSWER: 
You have not given me enough information. But let's take the simplest situation: the bucket is moving with constant speed when at the top (it could also be either speeding up or slowing down). In this case, the water has its own weight pointing straight down and the bottom of the bucket exerts a force straight down on the water. The sum of these two forces would be equal to mv2/R where m is the mass of the water, v is the speed, and R is the length of the rope. If it were speeding up, there would also be a force of the bucket on the water in the direction of the velocity. If it were slowing down, there would also be a force of the bucket on the water opposite the direction of the velocity


QUESTION: 
Most publications show the solar system with the planets orbiting around the sun in a disc-like plane. Why do the planets not orbit individually in many different circular trajectories? Like the atom with its many orbiting thingies. Is the universe laid out on a flat kind of area like a map or is it 3 dimensional?

ANSWER: 
It is believed that the reason is that the solar system formed originally as a disk of dust and rocks. Planets formed by rocks colliding and coalescing. But they all stayed more or less in the same plane. Pluto is the most out of plane plus it has a very eccentric (not circular) orbit. Most (not all) galaxies are also mainly in a plane. But, the universe in general is not. I should tell you that the picture of electrons (thingies) orbiting is very simplistic and, essentially, not correct. See an earlier answer.


QUESTION: 
Does a Cell Phone Charger waste electricity when it is plugged in a wall socket but not attached to any phone? Recently, people are encouraged to un-plug their chargers when not in use so as to save power and be "green". However, there is no flow, it is an open circuit, when not connected to any device. True?

ANSWER: 
It depends on the charger
—some continue using power. You can tell how big an offender yours is by seeing if it gets warm while plugged in. This is also true of numerous other things: tvs, stereos, computers, and so on. Any individual thing will consume what might seem like a trivial amount of power to you, on the order of a watt. But if there are like 200,000,000 such chargers in the U.S. and they are all plugged in, we are talking 200 megawatts. If you have the option on an appliance to turn power off rather than go to "stand by", do it.


QUESTION: 
Does the moon have rotational angular momentum, and if so, wouldn't that mean that the moon actually rotates on an "internal" axis?

ANSWER: 
Like all objects in our solar system, the moon rotates about an axis through its center of mass. The earth rotates on its axis once a day and revolves around the sun once a year. The moon revolves around the earth once every 28 days. Since the moon always presents the same side to the earth, it also rotates around its axis once every 28 days.


QUESTION: 
Understanding that gravity can bend light rays due to the curvature of space resulting from nearby mass, what would happen if instead of light ray traversing near the sun, suppose you pulled a wire taut along the same path...would it still exhibit the same amount of deflection? That is does curvature of space actually deform objects or does it just apply forces that the object can resist if stiff enough (for example, does pi for a physical stiff circular disc actually become different?).

ANSWER: 
It would behave quite differently from the light beam because it has mass. The wire would be pulled toward the sun near the sun so that if you imagned a particle moving along that wire it would appear to be deflected away from the sun. However, that is not the way a particle would move, in other words forget about your fictional wire. If you shoot a mass along the light trajectory it would be deflected in the same direction as the photons but not by the same amount since that would depend on its speed.


QUESTION: 
Would perfectly pure water form snowflakes when frozen, and if not, what if any geometric structure, would be formed ?

ANSWER: 
Although water normally crystalizes around a "seed" like a piece of dust, I do not think this is required. I feel quite sure that if you had a tiny speck of ice, a snowflake could form around it. The formation of snowflakes is a very complicated process and happens only under appropriate conditions of pressure, temperature, vapor density, etc. If you do a survey of many snowflakes you would find not all were the pretty symmetrical things you think of; for example, needles can form.


QUESTION: 
My friend and i were arguing wether Time Travel to the Past is possible. Here is my thought. Accepting that the age of the earth is apprx. 4.6 Billions years old, let just say that an alien civilization is at a distance that is equivalent to the time it would take the light travel in 4.6 Billions years. Would the aliens see the earth as it once was from the begining, and begin watching it's evolution?

ANSWER: 
The aliens would see the earth as it was in the past, but this is not time travel any more than watching a movie made in the 30s is time travel. Time travel is when you are actually physically present at a location in the past. The current known laws of physics allow time travel to the future but not the past.


QUESTION: 
Is it possible that time speeds up and slows down without our knowledge? If it did, there wouldn't be any way to detect it because time measuring devices (i.e. clock) would also slow.

ANSWER: 
If what you were talking about happens throughout the universe, then your conjecture has no meaning from a physics standpoint because you cannot measure it. If you are suggesting that maybe it just happens here, then we could detect by looking at clocks elsewhere.

That answer was predicated on the assumption that we measure time alone. But suppose that we measure it indirectly by applying Newton's second law. If we drop a ball, its acceleration is the force of its weight divided by its mass. But, if we measure this acceleration with clocks today and then again tomorrow when your proposed rate of time passage has happened, we will get different answers. So, from that perspective, lots of different things would have to also change for it to be unnoticed.

From the theory of special relativity, we know that time, whatever it is, is not a separate thing but is "tangled up" with the three dimensions of space.


QUESTION: 
Can a nanoparticle be shot a big distance, say 5 feet, on its own, or does it need a vacuum (to pull it) to get that far to it's destination?

ANSWER: 
Can you throw a particle of dust 5 ft? No. But you could "shoot" it in a jet of air which would carry it along with the air. A nanoparticle is much smaller than a dust mote but similar comments would be true for it. This reminds me of something I read recently about bacteria in water: they can move by swimming but the instant they stop swimming they stop moving because of the fluid friction. Same thing for tiny particles in air.


QUESTION: 
A friend of mine thinks that theory of relativity disproves evolution, because 1 second last same as 50 000 years (for example). He never studied physics or biology to any advance level. I therefore think he just misunderstood both theories. However I would like to hear physicist view of evolution.

ANSWER: 
This is about as ridiculous an idea as I have ever heard. Your friend has no understanding of relativity. To say that a second is the same as 50,000 years is simply not true. It is like saying that someone is both 30 years old and 1 year old. Time is different only in different reference frames but in one frame (like the earth where life is evolving), 50,000 years is 50,000 years.


QUESTION: 
why electric filed lines do not intersect?

ANSWER: 
Electric fields represent the force which would be felt by a unit charge. If, at a particular point the electric field lines crossed that would mean that the force on a charge would not have a single force on it, so which one would it be? There might be two charges each of which had its own electric field at some point, but the electric field at that point would be the sum (vector sum) of the two; this is the superposition principle.


QUESTION: 
atom share electron not proton in static electricity or electric charge.if I charge comb, with my hair why is sharing of electron not proton in b/w comb and my hair.please in detail.

ANSWER: 
Electrons are much easier to move around because the outermost electrons are bound by small energies. Hence it is easy to transfer them from one thing to another. Protons, however, are much more strongly bound. The force holding the proton in the nucleus is much larger than the force holding an electron in an atom.


QUESTION: 
In their experiments that revealed the wave-like nature of the electron, Davisson and Germer used an electron with a velocity of 4.35 * 10^6 m/s. Using the formula Lambda = Planck Length / momentum it can be shown that an electron of this velocity has a wavelength of 0.167 nm. Does this electron also have a frequency and how can I determine it?

ANSWER: 
The frequency of a particle is its energy divided by Planck's constant. Keep in mind that energy is total energy (that is, including rest mass energy).


QUESTION: 
I've seen the Heisenberg Uncertainty Principle stated in various generalities. For example:
1) (most specific) You cannot measure the velocity of an electron without affecting its location (or vice versa).
2) (more general) You cannot measure the velocity of "anything" without affecting its location (or vice versa).
3) (most general) You cannot measure any characteristic about anything, without affecting that characteristic.
Which is really true? I would think that 3) is too broad, and not true. I can measure the mass of an object by counting the number of molecules it contains, or measure an object's length by observing the light bouncing off it in a direction perpendicular to its length.

ANSWER: 
#3 is certainly false. A simple example would be electric charge which can be measured with accuracy without affecting the charge. #1 is certainly too restricted since the HUP is not confined electrons; so, true but not general. I guess that leaves #2. To my mind, however, it is better to state it as follows. The more accurately you know linear momentum (which is the mass times the velocity), the less accurately you can know position and vice versa. So, if you know the position of something very well, you are mostly ignorant of its velocity; but if you now measure that velocity so that you know it very well, the position after the measurement will be almost totally unknown. To make the HUP more general, we note that linear momentum and position are conjugate variables. Conjugate variables are two variables which, when one is measured the other is affected. There are many pairs of conjugate variables in physics, the other best known pair is energy and time. If a certain state in a system decays to the groundstate of the system in a certain time, then the energy of that state cannot be measured perfectly accurately, that is the state has an inherent width or uncertainty. The ground state of a system may be measured exactly since it lives for an infinite amount of time. Another example is in classical physics, frequency and time. To measure the frequency very accurately you must make your measurement over a long time. For example, if you listen to only 1/10 of a cycle of a sound wave, you will hear not the tone associated with its frequency but just a click, a mixture of many frequencies.

FOLLOWUP QUESTION:
I'm thinking of an inflated balloon sitting stationary on a table. I am certain that I can determine its momentum (zero) and yet still know its location. Also, if I open the balloon's knot, it will dance around randomly, making it more difficult to determine either its momentum or its location. I assume this is a misapplication of the principle (too general)?

ANSWER: 
The fact is that you cannot know
the momentum to be exactly zero. In fact, you could probably not even measure it to be zero to one part in a thousand. The problem is that your intuition is getting in the way of your understanding the HUP because you are trying to apply it to a macroscopic object and it is completely unnoticeable in that regime. To understand this, we have to get a little more quantitative. The HUP states that the product of the uncertainties in the momentum and position must be on the order of Planck's constant or greater. Now, Planck's constant is on the order of 10-34 kg m2/s2. So if your uncertainty in measuring momentum to be zero is 1/1,000,000 kg m/s, then you can measure its position to an accuracy of 10-28 m! If you work with elementary particles, however, the masses are so small (the electron mass is like 10-30 kg) that momenta of even rapidly moving particles are also small so the HUP becomes observable. For example, suppose you have an electron going 100 m/s and so with momentum of about 10-28 kg m/s and you measure this to one part in a thousand, 10-3x10-28=10-31 kg m/s. Then you may not know its position more accurately than 10-3 m, a millimeter, which is a gigantic distance in the subatomic world (an atom is like 10-10 m, a nucleus like 10-14 m).


QUESTION: 
I'd like to keep the fizz in my 1 liter bottle of club soda after opening. Which method will help keep more carbonation in the bottle for my next cocktail: 1.) open bottle, pour about one quarter into glass, immediately recap tightly, or 2.) open, pour same amount and then squeeze bottle until liquid is near bottleneck and recap tightly? The first involves a volume of air at normal pressure that replaced the poured liquid, the second removes the air and re-inflates the bottle with CO2. Again, which of the two keeps more carbonation in the bottle?

ANSWER: 
The second method ends up with a large amount of CO2 out of the liquid, not what you want. The first is much better since, although some CO2 will come out, not nearly as much as the second method because to do so would cause a large increase of pressure (you can tell some comes out by the slightly increased pressure evident in the PSST when you reopen it). Another tip: my wife told me the fizz keeps in better at room temperature rather than putting the opened bottle in the refrigerator. The reason for this, I figure, is that the cold liquid can disolve less CO2.


QUESTION: 
My understanding is that the central assumption of Special Relativity is that the speed of light is constant for all observers - and the same constant. Assume there are three observers in separate frames of reference, and the frames of reference are in motion relative to each other. Imagine two of the frames of reference (say "A" and "B") are railway carriages (not linked) on the same, straight track on the earth, and the third ("C") is at a point directly "above" the railway track, and stationary with respect to the earth. Assume the observer in C looks down on the track, and the track extends left to right (C's reference), with A on the left and B on the right. Assume A is travelling to the right (i.e. towards B) at 50,000mps, and B is travelling also to the right at 100,000mps (and that C is high enough for the observer to keep them both in view for the duration of the experiment). The observer in A shines a torch towards B. Do all observers (in A, B & C) measure the speed of the light emanating from the torch in A at (approx) 300,000mps? I assume that is what is meant by "the speed of light is constant for all observers". If that is so, why? And if not, what is the root of my misunderstanding?

FOLLOWUP: 
I think it can be explained by objects becoming shorter at greater speeds.

ANSWER: 
There are two ways to convince yourself that the speed of light is the same for all observers. The first, my favorite, is to show that this follows from what I consider the fundamental assumption of relativity, that the laws of physics are the same for all observers (see an earlier answer). The second is to note that, since a medium (called the luminiferous
æther in the old days) with respect to which light travels has never been found, that its speed must be a universal constant. Your explanation is logically incorrect because the reason moving sticks are shorter is that the speed of light is constant, so it is circular reasoning.


QUESTION: 
I am doing a presentation to a Toastmasters club on possible ways the universe could have come to be. I was given an interesting question "If we have a jar packed with free electrons and protons, what are the odds of even one electron and proton joining to form a single hydrogen atom" I know the velocity, angle and separation would have to be exact, but not good enough at math to calculate. My guess would be infinitesimal. but would like to give a little better and authoritative answer.

ANSWER: 
There is no answer to your question because the recombination rate depends on many things including density, pressure, temperature, etc. The recombination is not possible with a single electron and a single proton because energy and momentum conservation cannot be obeyed. Technically, the only way it can happen is if a third body, say another proton, participates, sort of like a catalyst. At low pressures (less than about 1/10 atmospheric pressure) recombination is nearly impossible because the probability of there being two protons and an electron in such a small volume as the atom is nearly zero. If it is in a jar as you suggest, most of the recombination will occur near the walls of the jar where the wall can play the role of the third body. At higher pressures there will be a significant recombination rate. If the temperature is high enough, then subsequent collisions will reionize the atoms and some equilibrium mixture of plasma and neutral atoms will develop.


QUESTION: 
Why are gas particles in constant motion?

ANSWER: 
This is the hardest kind of question to answer because it is based on an unfounded assumption, in this case that somehow it would be "natural" for them to not be moving. Actually, this notion is one of the tenets of Aristotelian natural philosophy, that the natural state of objects is to be at rest; like most of Aristotle's notions of the natural world, this is not correct. To understand how they move and with what distribution of velocities would require a whole course in thermodynamics. One of the most important findings is that the temperature of a gas is determined by the average kinetic energy of the particles in the gas, so the slower the particles, on average, are moving the colder the gas. The coldest possible gas (containing no kinetic energy) would be if all the particles were at rest. However, the uncertainty principle in quatum mechanics states that it is impossible to know the velocity of something with perfect precision unless we are completely ignorant of the position. So the idea of having a container of molecules all at rest is not possible.


QUESTION: 
Why isn't wind always blowing at a near constant rate? The earth spins at the same rate every day so it seems that the hot and cold air should be trying to balance each other out at the same rate as well? Why should there be a really windy day followed by a very calm day, shouldn't it always just be mildly breezy?

ANSWER: 
The speed of the earth's surface due to rotation on its axis is about 1000 mi/hr. So, you can't very well easily relate that to wind speed. Meteorology is a very complex science, but the main things which move air around are differences in temperature and differences in pressure. Air tends to move from low pressure to high, so you would expect it to all just even out. However, because we are on a rotating frame of reference, there is a "fictitious force" called the Coriolis force and the result is that the actual direction of the resulting motion of air is around regions of low or high pressure, not directly into or out of them. This is the cyclonic pattern you see on weather maps.


QUESTION: 
could we attach a molecule to a photon?

ANSWER: 
If you mean attach the two so that the photon retains its identity, then no because the photon has to travel with the speed of light and the molecule can't. If absorption of the photon is what you mean, yes, but the photon is now gone.


QUESTION: 
If a photon has no mass, how does it contain energy?

ANSWER: 
What makes you think that having a mass is a prerequisite to having energy? If you accept the conservation of energy, then when an atom decays it loses energy (its mass actually becomes slightly less). Where does the energy go? The photon carries it away.


QUESTION: 
Seattle does not usually get much snow but this year we did. I have noted a consistent pattern for which I have no explanation. On the uphill portion of streets, cars tend to create furrows in the snow down to or near to bare pavement far sooner than on the downhill. (You will even see on the same hill the uphill being bare and the downhill on that side being snowy, with the same occurring on the opposite side of that hill, so it's nothing to do with traffic volume.) I could understand it if the going was tough enough that cars were spinning their wheels somewhat going uphill, but that certainly isn't the case in the latter days of the snow and I would doubt that that occurred in earlier days enough to explain this.

ANSWER: 
I believe that you are wrong in your assumption that wheel spinning is not the explanation. I do not mean very rapid spinning, just very slight slipping due to loss of traction. An experienced winter weather driver will adjust the throttle to right about at the point where slipping will occur. And, drivers more inexperienced in driving in snow (probably many in Seattle) always tend to accelerate too much in snowy conditions.


QUESTION: 
After experiencing an exhilarating (and injury free) car accident, I have been trying to remember enough rotational physics to explain what happened. I was hoping you could confirm if I have the right approach. A car travels in a circle of fixed radius on a wet road. It increases its speed until it hydroplanes and spins out. At what angular rate does it spin? It seems to me it a conservation of angular momentum problem. However, as far as I understand it, a car driving in a circle has both orbital angular momentum relative to the center of the circle and spin angular momentum around its center of mass. (Isn’t this like the moon rotating once as it revolves once around the earth?) When the car spins out, I assume it travels straight (tangentially) with the same linear velocity, and spins (now faster) about its center of mass. If I know the radius of the circle, the car’s mass, linear speed, and moment of inertia around a vertical axis through its center of mass, I believe I can solve this problem by equating the spin + orbital angular momentum before and after the wheels lose contact with the road. Am I on the right track or am I spinning my wheels?

ANSWER: 
You are on the right track. Here is a simplifying fact for you, though: when you lose traction and begin moving in a straight line your orbital angular momentum is unchanged, so you can ignore that. So just calculate the angular velocity of the car about its own vertical axis through its COM just before you lose traction; that will be the same afterwards. You do not need any moment of inertia. All this assumes zero friction after breakaway.


QUESTION: 
are photon smaller than electrons and can the be considered the smallest sub atomic particle?

ANSWER: 
In the world of subatomic particles, size is a fuzzy notion. It is easier to try to define what we mean by size for a particle which has mass or charge; we do experiments to determine the diatribution of the charge or mass and then define the radius to be, for example, the point where the charge has fallen to half its maximum value. An electron, in this context, is the closest thing we have to a point particle. A photon, however, is as big or as small as you want it to be; it all depends on how you choose to observe it. This is the real world as dictated by quantum mechanics.


QUESTION: 
Why is radioactive decay dependent on the number of atoms or radioactive particles present in the sample? Does it have anything to do with the total energy of all the atoms in the sample?

ANSWER: 
For every radioactive nucleus there is a probability that it will decay in a certain elapsed time. If you have a thousand of them, then you will see them decay at a rate which depends on the probability tha any one will decay. If you have a hundred thousand of them, the rate at which they will decay will be 100 times the rate at which you observe the thousand to decay. Given the fact that the likelihood of any given nucleus decaying is the same as for all the others, then the number you actually see decay per unit time is proportional to the number of nuclei present.


QUESTION: 
Einstein said that space and time are connected. Specifically the faster you travel, the slower time passes. My understanding then, is that if you're completely stationary then you're in fact travelling through time at the speed of light. Is this correct? Also, if the Universe is expanding, gravity is pulling our Galaxy towards Andromada, our star is rotating around the galaxy core, or planet is rotating around our star then I would imagine we are travelling at a very rapid rate through the universe. If this is true, then does that mean time on Earth is actually going much slower than in deep space?

ANSWER: 
Your statement that time passes more slowly as you travel faster does not refer to the moving person but to another observer watching the clock go by. The correct statement is that moving clocks run more slowly. Your statement "you're in fact travelling through time at the speed of light" has no meaning to me. Your second question depends on what frame you measure the earth's velocity relative to; it moves fast as seen by some observerrs (e.g. distant quasars) and slowly as seen by others (e.g. the sun). You seem to have a biasc misconception that absolute speed has significance. There is no such thing as being at rest in an absolute sense. What matters in relativity is relative velocities between frames of reference. If you find one frame of reference in which the laws of physics as we know them are true, then exactly those same laws will be true in any other frame of reference which moves with any constant velocity with respect to the first; no one of these frames, called inertial frames of reference, is in any way preferred over the others.


QUESTION: 
My son wanted to know if the technology exists to build a hoverboard like the ones in Jak II video game, and also when hoverboards will be available and invented.

ANSWER: 
I am most familiar with the hoverboard which plays a central role in one of my favorite movie series, Back to the Future. This hoverboard is based on some sort of antigravity technology which does, in fact, not exist. This is probably the type of hoverboard featured in the video game your sone plays also. There is no prospect for its ever being invented since there is no scientific evidence that we can have a repulsive gravitational force. Howver, there are lots of various kinds of hover technologies which do exist. The simplest and the one around for the longest time is the hovercraft which relies on a cushion of air to hover. I rode a hovercraft ferry from Copenhagen to Malm
ö, Sweden in 1968. Also, there are mag-lev trains which levitate the trains by using magnetism. You can make a hoverboard using the hovercraft technology; for example, see http://www.youtube.com/watch?v=W4N-CLPXF8w To see more, do a Google search on making a hoverboard.


QUESTION: 
Wouldn't a longitudinal pressure wave be able to have some sort of influence on a photon?

ANSWER: 
You mean a sound wave? It is just air and air can have an effect on electromagnetic radiation. A photon can scatter from or be absorbed by a molecule in the air. Any normal sound wave would appear essentially stationary to a photon because the photon travels so fast that the wave would be traversed very quickly.


QUESTION: 
Why is the absolute value necessary in determining the probability from a probability amplitude function if squaring it typically undoes negatives?

ANSWER: 
The wave function is a complex function, that is it has real and imaginary parts. So it cannot represent a physically measureable quantity. To make it a real quantity we form the absolute square, the function times its complex conjugate which assures a real, positive function. Also, if this quantity is identified with probability, it would be meaningless if it were negative since negative probability has no meaning.


QUESTION: 
when two carts ahving the same mass and the same speed collide and stick together, they stop. What happened to each cart's momentum? is momentum conserved?

ANSWER: 
Momentum is a vector quantity, so the two cars have equal in magnitude monmeta but they are in opposite directions. Therefore, if the momentum of one is MV, that of the other is -MV. Hence the momentum is zero before and after the collision; it is conserved.


QUESTION: 
What propels a light beam? Does it slow down and disappear?

ANSWER: 
Nothing propels it, it is self-propogating once it is created. It cannot slow down because the speed of light is a universal constant. It cannot disappear because energy is conserved and light carries energy. If it interacts with something, though, the light may be absorbed and converted into another form of energy; for example, something on which the sun shines may absorb some of the light and heat up.


QUESTION: 
Just a curious question I've always had. I know that the surface of the earth is negatively charged and the earth is moving through space. A charge carried around is equivalent to a current. Ampere discovered that two current-carrying wires exert forces on one another. Therefore, since the earth is equivalent to a current (because it is a moving charge), shouldn't the earth exert a force on a current-carrying conductor in its vicinity? In other words, does the earth exert a force (besides gravity) on a current-carrying wire? Thank you for your help.

ANSWER: 
If you are traveling along with the earth, you do not see a current, just a charge. So, only a current external to the earth itself will experience such a force, not one at rest relative to the earth.


QUESTION: 
Why are metals like aluminum transparent to neutrons and why are metal used as coolants in fast reactors?

ANSWER: 
That is not really a single, well-focused question, is it? When you say transparent to neutrons you are saying small neutron absorption cross section. Different elements have different cross sections which depend on the structure of their nuclei. These cross sections also depend on the neutron energies and something which is a good absorber at low energies may be a poor absorber at high energies. The main reason liquid metals are used as coolants is that the systems do not have to be pressurized. You should read the article on liquid metal cooled reactors in Wikepedia.


QUESTION: 
What do you think about the current work being done by the CERN? I think the Large Hadron Collider is a waste of money, when you consider that billions of people on the earth are living under the threaten of starving. Will the result of this experiment benifit our daily life?

ANSWER: 
This is not an easy question. My own perspective is that what makes us human is our quest for knowledge and understanding. And, if you make a little study of the amount of money spent on science compared with the money needed to feed the world's hungry, the fraction is very small. A society devoted only to putting food on our tables, roofs over our heads, and clothes on our backs is a society, in my opinion, not worth living in. When the space programs were begun in the 50s and 60s many people said that it had no relevance to everyday life, but it has spawned the whole current high tech society we live in today providing employment for millions; because of science we have myriad things never dreamed of a half century ago. While I am sympathetic to the plight of the hungry, I suspect that if we stopped spending money on science we would not shift all those resources to the poor. A more appropriate question might be how limited resources for science should be allocated among the many scientific endeavors wanting support.


QUESTION: 
If a photon has no weight then how can it be affected by gravity? ex. light bending towards large masses and light being able to be sucked into a black hole.

ANSWER: 
The reason is that our modern understanding of gravity is the theory of general relativity shows that mass "warps" the space around it so that a "straight line" in warped space is really a bent line as we perceive it so light is really traveling in a straight line when we see it being bent around a large mass. We no longer think of gravity as being some mysterious force between masses. Read my earlier answers (1 and 2) on general relativity.


QUESTION: 
Many moons ago whilst at college we had a wonderful physics laboratorycontaining, among other things, a large gyroscope and a very sensitive set of digital scales. The gyroscope weighed about 1kg when stationary and weighed slightly less when spinning. The exact amount of the difference varied with the speed we could get the gyroscope spinning. The experiment was repeated some years later when I found some similar equipment. I have asked teachers and knowledgeable people but no-one can give the an answer as to why this happens.

ANSWER: 
The long and short of it is that there is no difference in the weight of a spinning gyroscope. If the scale does not read the weight correctly, then there is an error in the experimental method. There was, several years ago, a bit of a flurry over the purported weight difference for a gyroscope spinning in its two different possible directions, but subsequent experiments showed this to be an error. Incidentally, there actually is a slight increase in the mass because it has increased energy (
ΔmE/c2), but way too small to be able to measure with a scale.


QUESTION: 
I am curious as to the differing constants used in equations of the uncertainty principle. For instance I have observed (delta x)(delta p) => h (Planck's constant, (h-bar (Dirac's constant), and h-bar/2. Is there a form more commonly accepted...and whence comes the rational for the differences. (I am aware that the magnitude of the difference in constants is either pi or 2pi.)

ANSWER: 
There is nothing profound here. The problem is that the idea of uncertainity is a statistical idea and not terribly well defined. What does it mean
—within one standard deviation? within 90% likelihood? …? It depends on the nature of the experiment you are doing also. Note also that it is an inequality, so that confuses the issue of nailing it down too. It may be shown that the minimum uncertainty product is attained for Gaussian distributions of momentum and position uncertainties.


QUESTION: 
I am just starting an interest in physics, the universe in particular. My question concerns light. Is there a distance when light disperses? I know we see things in our sky that are billions of light years away. Does a beam of light continue on forever? How strong does it have be? It seems obvious to me that if I shined a flashlight at the night sky, aliens billions of light years away would never see it. If you don't have an answer could you please refer some resources that may help me.

ANSWER: 
Light, and all electromagnetic radiation like radio, x-rays, etc., is composed of photons. In truly empty space a photon will live forever. If it encounters an atom it can be absorbed but will then be reradiated in a different direction, or it can simply be scattered, or it might cause a chemical reaction which will cause it to disappear. The stronger the light source, the stronger the signal far away, but it will not simply disappear without something taking it away.


QUESTION: 
In ice hockey I know they freeze pucks to control the speed of a puck sliding on ice. Does tempertaure affect a plastic roller hockey puck sliding on a plastic surface at all? Would it slide faster or farther if it were chilled or heated?

ANSWER: 
No. The ice hockey puck slides on ice and, if its temperature is above freezing, it has a thin layer of water between it and the ice which lubricates it. There is no such similar thing if there is no ice.


QUESTION: 
What does or would it mean for a person to be ionized? I had a dream in which a ton of people including myself were yelling ionized... the dream seemed very real and in the dream the fact that we were ionized seemed very significant. I am not a science person and I don't know if this breaks the ground rules, but I would really like some insight into what it would mean for a human to be ionized.

ANSWER: 
Ionized means that atoms are electrically charged either by losing electrons (positive ions) or having extra electrons (negative ions). When you get static electricity and get a shock by touching a car or doorknob or something, it is because you are ionized and the ions want to become neutral. Nerve cells work by ions of potassium or sodium moving along and through cells. There is never a time when ions are not moving around in your body. And your body is probably never exactly neutral since ions are constantly being made by your rubbing on things.


QUESTION: 
I am a high school science teacher. I am trying to explain to my students why liquids exert pressure. One reason is due to their weight. But there must be another reason because there is a term in Bernoulli's flowc equation for "the pressure." For gases, I have told my students that pressure can be due to the weight of the gas (pushing "down" on something) OR to the gas particles striking the walls of the container. In fact, I have emphasized to my students that that the pressure mentioned in the Gas "Laws" is due to the gas particles "bouncing off" the container. So my question is this: Is there a second reason why liquids exert pressure (aside from any attempt to compress a liquid)?

ANSWER: 
The weight of the fluid above is one important factor, as you note; that is why the pressure gets bigger as you go deeper under the surface of water. You can increase the pressure in a fluid by pressing on it. This pressure increase is then transmitted to all points of the fluid. Here are a couple of examples:

  • Suppose you calculate the pressure at a depth of d=20 m below the surface of a lake using the standard formula P=ρgd. You will get about 2x105 N/m2. But if you measure it you will get about 3x105 N/m2. What is going on? The atmosphere is pushing down on the surface of the lake with atmospheric pressure which is about 105 N/m2, thereby increasing the pressure everywhere.
  • Look up how a hydraulic lift in a garage works. You have two moveable pistons, one large and one small. You push on the small one with a force F1 and, if the area of the small one is A1, you increase the pressure by P=F1/A1. But the pressure at the large one increases by the same amount and so P=F2/A2. And so, F2=(A2/A1)F1, a much bigger force.

By the way, regarding the atmospheric pressure thing: often when you measure the pressure with some guage, like the tire pressure in your car tires, you do not really measure the absolute pressure but what is called the guage pressure which is the amount by which the pressure exceeds atmospheric pressure. The measuring devices are calibrated to subtract atmospheric pressure. So, in my first example the guage pressure 20 m down is 2x105 N/m2.

FOLLOWUP QUESTION: 
I understood your answer but perhaps my question was poorly worded. In essence, why does a fluid moving at CONSTANT velocity exert a pressure independent of its weight AND independent of any attempt to compress the fluid?

ANSWER: 
I think the problem is that you are thinking about static fluids. When a fluid is in motion, the pressure in the fluid changes. To understand why you must understand the derivation of Bernoulli's equation. Bernoulli's equation is essentially energy conservation. The pressure is the work done on a volume element of the fluid (work = force x distance = pressure x area x distance) and that is how pressure gets into the equation. The velocity gets into the equation because of the kinetic energy (moving mass has kinetic energy). The third term is an altitude dependence which comes from potential energy. There are numerous examples of pressure being dependent on velocity:

  • An airplane can fly because of this effect.
  • Smoke inside a car will exit through a slightly open window.
  • A curveball in baseball curves because of this effect.

QUESTION: 
The speed of light is the same no matter what frame of reference it is measured in. Then if you have a photon traveling towards a moving spaceship, is the time taken for the photon to catch up with the space ship irrelevant of the space ships speed, because in classical mechanics, relative speeds is calculated by speed 1 - speed 2. But if the speed of light is always C then it does not matter how fast the space ship is traveling.

ANSWER: 
You are right, the speed of light is the same in all frames. However, there are very surprising consequences due to this experimentally verified fact: lengths change and clocks run at different rates in different frames. In particular, moving sticks get shorter and moving clocks run slower. (It is important that you realize that these mean that they don't just appear to be shorter/slower, they are.) Here is a numerical example: Suppose a spaceship is going with speed 0.8c away from you (c=3x108 m/s is the speed of light) and you shoot a pulse of light at it when it is 100 m away. The time it takes to catch the spaceship is 1.67x10-6 s. (Getting this number is a little tricky because the spaceship goes some distance during the time. Try it.) Now, the guy on the spaceship sees the distance between you and him at the instant you flash the light to be 60 m, not 100 m because, to him, that distance is moving with speed 0.8c. (The shortening factor is
√(1-v2/c2) where v is the speed.) Hence the time he measures between the flash and his receiving it is 0.2x10-6 s. So, the speed does matter.


QUESTION: 
When talking about Work, the units used are Joules which equal a Newton times a meter. Why is it that when talking about torque, the units are Newton's times meters, but they are just called Newton-meters. What is the reasoning that they have the same units, but you don't refer to the units of torque as Joules?

ANSWER: 
There is no profound answer, it is just conventional. But it makes sense to me, in a way. Energy is an extremely important concept in mechanics; it is reasonable to define a unit of energy which is called the joule in SI units. There are other energy measures such as the erg (dyne cm=(gm cm/s2) cm) and eV (1 electron volt=1.6x10-19 J). Torque is not energy, it is the rotational analog of force, so it would be confusing to measure it in joules.


QUESTION: 
Is proton stable ? what about electron ?

ANSWER: 
I discussed proton decay in an earlier answer. There is no evidence nor theoretical expectation that an isolated electron is not stable.


QUESTION: 
Is it possible to spin an object at the end of a string completely horizontally? What about gravity pulling the object down? I didn't think it was possible to have an object spin completely horizontally. If so, what does the free body diagram look like for that? Tension would be completely in the x direction? I thought that the object could never be spun absolutely horizontally because the y component of the tension would have to balance out the weight of the object. Does angular momentum play a part here?

ANSWER: 
You can do this with any toy gyroscope. However, it does not just sit there spinning, it precesses. You can see what I mean by precession at this link; this particular example is not exactly horizontal but it could have been. (Also, it is on a pivot point, not hanging from a string, but it would behave just the same.) The free body diagram would have a force up at the pivot (the normal or tension in the case of a string) and a force (the weight) down at the center of gravity. These would be be equal and opposite since there is no acceleration in the vertical direction. However, the object is not in equilibrium because there is a net torque about the pivot (due to the weight). Newton's second law for torques is that the rate of change of angular momentum is proportional to the torque. For your scenario (horizontal), the torque is in the horizontal plane and perpendicular to the axis and the angular momentum is in the horizontal plane and parallel to the axis; hence the effect is for the torque to change the direction of the angular momentum, not its magnitude. (That is, the direction of change of the angular momentum will be horizontal and perpendicular to the axis, resulting in the gyroscope remaining horizontal.)


QUESTION: 
If I hooked up a step down transformer(say from a toy) in reverse will it become a step up transformer ? I also need it to be DC.

ANSWER: 
In principle, yes. However, I cannot guarantee that it would work because it may be engineered in such a way that it would not work because it could not, for example, be able to take the power you might put into it. But, the whole project will not work because a transformer works on the basis of Faraday's law which is operative only for time varying magnetic fields and so DC is out of the question (unless you are willing to rectify the AC after the stepup).


QUESTION: 
I have just read about a story of Einstein. when he was 16, he had just studied Maxwell's Electromagnetic Theory. Einstein did a thought experiment that he rides along a beam of light with the same velocity of that of light. I do not quiet grasp what is the real significence of his thought experiment and how does his thought experiment lead to special relativity.

ANSWER: 
First, read an earlier answer which will explain the nature of electromagnetic waves. Now, if you run alongside the wave, the electric and magnetic fields will be static (not changing in time) but the wave shape will be varying with where you are. Maxwell's equations do not allow such a solution, if the fields are varying in space, they also must vary in time. He therefore concluded that nothing he could do could put him on pace with the light beam. From this he concluded that the electromagnetic wave was a special kind of wave in nature, one which does not require a medium in which to move, and therefore that all observers must see it moving with the same speed, regardless of their own speeds; this is the fundamental postulate of special relativity.


QUESTION: 
If projectile motion states that a bullet from a gun will fall at the same rate as a dropped bullet then how can a sniper laying on the ground fire a 50 cal. and like three seconds later it will hit its target? Does the bullet rise as it leaves the barrell or does he have to shoot down hill?

ANSWER: 
The fact that the bullet drops makes marksmanship a science to perfect. You must aim above that which you want to hit to correct for this effect. Sometimes this is done by sophisticated long-range rifle sights which you adjust to the distance of the target. If the distance is small enough, the drop will be small enough to be negligible. With slower projectiles, like a snowball, for example, you clearly need to throw it up some to hit your target more than a few yards away. It is also very evident for long range artillery which must be aimed very much above the position of the target.


QUESTION: 
What about 2 objects travelling towards each other, each at 1/2 the speed of light? Relative to one another, doesn't that equal what would be considered the speed of light, from one perspective or the other? Does it matter?

ANSWER: 
This question has been answered before.


QUESTION: 
Why do the planets around the sun orbit, instead of just going straight into it?

ANSWER: 
This is such a basic question that I cannot answer without giving you a whole course in introductory physics. I believe that you can begin to understand by studying Newton's mountain, how he envisioned the possibility of earth satellites. There is a very nice interactive demonstration on the web.


QUESTION: 
since matter is composed of chaotically, rapidly moving particles (or whatever they are) why do solids keep their shape, why dont they "evaporate"? a corollary question: if you clamp two pieces of wood together, why dont they join into one piece?

ANSWER: 
In a solid the particles (which are atoms) are locked in place by forces they feel from their neighbors; they are not moving around chaotically. Under the right conditions, solids do evaporate but that is not what it is called, it is called sublimation. The reason that two pieces of wood clamped together do not bond into one is that the surfaces are not smooth to an atomic scale so few atoms in one block will bond with those in the other and there are inevitably contaminants on the surfaces, even air, which prevents them from getting close enough to each other. In fact, two pieces of metal very smooth and flat and clean in a vacuum will adhere to each other and affect almost a weld.


QUESTION: 
Light bends. Einstein said the the curvature of space bends light but only under immense gravity. How does immense gravity bend the fabric of space to allow light to bend?

ANSWER: 
Immense is a qualitative term. It is simply not true that only "immense gravity" warps space-time; any gravity does. It is just that large masses cause larger warping than small masses. To ask how this happens is to ask how the theory of general relativity leads to this. You cannot understand completely without knowing the theory. It is, nevertheless, worthwhile to know this final result that mass warps spacetime because we believe general relativity to be true because it has passed the tests of its predictions (like gravity bending light) being true.


QUESTION: 
Why does a balloon decrease in size when it gets colder?

ANSWER: 
It doesn't, necessarily. To a good approximation, PV/T is a constant where P is pressure, V is volume, and T is temperature. If the pressure remains constant, then V/T is a constant and decreasing temperature means decreasing volume to maintain a constant ratio. Keep in mind that T is absolute temperature where, for example, room temperature is around 3000. If the pressure changes also, all bets are off.


QUESTION: 
This is a question about the physics of motorcycle cornering.My question If you are riding a 400 lb motorcycle and go thru a corner at 80 mph ,would the g forces be the same as a 200 lb motorcycle going thru a corner that is half the radius at 40 mph.

ANSWER: 
The force required to keep an object in uniform circular motion (centripetal force) is mv2/R. So, in your case, you halve the mass, halve the speed, and halve the radius, so the new force is (m/2)(v/2)2/(R/2)=
¼mv2/R. So the answer is no, you have it wrong.


QUESTION: 
If sunlight can be conceived as a beam of photons, each of which carries a certain amount of energy and momentum, why don't we experience (or feel) any recoil as these particles collide with our bodies when, say, we're at the beach?

ANSWER: 
You do not even need to invoke photons to ask this question. Electromagnetic waves carry energy and momentum also. It is because, under normal intensities of light, the momentum carried is so small that even if all the momentum were transferred to you (as it is when you absorb it), the force is too small to feel. It is like asking why you do not feel the impact of a single atom.


QUESTION: 
Why the orbit of Mercury around the Sun cannot be all accouted from the Newton's Law of Gravity, because there is a disprency between observation and Newton's Law.

ANSWER: 
The orbit of Mercury is an elipse and the semimajor axis of this elipse precesses in time and this should not happen in Newtonian physics. However, this is predicted by the theory of general relativity and is one of the conerstone tests of that theory.


QUESTION: 
I've always known that water and ice occupy different volumes because of their densities. However, do they really "occupy" different volumes? In other words, should we really state that the vacuum spaces (more so in ice) are are actually being occupied by ice or water or whatever?? If you only include the actual molecules and not the empty spaces, then both water and ice should have the same volume and weight.

ANSWER: 
There really is no such thing as empty space in a solid or a liquid. All the atoms overlap each other. You should not think of atoms or molecules as hard spheres or other shapes with definite volume. Where they start and end is fuzzy.


QUESTION: 
why does nuclear fusion give off more energy than fission?

ANSWER: 
You shouldn't really say more energy, you should say more energy per unit amount of fuel. The reason is that a higher percentage of mass is converted to energy in fusion because of the differences in binding energies of very light and very heavy nuclei.


QUESTION: 
to split a nucleus would you need its temperature to be higher than the temperature required for nuclear fusion?

ANSWER: 
Since fission is induced by capture of neutrons, it can happen at room temperature. For fusion, you must get the two nuclei close to each other which requires that you give them high velocities; one way to do this is to have a very hot plasma.


QUESTION: 
How come the electromagnetic force is the only force that is attracted to its own negative force, and the strong force, weak force, and gravitational force are repeled by their negative selves?

ANSWER: 
"
attracted to its own negative force" has no meaning. And it is futile to try to ask why two forces are "different" because each is fundamental. One reason which might satisfy you is that there are two kinds of charge but only one kind of mass, but why like charges repel and like masses attract is just because that is the way the world works.


QUESTION: 
Will a rubber band stretch the same distance everytime the same amount of wirght is attached to it?

ANSWER: 
No, a rubber band is not a very good spring. When stretched, it tends to get damaged and not return to its original shape exactly. If you attach very small weights to it, it will behave fairly reproducably.


QUESTION: 
No one has been able to explain to me why a heavier person or person(s) seems to be able to go faster sliding down a hill on a toboggan, sled, or skis, even though, theoretically, the mass should not affect the acceleration.

ANSWER: 
See several earlier answers to this question.


QUESTION: 
Can final momentum be found using P=mv, or is there a separate formula for final momentum?

ANSWER: 
Well, sure you can find it this way if you know the mass and final speed. However, we generally write that the impulse is equal to the change of momentum and so the final momentum is the initial momentum plus the impulse. Impulse, by the way, is the average force times the time over which it is applied.


QUESTION: 
This problem was given as as a take home chalenge. I've thought about it many ways. How would you go about solving this??? I'm obsessed with this please help me. I need to figure out how to make a "lead balloon" fly and here's what I'm given: Temp = 300 K Pressure = 105 Pa (1atm) inside and outside the balloon Thickness of lead foil that makes up the balloon = 0.1mm Density of lead = 104 Kg/m3 Mhelium = 4amu Mair = 29 amu V= 4/3πr3 What is the Volume needed to support itself?

ANSWER: 
Although I do not do homework, I can give you some hints. The one unknown in this problem is R, the radius of the sphere. First you must find the mass of the balloon plus helium in terms of R. The mass of the lead is the volume x density = area x thickness x density and the area is 4
πR2. The mass of the helium is volume x density of helium where volume you know in terms of R; the density you will have to look up somehow, but one way is that it is 4/29 that of air at the given temperature and pressure. Now the weight, in terms of R, will be the sum of the masses times 9.8 m/s2. The buoyant force must equal that weight and so the weight of the same volume of air must equal the total weight of the lead plus helium; this will give you an equation which you can solve for R.


QUESTION: 
If the light (electromagnetic radiation) can be slowed down through the medium (eg. air, glass, water, etc), then is the propagation of magnetic field also slowed down in a same factor. Take an example, if we apply current through a coil to magnetize an iron core, then an iron on the other side of medium will be affected with specified delay in the same factor of lightspeed in this medium. Is it right?

ANSWER: 
This is an interesting question. There is no simple answer because, among other things, a material like glass exhibits dispersion which means that light of different colors travel with different speeds; so there is no unique "slow down factor" as you imply. There is a simple law in relativity
—no information may be transmitted between two points at a speed faster than the speed of light in vacuum. If there is a material through which you must transmit the information, this is still the law and the speed with which the information travels through the medium depends on how you choose to send the information. If you use light, the information will travel at that speed. If you use something other than EM radiation, like, as you suggest, a magnetic field, than the speed with which the field will propogate through the space depends on the magnetic properties of the material. Since a material like glass has very little effect on magnetic fields (unlike a much larger effect on electric fields), I would guess that the information would travel through much more quickly than it would with radiation.


QUESTION: 
why do nuclear fusion reactions result in the emission of EM radiation?

ANSWER: 
Two reasons are: nuclear reactions leave the residual nuclei in excited states and gamma rays are emitted when they deexcite; and the fusion adds considerable thermal energy to the system resulting in black body radiation. Most of the visible light from the sun, for example, is black body radiation.


QUESTION: 
Can EM radiation move any form of matter, even neutrons?

ANSWER: 
For EM radiation to have any effect on something, the something must have either electric charge distribution or electric current distribution. An object need not have a net electric charge; for example, an atom is uncharged but has a charge distribution (regions of positive and negative charge which happen to sum to zero) and it is affected by EM radiation. A neutron has a magnetic moment (meaning it looks like a tiny magnet because of current distributions in it) and it also has, like the atom, no net charge but a charge density, so it will interact with EM radiation.


QUESTION: 
Starting with a sphere of water placed in space, would it remain as a sphere, or disperse? ie, are there any internal forces which would influence its destiny?

ANSWER: 
It sounds like you are interested in the lack of gravity on the water and not the lack of air which is implied by your use of the word space. So I will imagine a sphere of water inside a spaceship which is pressurized to normal atmospheric pressure. The water, like any liquid has two important interatomic forces, those between the atoms in the volume and those between atoms on the surface (which differs from the former in that there are fewer nearest neighbors on the surface). These are attractive forces and would hold the sphere together; the sphere would remain a sphere.


QUESTION: 
What is state of matter is fire?

ANSWER: 
It is mainly gas. Because chemical reactions are going on in the fire, there is a small amount of plasma in the flame. Also, there will be small amounts of solid in the form of soot and other small particles.


QUESTION: 
A coworker and I have a disagreement over the law for the "Conservation of Matter". We both agree on.... "Matter cannot be created or destroyed, only changed in form. The mass that was present before a chemical or physical change equals the mass that is present after the change" But I add, "...except in a nuclear reaction." upon which he disagrees. Most online definitions seem to support his stance, although I have found some to support mine. Who is right.

ANSWER: 
Your "conservation of matter" is an antiquated idea. When you burn something, let's say carbon, by combining it with oxygen, energy is released. Where does this energy come from? The simple fact is that if you were to weigh the carbon dioxide and compare that weight with the carbon and oxygen you started with you would find less mass. Unfortunately, this is an impossible experiment to do because chemistry is such an inefficient way of producing energy that the mass change would be incredibly tiny. Suppose that you get 1,000,000 joules of energy by burning a few pounds of coal; this corresponds to a mass change of 106/(3x108)2 which is about 10-12 kg. This comes from E=mc2. Imagine trying to measure this mass change if you burnt a few pounds of coal to get this million joules of energy! In a way it was lucky because so much of 18th and 19th century chemistry is based on this conservation idea. In a nuclear reaction, as you note, the energy conversion is much more efficient and masses change by measurable amounts, something like 1%. So, the final accounting is: you are wrong once and right once and your coworker is wrong twice. But, I would give you both a marginal pass on the first because we learn this in chemistry courses and it is almost right.


QUESTION: 
Suppose you roll a bowling ball into a pillow and the ball stops. Now suppose you roll it against a spring and it bounces back with an equal and opposite momentum. (a) Which object exerts a greater impulse, the pillow or the spring? (b) If the time it takes the pillow to stop the ball is the same as the time of contact of the ball with the spring, how do the average forces exerted on the ball compare?

ANSWER: 
Since impulse is equal to the change in momentum, the spring delivers more impulse (twice as much) because the magnitude of the momentum change by the spring is twice the initial momentum. Since impulse is average force times time interval, if the time intervals are equal then the pillow exerts half the average force of the spring.


QUESTION: 
soliders are asked to break steps while crossing a bridge why????

ANSWER: 
The rhythmic beating of many feet may cause the bridge to vibrate in resonance with the marching and fail.


QUESTION: 
a bob of the simple pendulum is a hollow sphere filled with water.how does the period of oscillation change if water begins to drain out of the sphere.

ANSWER: 
The period of a simple pendulum is independent of its mass and so, reducing the mass of the bob will not change the period of the pendulum.


QUESTION: 
If you're in a ship travling close to the speed of light could you fire a gun?

ANSWER: 
Strange things can happen in relativity. Suppose that you are in the space ship going, say, just 1 mile/hour slower than light speed past me. In your frame of reference everything appears perfectly normal, the laws of physics are just the same as mine. So if you fire a gun in the direction in which you are traveling it works just the same as my gun; suppose that you measure the speed of the bullet to be 500 miles/hour. Then I would see the bullet travel with a speed of 499 miles/hour faster than the speed of light, right? Wrong! I would measure the speed of the bullet to be faster than your speed but slower than the speed of light.


QUESTION: 
Time, as I understand it, is a concept. "it took me three minutes to cook an egg". Those three minutes are just my brain measuring how long it took to cook that egg. So time is a concept and not a thing. It has no mass. OK? How can time be affected by gravity? Please understand that I accept that it does. It has been proven, I am sure. I just don't understand how it can be. Time is not a thing, it's a concept. How can it be affected by gravity?

ANSWER: 
Your question contains much ill-defined terminology. What is a "thing" and what is a "concept"? What about a quart, is it just a concept? "It took me 4 quarts to fill that gallon." And, time is not just your brain measuring something, it is something which can be operationally defined completely objectively; brain-measured times are notoriously inaccurate. What we have learned from the theory of special relativity is that time and space are inextricably tied up with each other, that is, the concept of length and the concept of time are not separate concepts but interdependent. The upshot of this miraculous discovery is that time does not run at an immutable rate but proceeds with different rates for different observers. For example, if the egg cooker were flying by you with a speed very large (comparable to the speed of light), you might see it take three days to cook that egg; but if you asked the cooker, she would still say it took three minutes. General relativity is the extension of special relativity and is the theory of gravity. In essence it says that the presence of mass warps the space around it and, since time and space are, as I explained above, interdependent, time is also affected; physicists call this the warping of space-time. That is why gravity affects time. Also, in general relativity it is not required for something to have mass to be affected; the best-known example is that light, which has no mass, is bent by gravity. There is actually an everyday application where the effects of gravity on time must be corrected for. GPS systems, which rely on signals from satellites to calculate positions, must make relativistic time corrections without which they would not work.

FOLLOWUP QUESTION: 
A quart, since you used it in an example, is a "thing". Even without the container, a quart exists as a thing that I can see and measure and use. Light, if my caveman mind understands, can be a particle and a wave. When it's a particle, I could hold it, measure it and use it. So, it can be affected by gravity. It can be slowed and bent and manipulated. Time has absolutely no way of having mass. Hence, in my mind, it can not be affected by gravity. Maybe my definition of gravity is wrong. I think of gravity like a magnet. A mass will have gravity. Gravity, by definition, will attract mass. Will it affect waves?

ANSWER: 
I am still confused by "thingness". A photon, the particle form of light, has no mass, but evidently it is still a "thing". (Just because light can be a particle does not mean it can have mass, as you seem to imply. It does not have mass. And, light most certainly cannot be "slowed".) Let me step back from my quart analogy and instead suggest considering the distance between two points in space at a particular time. Is that a "concept" or a thing? If a thing, then how does it differ from time which is the "distance" between two events at the same place in space? And, can the distance between two points in space be affected by gravity? The answer is yes, space is affected by gravity; and, as I explained in my original answer, space and time are all wrapped up with each other and time, like space, is affected by gravity if space is. Yes, your "definition" of gravity as being just a mysterious force (like a magnet) which acts on objects which have gravitational mass is incorrect. As I tried to explain in my original answer, our best understanding of gravity is general relativity in which the presence of mass warps spacetime.

No amount of "philosophizing" will make you understand this. You need to learn the theory of special relativity to understand how time and space are coupled. It is not as hard as you might think, requires only algebra and there are numerous books for the layman explaining relativity.


QUESTION: 
Rutherford was able to determine the nature of alpha particles by recognizing that they have the same atomic spectrum as helium. However, if alpha particles are just the nuclei of helium atoms, then in the absence of electrons, what produced the spectra?

ANSWER: 
He put his alpha source in a vacuum and waited a while. Alpha particles would acquire the necessary electrons and, when there were enough of them, he could do the requisite spectroscopy.


QUESTION: 
My understanding is that scientists were able to determine the chemical make-up of the sun by way of spectral analysis of the sunlight. Specifically, a spectroscopic analysis of the sun shows the line spectra for hydrogen and helium and allows us to conclude that the sun is basically hydrogen and helium. However, when sunlight passes through rain drops we see a continuous spectrum of light, not the discontinuous spectrum of hydorgen.

ANSWER: 
Sunlight is basically white; when you analyze sunlight you do not see spectra of hydrogen or helium because the sun is not composed of atoms radiating, it is composed of a plasma. So, light from the sun does not come from atomic transtions in bound atoms. There are, however, some dark lines in the white spectrum; these result from hydrogen atoms which do exist in the sun's atmosphere which absorb light of the right wavelengths.


QUESTION: 
What is the form of energy that is released by nuclear fusions in the form of, electromagnetic radiation?

ANSWER: 
Well, fusion fuels the sun so look at the sun: an enormous amount of electromagnetic radiation comes out, lots of neutrinos come out carrying kinetic energy, the sun is very hot, that is the particles that make it up have very large amounts of kinetic energy.


QUESTION: 
I can recall having read that a all gravitating bodies have a terminal velocity regardless of the drag effect. I wasn't able to find the cause of this type of TV on the web. What causes an object to reach terminal velocity while in free-fall on say, the moon for instance, which has no atmosphere..? And is it in any way related to escape velocty?

ANSWER: 
I do not know where you read this but it is wrong. With no air friction there is no terminal velocity. Terminal velocity means that eventually the speed becomes a constant and acceleration ceases. This never happens if weight is the only force. It is true that if something is dropped from very far away that it will have a specific maximum speed when it hits the surface of the planet (which happens to also be the escape velocity), but this is not a terminal velocity.


QUESTION: 
if light bends around a planet or something it creates an arc like this (. So isnt the inner side of one unit of light moving slower then the outer side? Think of a bicycle wheel, the tire moves faster in relation to the axle. Im almost sure that its a relativly easy thing that im just missing but please help and thanks again.

ANSWER: 
What is "one unit of light" you refer to? Think of the light as being made of many photons traveling side by side. After getting bent by a massive object, those passing closer to the object have less far to travel so they get around in a shorter time than those farther away. So they end up not being side by side after the bending. Your mistake is to think of them rigidly attached to each other like the parts of the bike wheel but they are not.


QUESTION: 
Atomic hydrogen produces well-known series of spectral lines in several regions of the electromagnetic spectrum. Each series fits the Rydberg equation with its own particular n1 (n sub 1) value. Calculate the value of n1 (by trial and error if necessary) that would produce a series of lines in which: (a) The highest energy line has a wavelength of 3282 nanometers. (b) The lowest energy line has a wavelength of 7460 nm.

ANSWER: 
The relevant equation for atomic hydrogen series is 1/
λ=R[(1/n12)-(1/n22)] with R=10.972x106 m-1. The highest energy transition of a particular series is for n2=∞ and so n1=√()=√36=6. For the lowest energy transition, n2=n1+1, so, after a little algebra, n12(n1+1)2/(2n1+1)=Rλ=81.85; this is a quadratic equation in n12 and could be solved analytically, but it is probably easier to just make a couple of guesses. It turns out that n1=5.


QUESTION: 
Why do the tires on a moving car sometimes appear to be turning backwards? I teach 5-8 graders and would like to be able to explain it to them!

ANSWER: 
I presume that you are talking about wheels you see in a movie or on TV. I have previously answered this question. The main reason is that movies or TV come to us as a series of still pictures at a particular time interval between pictures whereas everyday life comes to us in a continuous stream. This same effect can be seen using a strobe light.


QUESTION: 
Because energy makes up matter, could energy units be used to measure mass as well as weight?

ANSWER: 
Weight is a force, so let's leave that out of your question. The answer is, yes and in fact nuclear physicists and particle physicists usually specify the mass of a particle by specifying its rest mass energy (mc2); for example, the "mass" of an electron is 511 keV (keV is kilo electron volt, a unit of energy).


QUESTION: 
If there is a force, that agrees with the inverse square law, lets just say the electromagnetic force that is being created by the interaction between two electrons, and if the distance between these two electrons is increased, would the intensity of this EM force, which is being created by the two electrons, decrease by a factor of the distance increased squared? If this is true, is this exactly, and the only way how the inverse square force works?

ANSWER: 
"Intensity" of the force is the wrong word; the strength of the force is proportional to 1/r2 and so if the distance doubles the force gets smaller by a factor of 4. And yes, the Coulomb force is, as accurately as it has ever been measured, an inverse square law force. See an earlier answer for more detail.


QUESTION: 
I have a student who suggests that chemistry is not a true science and that everything in chemistry can be explained using quantum mechanics. He further states that everything in physics is derived from deductive reasoning whereas everything in chemistry is derived from inductive reasoning. Do you have any insight in this matter that might be helpful?

ANSWER: 
At the heart of things, everything is physics. It is truly an overstatement to say that chemistry is not a "true science" just because it is not the most fundamental science. I don't get his deductive/inductive arguments, but I can tell you that physics and chemistry both depend on experimental observations to acquire data upon which to base the science.


QUESTION: 
Can magnetic radiation be reflected like light? If so, what material would be suitable?

ANSWER: 
There is no such thing as "magnetic radiation". Electromagnetic radiation is always both electric and magnetic fields. The radiation can be be generated by either oscillating change distributions (electric) or oscillating current distributions (magnetic), but in both cases the radiation is electromagnetic.


QUESTION: 
Could you explain the origin of the inverse square law that occurrs in nature? Why can't Newton's law of gravitation vary as R^3 or R^5? How did Newton deduce this so long ago? Could it be because of the geometry of space when something radiates from a central point source...as a sphere gets larger & larger radii?

ANSWER: 
Let us start with the historical question of how Newton could deduce the inverse square law for gravity and that will take care of your questions of why it cannot possibly be some other power law. There were data available to Newton which described the motion of the planets; these were Kepler's three laws. These can only be described by an inverse square law. So once again we see that, at its heart, physics is an experimental science and data dictate theory. Is there a deeper reason? As I see it, that is part of your question too. The electrostatic force is also an inverse square law (Coulomb's law) and in this case the theory of quantum electrodynamics correctly predicts the 1/r2 dependence of the force. One reason is that the quanta which transmit the field, photons, are massless; forces like the nuclear strong interaction are mediated by particles with mass and the force is a much shorter-ranged force. For gravity, it is, as you suggest, related to the geometry of space (the area surrounding a point increases like r2 as the distance r increases) as best we understand. General relativity, our best theory of gravity, correctly predicts the force to be inverse square law. Although there is no theory of quantum gravity, the mass of a graviton, if it exists, is expected to be zero because the force law is what it is.


QUESTION: 
Why is silver wire a better electrical conductor than gold even though both have a single valence electron?

ANSWER: 
There is no simple answer to this question. The mobility of electrons in any material depends on lots of things other than is there a single valence electron. It depends on the atomic structure of the particular atom (what orbital is the valence electron in, how strongly bound is it, etc.) and also on the crystal structure of the bulk material.


QUESTION: 
The Bohr model of the atom is rarely mentioned in reference to quantum gravity, yet it is a quantum description of simple orbitals. What is the principal problem? If the transition frequency between 2 atomic orbitals defined the period of an even acceleration of the electron as it moves between orbitals, then would this not parallel gravitational orbitals? (for if the photon is a unit of momentum spread over time, then it could not instantaneously transfer this momentum to the electron but instead would slowly accelerate the electron to the next orbital over the length/duration of the photon wavelength)

ANSWER: 
Well, I have never seen the Bohr model mentioned in this context, and for good reason. The Bohr model is essentially totally incorrect but it did provid a good starting point for understanding atomic structure and for discovering rigorously correct quantum theory.


QUESTION: 
I am a student at the University of Canterbury in New Zealand. As part of an assignment I am currently undertaking I must seek out experts in a particular field and put forward a question related to their area of expertise. I was wondering if you would be able to spare some of your time to answer a question that will help me to complete my assignment and that has been leaving me quite perplexed. My question to you is this: Why don’t the protons in the nucleus of an atom spin about like the electrons do?

ANSWER: 
By "spin about" I presume you mean move in orbits. Of course, the first thing you should appreciate is that the model of electrons moving around in well defined orbits is qualitative, at best (see earlier answer). Now, to answer your question: in fact, protons (and neutrons) do move in orbits inside the nucleus. This is referred to as the nuclear shell model and does a very good job describing the properties of nuclei. It is not quite so obvious as an atom because the electrons have a single center of the force, the nucleus itself, around which to orbit. The sum of all the other particles in the nucleus provide the force seen by each individual particle which cause it to orbit. {One additional detail: There is one crucial element for nuclear orbits not important in electron orbits and that is called the spin-orbit force, a force which depends on the direction of the proton or neutron spin relative to the direction of its orbit.}


QUESTION: 
If the earth rotates from East to West shouldnt a plane always arrive at its destination much quicker if traveling east, since the earth is rotating in the direction opposite the plane?

ANSWER: 
Because an airplane flies relative to the air and the air rotates with the earth, so the earth might as well not be rotating at all from the airplane's perspective. This is not literally true because of winds and if an airplane has a tail wind it will fly faster relative to the ground than if it has a head wind. The prevailing winds and the jet streams tend to blow from the west and so a trip from the west coast to the east is usually quicker than the trip in the other direction.


QUESTION: 
Are the eliptic orbital planes of all our solar systems planets on the same 2d plane, or are there angles between the planes of the planets orbital elipses' ?

ANSWER: 
Orbits of the planets are nearly coplanar. Inclination, measured relative to the plane of the earth's orbit, are all less than 3.50 except for Mercury which is about 70. Pluto, no longer considered a planet, has an inclination of 170.


QUESTION: 
Would one mole of baseballs fit into our Earth's Moon? I have found that a mole equals 6.02 x 10 to the 23rd power. I am at a loss as to how to find an answer to this question. What would be the equation (s) needed to solve this problem given the diameter of the baseball is 3inches and the diameter of the Moon is 136771430.4 inches (3474 km)?

ANSWER: 
Calculate the ratio of the volume of the moon to the volume of a baseball; that is the number of baseballs. This makes a slight error because there is some volume wasted between balls and that depends on how they are packed.


QUESTION: 
I've often seen emergency vehicles parked next to a long building or long wall and observed that the rotating light slows down perpendicular to the vehicle then accelerates rapidly into the distance as it travels along the wall. This got me thinking; if the wall was straight and infinite and the light was a laser powerful enough to illuminate for a million miles, what happens to the spot of light as the angle of the beam approaches parallel and the illuminated spot is moving along the wall at near light speed or faster than light speed. I believe that theoretically the spot's target could move faster than the speed of light. What would an observer who could keep up with it see?

ANSWER: 
Yes, the end of a beam of light can move faster than the speed of light. However, the end of the beam is not really anything, it is just a location. What is forbidden is to have any material object move equal to or faster than the speed of light or to transmit any information faster than the speed of light. The sweeping beam cannot transmit any information between two locations of its trajectory. Think of looking at a distant star and then shifting your gaze to another star which is 1 light year from the first; the point you are looking at moves at a speed greater than the speed of light (unless it takes you more than a year to shift your gaze).


QUESTION: 
Hi could you help me understand what duration is. I understand our common everyday useage as in the amount of time it takes for an event to happen say a rocket to get form a to b. Specifically what I mean is physicists are suggesting that duration is best understood as a continuum of instants. This confuses me. Is it right to think of this as a continuum of interrelated instants, where each instant is dependent on the previous and that change cannot occur instantaneously. Does this make sense?

ANSWER: 
Your terminology is not really accurate, that is a "continuum" implies continuous values of something, not discrete values. But I know what you are getting at. I have previously discussed the Planck time and Planck length. Even if we consider a continuous time rather than discrete time intervals, no time change occurs instantaneously.


QUESTION: 
If someone is playing soccer on a moving train, and the man kicks the ball at 2km/h, while the train is traveling at 100 km/h, to the outside observer standing on the side of the road, how fast is the ball moving? is it 100 km/h or 102 km/h?

ANSWER: 
It depends on the direction of the kick. If it is in the same direction as the train is moving, then 102
km/hr. If it is in the opposite direction, then 98 km/hr. If in other directions you need to add vectorially to find the speed; for example, if perpendicular to the train velocity then the ball will have two components, 100 km/hr in the direction of the train and 2 perpendicular to it so the speed will be √{22+1002}=100.02 km/hr.


QUESTION: 
In the Lenard's experiment of photoelectric effect when a light of suitable frequency falls on a photo conducting material it results in emission of electrons and flows as photo electric current.So the element will have more proton and it's unstable.but it is not said in texts about the after effects of that photo conducting material.it will try to react with glass or when it comes in contact with outer atmosphere.Or does such a material of high positive charge exists. Also how can a photo electric current flow when there is no electron ie, when all the electrons(billions of electrons)of the whole metal had flown.I asked so because in its graph it is constant after saturation current.No falling down is shown. Why doesn't photoelectric current increase above saturation current even tho we apply more and more accelerating potential for a particular frequency.

ANSWER: 
The photocathode is not just an isolated piece of metal, it is part of an electric circuit so that when electrons are ejected they are replenished so that the metal stays essentially neutral. If the electrons were not replenished, the work function would increase so that, eventually, the photons would have insufficient energy to remove any more electrons. However, it would never be possible to get anywhere close to removing all the electrons from the metal.

FOLLOWUP QUESTION: 
which part of that electrical circuit provide electrons.in the diagram it is not shown.the diagram which we have studied consists of a commutator.there there is no actual contact between the elements in that junction.then how come electrons get neutralized

ANSWER: 
Electrons are emitted from the uppre plate, go to the lower plate, and then return to the upper plate through the ammeter (A).


QUESTION: 
I've read that if an atomic nucleus was scaled up to the size of a period in size twelve font, the nearest orbiting electron would be 3m away. I've also heard that a nucleus the size of a baseball at 'home plate' would have its nearest electrons orbiting at 2nd base. Is there a specific ratio involved for 'size of nucleus': 'distance of electrons?'

ANSWER: 
The nearest orbiting electron is not a very good yardstick to get a feeling for nuclear sizes because it depends on what atom you are talking about. Lead will have a much smaller "nearest orbit" than helium will. The reason is that the electric force on the innermost electrons is much stronger as the charge of the nucleus increases. What is more meaningful is to compare the size of the nucleus with the size of the atom (which quantifies the largest rather than smallest orbits). An atom has a diameter on the order of 10-10 m whereas the nucleus has a diameter on the order of 10-14 m. So the size ratio is ~1:10,000. If the atom is the size of a football field (100 m) the nucleus is the size of 100x10-4 m=1 cm, about the size of a gumball.


QUESTION: 
If the acceleration due to gravity were somehow doubled to 19.6 m/s/s what would happen to your weight?

ANSWER: 
It would double.


QUESTION: 
Wasn't it once historically proven that EM radiation consists of no particles? If so then why do we still consider EM radiation as consisting of particles known as photons?

ANSWER: 
Historically it was not known what light was. Newton favored the notion of thinking of light as particles. However, the famous Young's double slit experiment in the 19th century clearly showed that light is a wave. When Maxwell's work later in the century showed how light is an electromagnetic wave, setting a firm theoretical foundation for the wave-like nature of light, it was thought that the question was settled. Then came the 20th century and Einstein showed that an experiment called the photoelectric effect could not be understood unless light was a particle; then another experiment, Compton scattering, came to the same conclusion. So, which is it? The modern view is that it is both and what you observe is what you look for. This is often referred to as duality
(see earlier answers, 1 2 & 3)and applies also to what we normally think of as particles; for example, an electron will behave like a wave if we look for wave properties. This is just the way nature is!


QUESTION: 
How does the repulsion of like charges occur, and the attraction of opposite charges occur?

QUESTION: 
The repulsion force is caused by the electromagnetic force between particles, this process involves (for example) to electrons meeting, one of them will exchange a photon, and somehow this results in the EM force which repels them. How? Does the photon push the other electron away and also is it absorbed by the electron that was pushed?

ANSWER: 
For a discussion of photon exchange, see an earlier answer.


QUESTION: 
If a planet/sun were considerably larger than it is, and the core was considerably more dense as a result, would the ommisions from the nuclear fusion at the core be of a shorter wavelength than gamma, or is gamma the shortest - or just the shortest known so far ??

ANSWER: 
First of all, "gamma ray" is a qualitative term, not a specific quantitative term, which refers to the most energetic electromagnetic radiation. Normally they come from nuclear reactions of some sort. Lower energy gamma rays from nuclei sometimes overlap what would be called x-rays if they came instead from atoms. Secondly, a gamma ray resulting from a fusion reaction has its energy determined by the reaction, not the environment (like density as you suggest). The energies for the gamma rays would be unchanged. It could be that some nuclear reactions would happen in a denser environment with greater probability, but the main stellar fusion reactions would be unchanged.


QUESTION: 
Why are strong and weak forces short ranged? Could they, under any circumstances change their range of action?

ANSWER: 
Because the quanta which mediate the forces are massive. The electromagnetic interaction is mediated by photons, massless particles, and is therefore long range.


QUESTION: 
Hi I'm a student at West High School, SLC, UT. and I've been struggling on a physics lab I need for school and I can't get any help from my teacher because he's not allowed to "assist" me in my labs, but he did say I could use an outside source. I recently did an experiment to prove the spring constant K using T=KX, weight and a spring blah blah. The lab itself would have been very simple if I had performed it any way but this. To measure the constant I collected varying weights (w) (dumb bells) of 3lbs (1.36kg), 5lbs (2.27kg), and 10lbs (4.54kg), but I couldn't get the weight to balance on the spring without using something externally to support it, compromising an accurate measure of force. So I decided that I would rest half of each weight on a raised surface and rest the other half on the spring. I measured the force using Newton's 2nd Law F=ma assuming ma to be half the weight total because only half of the total weight was on the spring. Well, when I finished taking my data I substituted F for T and simplified to find K, so F/K=K, and each of my answers were vastly different, not constant, and incorrect to the known constant 9.6. I looked over my lab and realized that the deflected spring would need to be exactly equal to my raised surface in order for the weight on the spring to be exactly half of the total. I would have done the lab differently but I didn't have time to retake my measurements, so I committed myself to find the error using my data. Now it's been almost a week and I've been racking my brain to find some sort of equation that will tell me the added weight on the spring, but to no avail. Question: So the question I have may be insanely simple, but I can't find it anywhere on the internet, nobody I ask knows, and this is the next step. I would just like to find a method or some equation(s) that can help me calculate the weight (force) when a single mass is being measured on two surfaces of different heights. So far what I've come up with is 1/2 w+wx where x is something relating to the difference of height between my raised surface and the deflected spring height or the tan/sin/cos of a theta measure, something along those lines. If I'm totally wrong I tried.

ANSWER: 
You have really not given me enough information regarding exactly how and where you attached the spring to the weight. If it was what I think of as a "dumb bell", two equal weights connected by a rod which is uniform; and, if you supported one weight by a horizontal surface; and, if you attached the spring to the other weight; and, if you took care that the spring was vertical for each measurement, then your assumption that half the total weight was stretching the spring would be correct and your data should have looked fine. (And, it does not matter what angle the dumbbell makes with the horizontal.) But, I suspect that what you did was attach the spring near the center of the dumbbells. Since the center of the dumbbell is close to center of gravity, this would lead to bad results because you have no idea what fraction of the weight was being held up by the surface. I am afraid that the best you can possibly do is plot your data (stretch as a function of weight of the dumbbell and hope that the data fall roughly on a straight line. If they do, then you should be able, since you know the spring constant which you are trying to measure, to deduce the approximate fraction of the weight held up by the surface. Probably, though, the fraction was different for each dumbbell, so your data are basically worthless. Sorry about that! Incidentally, you did not use Newton's second law because everything is at rest and the acceleration is therefore zero for this experiment.


QUESTION: 
Why is it when EM radiation is of long wavelength it is better to think of the radiation as consisting of waves, and when it is of short wavelength it is better to think of it as being made of photons? Also do photons travel in wave like motions?

ANSWER: 
I do not know what you mean by "
better to think of…" EM radiation is both particles and waves (duality, see earlier answers, 1 2 & 3) and the way it behaves depends on the experiment you do to observe it. It may be that shorter wavelengths are more often seen as single photons, for example light comes from atoms one photon at a time and gamma rays come from nuclei one photon at a time.


QUESTION: 
As the sun burns and turns its hydrogen into helium over time, is its gravitational pull increasing or reducing over time ??

ANSWER: 
Since the sun is radiating energy, its mass must be getting smaller. Therefore its gravitational field is getting weaker. Note that this is a very small effect, however, because the mass is so large to start with and a lot of energy takes only a small amount of mass.


QUESTION: 
During the proton proton chain reaction in the sun, neutrinos and gamma ray photons are released . . . How are the neutrinos and gamma ray photons charged (+ve or -ve) ?? And where do the electrons come from, (which annihilate with the positrons, to produce the gamma ray photons) ?

ANSWER: 
Neutrinos and photons are uncharged. When a star first forms it consists mainly of hydrogen which consists of protons and electrons. When the star gets hot enough, the electrons and protons separate and form a plasma (a mixture of unbound electrons and protons). So there are plenty of electrons. The star is electrically neutral, that is its net charge is zero. A positron will come from a reaction which converts a proton into a neutron, for example two protons goes to deuteron (one proton and one neutron, bound), a neutrino, and a positron.


QUESTION: 
It stands to reason that no matter how far we went into the depths of our universe we would never find an end or wall or boundary of any sort since its my understanding that space is infinity large. My question is that since space is infinity large wouldn't the inverse stand to be true as well. I imagine it like this If i have a piece of cheese and I cut it in half then again and again and again etc. At what point am I no longer able to cut that object in half? Assuming I had a device that was able to bisect an object of any size doesn't it stand to reason that I can divide something an infinite number of times? Is there ever a point where I reach an object which can bisected?

ANSWER: 
I believe that you are wrong at both ends, big and small. The universe is finite; its existence defines what space and time are and "outside" our universe there is nothing, no space, no time. There is no evidence that electrons or quarks, for example, can be bisected; you cannot bisect something an infinite number of times.


QUESTION: 
What is a dimension? Is it a sub-structure or a constituent of a universe? Is it itself a universe? And if so could our universe be a dimension of another universe, and also, could the four known dimensions of our universe be other universes, perhaps with life in them or intellectual beings? How have phsyicists come to the strong possibility that space and time are intertwined and there are four dimensions: space (3) and time (1).

ANSWER: 
Let us go back in time to the time of Ren
é Descartes (1596-1650). Legend has it that one day he was watching a fly buzz around his room and pondered the question of how many numbers it takes to specify uniquely where the fly was at any given instant. Of course, the answer he came up with was three (the distances from the floor, side wall, and front wall). Fast forward to Albert Einstein (1879-1955). His theory of special relativity clearly demonstrates that time is not a separate entity from the three dimensions of space, they are intertwined in a four-dimensional spacetime. One can extend the mathematic of three or four dimensions to higher dimensions and devise a complete mathematics of an N-dimensional space where N is any number; but is this just mathematics or is there some relation to the real world? Today physicists ponder whether there might be more dimensions to our world than we currently perceive and this leads to new theories of nature like string theory. So far there is no evidence for higher dimensions. Clearly, I cannot speculate on your questions of what higher dimensions might be since there is no evidence.


QUESTION: 
After the Big Bang where did atoms come from?

ANSWER: 
Protons and electrons were created by the big bang. When everything had cooled down sufficiently, they combined into hydrogen atoms. Over time these hydrogen atoms would coalesce into clouds which would, under the influence of their own gravity compress and form stars. These first stars lived out their lives creating heavier atoms until the stars had exhausted energy available from fusion and then exploded in events called supernovae which scattered and later became the stuff form which planets were formed. All the atoms in your body were forged inside stars.


QUESTION: 
Please explain exactly how magnetic and electric fields are produced. Aslo I have read that with EM radiation the electric wave when propagating produces a magnetic field wave and this then produces another electric field wave. Is this correct about EM radiation? If so how exactly does this work? Are the electric and magnetic fields already considered as propagating waves. What is the exact structure of EM radiation waves?

ANSWER: 
Two recent answers address your questions. First go here to read a discussion on what cause electric and magnetic fields. Next go here to read a discussion of what an electromagnetic wave is. Note that, as you suggest, an electromagnetic wave, once started, requires no sources (charges or currents) to keep it going since the changing electric and magnetic fields induce magnetic and electric fields respectively.


QUESTION: 
Will an object that initially sinks below the surface of water (such as in a lake or ocean) ever achieve a state of neutral buoyancy so that it will be suspended at some depth in the body of water and not continue to sink to the bottom? (this is not the same as the situation of a submarine achieving neutral buoyancy by adjusting its density through expulsion of water from ballasts). Assuming that an object retains its shape and density, does the density (not pressure) of the water change enough, or at all, with increasing depth so that the buoyant force on the object increases with depth, allowing some objects to achieve neutral buoyancy at some point in the body of water, so that the object will eventually be suspended?

ANSWER: 
Almost certainly not. Inasmuch as water is very incompressible its density will be almost the same at any depth and it is the density which determines the float/sink status. The density can vary with temperature and if the density of your object were extremely close to but slightly larger than the density of the water at the surface but equal to the density at some deeper depth (due to a different temperature there), it could happen.


QUESTION: 
Is there a formula or a way to calculate how fast a rigid, light weight, vessel, filled with just sea level air pressure, would travel upward at different depths of the ocean? In other words, if you made a crush proof, empty, sphere (5 ft in dia. for example) filled with just sea level air pressure, and somehow got it down to the bottom of the ocean, say at 1000 ft, then let it go, how fast would it travel at that depth and would it keep the same speed all the way up to the surface or would it slow down as it got closer to the surface?

ANSWER: 
This is essentially the same problem as calculating the terminal velocity of an object moving through any fluid. For a really detailed discussion of the problem of trying to compute the terminal velocity of an object in air, see an earlier answer. Because water is, for all intents and purposes, incompressible, I will assume that the density of the water is independent of depth; this is reasonable but not exact mainly because the the density does depend on temperature which decreases with depth. I will also assume that the viscosity which determines the frictional force is independent of depth, also a reasonable approximation but not exact. So, for water there is a force on a moving object due to friction moving through the water (depending on the viscosity of the water); this force points down because your object is going up. There is a force equal to the objects weight; this force points down. There is a buoyant force, equal in magnitude to the weight of the water displaced by the object; this force points up. The friction force gets larger as the speed gets larger and eventually the three forces add up to zero and the object moves up with a constant velocity which is called the terminal velocity. So a pretty good answer to your question is that it achieves a constant speed and continues all the way to the top with that speed. To make exact predictions is difficult even for the ideal case; to make corrections for viscosity and density variations would require detailed knowledge of the temperature vs. depth.


QUESTION: 
Suppose you could travel near the speed of light (99.999999...% c) and you fired a bullet while traveling at that speed, would the bullet travel at or above the speed of light?

ANSWER: 
The speed of the bullet would depend on who measured it. If the person who fired the gun measured it, he would find the speed to be the muzzle velocity of the rifle. If an observer who was being passed by the speedy space ship measured it, she would see the bullet going faster than the space ship but still less than the speed of light. Nothing can go faster than the speed of light; no material object may go as fast as the speed of light.


QUESTION: 
HOW CAN I MOVE A BRICK USING A MOUSE TRAP? I NEED SOME POINTERS ON HOW TO START MY PROJECT IT WOULD BE GREATLY APPRECIATED IF YOU COULD POINT ME IN THE RIGHT DIRECTION.

ANSWER: 
Archimedes said:
“ΠΑ ΒΩ ΚΑΙ ΧΑΡΙΣΤΙΩΝΙ ΤΑΝ ΓΑΝ ΚΙΝΗΣΩ ΠΑΣΑΝ.” Roughly translated, this is “Give me a place to stand and with a lever I will move the whole world.”


QUESTION: 
How would you explain centripetal force? What is centrifugal force?

ANSWER: 
I would define centripetal force as the force which acts perpendicular to the velocity of an object moving in a circular path. Since a particle moving in a circular path is always accelerating (because the velocity is changing because the direction is changing) this is the force which enables it to do so. For example, consider a stone twirling around your head on a string. The string exerts the centripetal force; if the string breaks the object no longer moves in a circle because the force disappears. Regarding centrifugal force, there is no such thing (technically speaking). It is what is referred to as a fictitious force. Sometimes, if we are in a frame of reference which is accelerating, we add these fictitious forces to make calculations easier. For more details on centrifugal force, see an earlier answer.


QUESTION: 
On the drive to pre-school this morning, my 4 & 1/2 year old son asked me why it gets colder the higher we go on/above the earth, when it is actually closer to the sun there. 

ANSWER: 
Well explaining this to a budding scientist who is so young will be a challenge, but there are few things more important than encouraging children to think and inquire and question. Important point number one is to get your son to appreciate how far the sun is from the earth. It looks like it is up there just out of reach, but the distance is 93,000,000 miles; to put this in perspective, the earth has a diameter of about 8,000 miles and so it is more than 11,000 earths stacked up to get to the sun. Now, how far is he ever likely to go above the earth? Maybe 40,000 feet, about 8 miles, in a commercial airplane. How does 8 miles compare with 93,000,000 miles? Pretty insignificant, eh? Here is how insignificant: If we had a really powerful electric heater 2000 feet (like half a mile) away we would feel some heat but how much more would we feel if we moved closer by one breadth of a hair? That is roughly equivalent to 8 miles compared to 93,000,000 miles.

Now, why is it colder as we go up? This is a little trickier for a four year old but the principle is the same which makes a refrigerator work, if a gas is expanded without any heat flowing into or out of it, it cools. At higher altitudes the air pressure is lower, the air is thinner (which is why it is very hard to breathe at the top of a very tall mountain, or why commercial airliners have to be pressurized). So, warm air near the earth rises and as it goes up it must expand because the pressure gets lower and very little heat goes into or out of it because air is a pretty poor conductor. So it cools.


QUESTION: 
Why does a magnegtic field moving in relation to any metal create an electric current, but the reverse is only true for a few metals. I realise that although I could explain in terms of some metals having domains etc. I don't really understand the relationship between electricity and magnetism - why do magnets cause a flow of charge, but a flow of charge only sometimes creates magnets?

ANSWER: 
In fact, the laws of physics are perfectly symmetric: a changing magnetic field causes an electric field and a changing electric field causes a magnetic field. The first of these is called Faraday's law and the second is part of Ampere's law. You seem to think that only a permanent magnet is magnetism. In fact, any moving electric charge causes a magnetic field. The most common source of magnetic fields is simply an electric current. Here are some facts about electric and magnetic fields:

  • electric charges cause electric fields,
  • electric currents (moving charges) cause magnetic fields,
  • changing magnetic fields cause electric fields, and
  • changing electric fields cause magnetic fields.

QUESTION: 
Now, in my understanding of the wave-this is a probability wave. the particle exists in any one of probalistic locations that are a wave matrix. From what I understand- a photon travels as a wave, but arrives or is located or "observed" as a particle (photoelectric effect etc). I also understand the h Plancks constant and that energy is continous but rather in discreet packets, hence quanta. My question is this. It almost sounds from the way quatum mechanics are described that the "probability wave" is just that, not a "real wave". But we have physical phenomena that make it obvious that this wave has physical attributes- spectrum, wave interference etc. So, how can once describe the "probability" wave also as a physical wave- which it must be because of our observations. Not to confuse things more, but the weird interference patterns that one gets by shooting a photon one at a time over a long period of time showing interference patterns, as though "the particle knows the other was there" type stuff. Would not the speed of light answer this seemingly strange result of interference pattern? ie time is different for the observer and the photon. 100 photons being released at once vs 100 released one by one over say 3 hrs- wouldnt the 3hrs be irrelevent with respect to the photon, or basically be the same thing as releasing them at the same time?

ANSWER: 
The wave function is not, to use your words, a "real wave", that is there is not some waving medium as, say, water waves or sound waves or light waves or
… It is incorrect to confuse the wave function of a photon with the electromagnetic wave which is the photon's alter ego. One of the most important differences between a wave function and a "real wave" is that the wave function is a complex function. Your point about the rate at which clocks run for the photon is an interesting one. However, photons do not have a "point of view" and do not carry clocks. I think that specialists in special relativity do not state that time stands still for photons.


QUESTION: 
It takes 23 hours 56 min & a few seconds for earth to complete one rotation.Take it as 23hrs & 57 min.So there is a loss of 3 min daily in the watch or clock.But our watches and clocks are of 24 hrs(or 12*2). So after 240 days ther is a loss of 12 hours ie, when it is 12 noon actually the watch or clock will show 12 midnight yesterday.It is not happening.Why is it so?

ANSWER: 
It is because the earth also travels around the sun in its orbit. Hence, the time for the sun to go from exactly overhead to that position again is not the time it takes the earth to rotate once on its axis. The 3 minute difference makes sense: [(3 min/day)/(24x60 min/day)]x[365 days/year]=0.76 days/year, about three fourths. That means we are in error by about one quarter of a day at the end of one year. Hence, we need to add one day every fourth year to make it all come out right. Since it is not exactly 3/4 we do not add the extra day once each century.


QUESTION: 
Is sunlight convergent, divergent, parallel or irregular

ANSWER: 
For all intents and purposes it is parallel (plane wave) because it is far away compared to the size of any experiment we might do. In fact, though, it is divergent since the wavefronts expand as spheres.


QUESTION: 
What is the energy balance(=Released from-Required for) in the process of nuclear-Fission of Hydrogen and nuclear-Fusion of Uranium.(Why Fission of uranium and Fusion of Hydrogen is so much talked about and not 'vice versa').

ANSWER: 
For nuclei lighter than iron, fission results in a loss of energy, not a gain; that is you must add energy to cause fission. For nuclei heavier than iron, fusion results in a loss of energy, not a gain; that is you must add energy to cause fusion.


QUESTION: 
1. I have red that in troposphere temperature increases with height, then why doesn't the ice and snow in mountains melt?
2.Light is an electromagnetic wave, then why doesn't it get ionized in the ionosphere?
3.the rest mass of a photon is zero, it moves in the velocity of light.then according to einstein's equation m=m0/(sq:root of(1-v2/c2))& v=c so denominator will become 0 so it will have infinite mass.if it was happening we will die due to infinite mass on us when light fall on us.Why is it not happening?

ANSWER: 

  1. Temperature decreases until about 20 km altitude, higher than any mountain. See Wikepedia for a graph of temperature in the troposphere.
  2. An electromagnetic wave has no electric charge, so how could it become ionized?
  3. I have previously answered this question.

In future, please abide by the site groundrules and submit single questions.


QUESTION: 
As I understand it, gravity is many many orders of magnitude weaker than other forces in the Standard Model. I heard one theorectical explanation for this is that maybe the force of gravity is diffused throughout higher dimensions so it appears relatively weak in the 3D universe we perceive. On a similar topic I understand that "dark energy" is also many magnitudes weaker than theories say it should be. What do you think of the possibility that dark energy is also diffused throughout higher dimensions, thus explaining why it appears weaker than it should be in our 3D universe? (If necessary I could look up the exact ratios of gravity to strong, weak, and electromagnetic force and I could look up some similar stats on dark energy predictions vs observations but I don't think this is necessary to answer my question.) I suspect this is a tough question so I'll donate some money too.

ANSWER: 
You should wait for an answer before you send money! You will probably not like my answers because I have little use for string theory or related multidimensional theories. It is not because I think they are stupid or wrong, in fact the ideas are clever and provocative. However, there is absolutely no connection at the present time between these theories and the reality of nature. Most string theorists will tell you that they have almost no hope of introducing any experiments which might test the validity of their ideas. It is certainly true that gravity is the weakest of nature's forces and by a long stretch. However, there exists no quantum theory of gravity and hence it has no real connection with other forces. Gravity is very well understood via the theory of general relativity. I am similarly not very sympathetic with the current use of the terms dark matter and dark energy as being some kind of statements of experimental facts since nobody has any idea what they are. It is certainly true that, given our current understanding of physical laws, experimental astronomical data suggest that there is much more matter in the universe than we observe directly as normal mass (dark matter?) and that the universe is expanding at an increaseing rate (dark energy?), but we do not really understand why these things are happening. Regarding "dark energy", I know of no accepted "theory" of it. When Einstein developed general relativity he believed that the universe was static and so he inserted something called the cosmological constant which would have the effect of a repulsive force to balance the gravitational force trying to collapse the universe. He later abandoned this (calling it his biggest blunder) when Hubble showed the universe to be expanding. The recent experiments showing the accelerating expansion have led some cosmologists to reintroduce the cosmological constant which is the only "dark energy theory" I am aware of; in any case, the constant is empirical and not really predictive of anything. My own suspicion is that these anomalies tell us that we do not really understand gravity as well as we thought we did.


QUESTION: 
you are standing on a bathroom scale in an elevator when suddenly the cable breaks and he elevator fakks freely down the shaft. how does the force of gravity that Earth exerts on you change over the same time interval?

ANSWER: 
The force of gravity does not change at all. The force of gravity is called your weight. When you are free falling you feel weightless but are not. (Actually, if you want to split hairs, the force increases slightly because you are getting closer to the center of the earth but this is really a small effect, on the order of 0.001%.)


QUESTION: 
Does an electron moving in its orbit take its entire charge along with it or does its charge gets spread in some fasion throughout its orbit?

ANSWER: 
It is an oversimplification to think about literal electron orbits. See the answer to a previous question. The answer is that the charge, like the electron itself, is smeared out around the atom.


QUESTION: 
I "understand" the concept that if a person accelerates close to the speed of light (away from earth) and then returns that the traveler will have (in essence) traveled into the future when he has returned to Earth. My question is very elementary. Assuming that a craft could be build that could accelerate a person to (close to) the speed of light, could a human being physicially withstand the forces of acceleration necessary to move him from standing still to "light speed," decelerate or turn around, return to earth and decelerate to allow reentry into the Earth's atmosphere? Or, is the speed of light so fast that a human being could not be accelerated to this speed in the course of a normal lifetime?

ANSWER: 
This is an excellent question and one never considered by writers of sci-fi movies. "Take her to light speed, Scotty" is accomplished almost immediately and, with the required acceleration, everyone aboard would be flattened to the floor to bloody pancakes. To put it in perspective, let's calculate the time to accelerate to just half light speed with an acceleration of 5g which is about 50 m/s2. Even this acceleration would be extremely uncomfortable while not fatal. So, v=at=50t=1.5x108; solving for the time I find t=3,000,000 seconds, about a month. This is one of the serious barriers to space travel outside the solar system.

FOLLOWUP QUESTION: 
Regarding the question about the effects on the human body on acceleration to near light speed: If 5G acceleration would get a space ship to near light speed in about a month, does this mean that 1G acceleration would reach the same speed in about 5 months? If so, could you use the 1G to simulate gravity on the ship with no adverse effects? There's obviously a lot of problems with this idea such as fuel, mass, etc. But if you feel like answering it, please do so. If you think this is a dumb question, it's because I'm not very good in math. I just assumed that 5G is 5 times more than 1G. But maybe it's not because there's an exponent in the equation?

ANSWER: 
First, half light speed is not "near light speed". Relativistic effects do not become really significant until you get to more like 90% the speed of light. Acceleration is tricky in relativity because it is obvious that, if you cannot exceed (or reach) the speed of light, then it is not possible to maintain some constant acceleration for an indefinitely long time because you would accelerate right past the speed of light. That is one of the reasons I chose to only accelerate to half light speed in my example
—so I could do a rough classical calculation without too much error. Since (approximately) v=at, the answer to your question is that yes, if you reduce the acceleration by a factor of 5 you would increase the time by a factor of 5. As you note, the source of energy to accelerate to here this speed, not to mention the amount of energy to get significantly closer to light speed, is a real problem. If you want to pursue this some more I would suggest looking at an earlier answer.


QUESTION: 
As I understand it, the P in PET scan stands for positron, which I further understand is a form of antimatter. If matter and antimatter come into contact with each other, don't they annihilate each other? If so, how are positrons isolated from the matter in the scanning equipment, our bodies, etc?

ANSWER: 
The annihilation is what makes this imaging possible. The source of positrons is a radioactive nucleus which undergoes positive beta decay. A solution which contains these emitters is injected or injested and when a positron is emitted it essentially immediately finds an electron in an atom of your body with which to annihilate. When this happens, two photons are emitted; these photons are detected and their trajectories are traced back, using a computer, to where the annihilation took place and images are constructed from many such events.


QUESTION: 
I notice that sometimes there is no any or very weak sound while I open a new sealed bottle of carbonated water? But more frequently, that sound is very loud. Why is that happened, even it is the same type of water and the same manufacturer?

ANSWER: 
Carbonated water is essentially water with carbon dioxide disolved in it. When it is bottled the little bit of air in the top is at atmospheric pressure. If everthing is the same as when it was bottled, when you open the bottle there is little fizz. But, if you shake it up it causes the carbon dioxide gas to go out of the water and into the little pocket of air at the top of the bottle and the pressure in that air therefore increases because there is more gas in the same volume. Now if you open it there is a fizz as the gas escapes. Also, if you store it in the refrigerator it will tend to fizz when you open it because cold water can hold less carbon dioxide than warm water.


QUESTION: 
Suppose John and Matt are on opposite ends of an extremely long plane, say 500,000 miles long. Both of them are holding onto one end of any straight, taut rod or rope. John then pulls on his end of the rod/rope. This is where I am confused about what would happen. I cant be sure about this but I am under the impression that Matt cannot get instant feedback on John's pull because that would mean some kind of information was being transferred at speeds greater than the speed of light, which is clearly not possible. If Matt's end of the rod can't shift instantly away from him then what happens to the rod?

ANSWER: 
I have previously answered this question (actually for a push instead of a pull but the idea is the same). The crux is that no rod or rope is unstretchable or incompressible and when you pull on one end you are pulling the atoms apart a little and this stretch is transmitted like a wave (think of a slinky) along the length of the rope or rod with the speed of sound in that rope or rod.


QUESTION: 
If you stopped an Earth satellite dead in its tracks, it would simply crash into the Earth.Why, then, don’t the communications satellites that hover motionless above the same spot on Earth crash into the Earth?

ANSWER: 
A communication satellite just appears to be standing still because it stays exactly above a point on the equator. The point on the equator goes around once every 24 hours and so does the communications satellite. Here is a cool site where you can see the locations of the satellites. The ring of satellites about 6 earth radii out are the communication satellites.


QUESTION: 
If i was to drop a ball at say 30,000ft in a plane, would it travel faster because it has more gravitational potential energy?

ANSWER: 
You cannot judge how fast something will go by its potential energy. I believe you are asking the wrong question since something dropped does not simply travel with some speed, rather it speeds up with some acceleration. So let's slightly change your question and ask whether the acceleration at 30,000 ft would be larger than the acceleration at the surface of the earth because the potential energy is larger up there. The answer is no, the acceleration would be approximately the same; if you have ever tried to play catch in an airplane you would agree that it is just the same as on the ground (of course the plane must be traveling in a straight line with constant speed). Now I will take it one step further: in fact the acceleration at 30,000 feet will be very slightly less than at the surface because the force, which causes the acceleration, is slightly smaller because the distance to the center of the earth is slightly larger. This difference would be difficult to observe because this altitude is very small compared to the radius of the earth (which is about 4000 mi compared to about 5 mi for 30,000 feet). However, if your plane were at an altitude of 4000 miles the acceleration would be about 1/4 what it is on the earth (the gravitational force is inversely proportional to the square of the distance).


QUESTION: 
I would like to know if there is a theory(s) to a concept I've been thinking about, and would like to apply theoretically to another area of study if it can be explained. Here goes: Let's say I have a sharpened pencil that is sticking point side up at exactly 90 degrees. Hypothetically, I believe, I could take a dinner plate and place it on the pencil point at an exact point below the plate and it would balance. (To keep things simple, we'll say that it's a circular dinner plate.) I'm now going to start to add food to that plate. Again, with each item of food I place on the plate, I could move the pencil below it and it would still stay balanced. What is this called, and is there an area of study that I could learn more about it?

ANSWER: 
There is no "theory" per se, it is just part of elementary classical physics. Every assemblage of mass has what is called the "center of mass" or "center of gravity". It is straightforward to calculate where this point is; it is like calculating a weighted average. For example, if you have two balls, one twice the mass of the other and 3 meter apart, the center of mass is 1 meter from the heavier mass (and 2 meters from the lighter mass). Once you find the center of mass, then the assemblage of masses will balance about a point directly below it. Here is a proviso, however: in the real world it does not work because you cannot place the "pencil point" with sufficient accuracy and because you cannot know where the center of mass is because of the Heisenberg uncertainty principle. Note, though, that you can balance a stick on your hand if you can move your hand around.


QUESTION: 
Is Space-Time quantized? –i.e. not a continous variable in Quantum Physics like Mass-Energy?

ANSWER: 
I have previously answered this question. Your statement about energy being quantized is not necessarily true: energy is quantized for bound systems but not for unbound systems.


QUESTION: 
As I understood well from the "Standard model of Particle Physics", the attraction or repulsion between masses are in fact an interchange of bosons. E.g. the electromagnetic force (or interaction) is in fact an exchange of fotons between the "charged" particles. What I do not understand is the attraction between opposite charges or repulsion betwen like charges. How do fotons "do" define an attraction or repulsion. Probably the same answer applies for gluons or W and Z bosons. Can yo uplease help me understand this mechanism?

ANSWER: 
The idea of particle exchange is only a qualitative, "cartoon" way of trying to represent quantum field theory. I had an earlier answer which discusses all this is some detail.


QUESTION: 
When we split an atom we can produce a nuclear reaction, which can be destructive in the case of a bomb or constructive in the case of energy production. If we were to collide and smash atoms together, in the case of CERN’s experiment in Europe, I would like to know from your expert viewpoint (or opinion), why would we not expect the same destructive outcome as splitting an atom? In each case the use of force is being applied to the atom. What is the difference between the two actions of “splitting” and “collide and smash” in that splitting causes a nuclear reaction and the other anticipates safe observations?

ANSWER: 
There is nothing at all destructive about a single nucleus undergoing fission. Nuclear fission is often characterized as releasing a large amount of energy. What this means is that a significant fraction of the mass energy is converted into heat (and other) energy. But "significant" is like only 1% of the original mass, so if the mass of the nucleus is say 5x10-25 kg then the energy released by its fission would be about mc2=(5x10-25)(3x108)2/100=4.5x10-10 J. To put this into perspective, 1 J is approximately the amount of energy required to lift 1 kg to a height of 10 cm. So, any single event is tiny in absolute terms. A bomb or a power plant works because a huge number of nuclei participate. Furthermore, at the LHC things go the other way: particles with very high energies are smashed into each other to create new particles, energy is turned to mass rather than mass to energy.


QUESTION: 
If a supernova kicks out a proton and that proton were to enter earths atmosphere and the hit a person, What if anything would happen to him.

ANSWER: 
A single proton will do no significant damage to a person regardless of where it came from or how fast it is going.


QUESTION: 
The naturalist, Steve Grand, has said: "Think of something from your childhood. Something you remember clearly, something you can see, feel, maybe even smell, as if you were really there. After all, you really were there at the time, weren't you? How else would you remember it? But here is the bombshell: you weren't there. Not a single atom that is in your body today was there when that event took place. Matter flows from place to place and momentarily comes together to be you. What ever you are, therefore, you are not the stuff of which you are made." If this is true, can you explain the fundamental physics behind this atom transfer or exchange?

ANSWER: 
This boils down to the well-known biological fact that our bodies are constantly discarding cells and replacing them with new ones. One such case is one which I am familiar with since I have osteoporosis: the bones in our bodies are constantly having bone removed and bone built and, if the rates get out of synch the result can be long-term bone loss. After our initial growth, for the most part the process is one of replicating what is already here. In that sense it is not surprising that we still remember events from our childhoods since the neurons responsible for the memories have simply been copied; it is kind of like copying the data on the hard drive of one computer onto a new computer. The statement that "[n]ot a single atom that is in your body today was there when that event took place" is certainly inaccurate, however. See my earlier answer estimating the number of atoms your body must share with atoms which were once part of somebody else; the same arguments could be made vis á vis atoms coming back to you after you lost them.


QUESTION: 
This is a question about the Large Hadron Collider. Those who don't think the experiment will produce "killer black holes" point to the fact that we are still alive: since such collisions occur all the time, killer black holes would have already been produced. But if the experiment being conducted is something that already happens in nature, why is there a need to perform it? Couldn't we get better results by observing the effects of particle collisions that occur naturally?

ANSWER: 
In a recent New York Times Op-Ed piece by Brian Green he points out that the earth is being constantly bombarded by cosmic rays with far more energy than available at the LHC, and no black holes have caused any damage during the several billion years the earth has been here. I have previously answered the question regarding why we don't just use cosmic rays instead of building a multibillion dollar accelerator.


QUESTION: 
if on an airplane at the point of takeoff a passenger stands up and jumps in the air would he be pushed to the back of the plane or would he come straight down?

ANSWER: 
At the point of takeoff the plane is accelerating forward. If that is the case, the passenger would appear to be pushed back (but in fact the back is accelerating toward him).


QUESTION: 
why can't the fastest cars accelerate for more than a few seconds?

ANSWER: 
The main reason is that there is an upper limit on the rate (rpm) that the engine can operate. Also, the air resistance experienced by the car increases rapidly as speed increases.


QUESTION: 
since hydrogen is the most common element in the universe, do we have a system of measure based on its width and weight? and thus a universal language?

ANSWER: 
First of all, an atom does not have a well-defined size (it is like a "fuzzy" object), so you cannot have a length unit based on its "width" since it doesn't have one. However, one can take a length from the Bohr model of the atom characterizing an atomic size (an approximate radius of the atom) and use it as a length unit; this length is called the "Bohr radius" and has a value of 5.292x10-11 m. Although one could base an atomic mass scale on the hydrogen atom mass, it is not done because it is not convenient to measure since hydrogen likes to form H2 molecules. The atomic mass scale (the unit of mass is called the amu) is instead based on the mass of 12C; one amu is defined to be 1/12 of the mass of a 12C atom. As you see, this is very close to (but not identical to) the mass of a hydrogen atom, so your idea is close to what is actually done.


QUESTION: 
I am a physics student trying to undertand QFT. I have a basic question. what is the physical interpretation of a quantum field? I mean, in terms of quantum field theory it seems like a field is just an operator in a hilbert space, right? Does it actually represent a physical object?

ANSWER: 
Any question about quantum field theory is too technical for the purposes of this web site which is meant to answer laypersons' questions. However, we can address the question of whether the field is "real" or just some kind of mathematical construct. In introductory physics courses we often say that the field is just a representation of forces present, in a sense you could say it isn't "real". However, in more advanced studies we find things like the field actually has energy content, for example an electromagnetic field has an energy density. Thus, a philosopher or logician might say that such an entity is indeed "real". I would never refer to it as a "physical object", however.


QUESTION: 
Conversion of mass to energy (fission) has been demonstrated many times in laboratory and field tests. Has conversion of energy to mass also been demonstrated in laboratories?

ANSWER: 
Yes. A couple of examples:

  • A very energetic photon (massless) can spontaneously turn into an electron-positron pair; this is called pair production.
  • The mass of a nucleus is always less than the sum of all the constituent proton and neutron masses. Suppose you remove a neutron from a nucleus; it will take work because that neutron is bound in the nucleus. Hence, the final system of neutron and the original nucleus minus one neutron has a greater mass if both objects are at rest. So let’s just say that the nucleus, having a mass smaller than the sum of its parts, is an example of converting energy into mass because there is more mass after you disassemble it by adding energy (doing work).

QUESTION: 
if you could bore a hole through the centre of the earth to the other side, then dropped something like a pebble into it, what would gravities affect on it be as it neared and reached the centre? Would it eventually balance out and reach an equalibreum and just be suspended in mid air so to speak?

ANSWER: 
When at the exact center the pebble would have zero force on it, that is if you put it there it would just sit there at rest. If you dropped it into the hole, the stone would be going very fast when it got to the center and, neglecting air friction, would go all the way to the other side of the earth before coming back. With air friction it would eventually lose all its energy at be at rest at the center. For a little more detail, see two (#1 and #2) previous answers.


QUESTION: 
If you take a band-aid in its sterile paper packaging into a dark room and quickly pull the sterile paper cover apart by the tabs a green light appears at the point where the adhesive is separated. This can be repeated many times. What is the particular nature of the adhesive bond that emits light in the visible spectrum when the bonds are broken?

ANSWER: 
I suspect you are simply seeing the arcing of simple static electricity. For example it is well known that pulling a piece of scotch tape off what it is stuck to will result in the tape having a net electric charge.


QUESTION: 
Does the earth gain mass from sunlight? Plants (and animals I guess) convert energy from sunlight into matter, right? Does this enlarge the Earth's mass over time?

ANSWER: 
Yes. I estimate roughly that if all the energy from the sun striking the earth were absorbed the mass of the earth would increase by about 1 kg per second. But, the mass of the earth is about 6x1024 kg, so this is pretty negligible. Also, not all the energy stays here but the earth radiates some of it away. I would expect the mass gain from meteorite collisions to be bigger than from the sun's energy. It is not necessary to have plants and animals do the conversion. A rock warming up would have (very small) mass gain.


QUESTION: 
The circumference of the earth at the equator is 24,901.55 mi, but, if you measure through the poles it measures 24,859.82 mi. Why is it so? Is this difference due to rotational (centripetal?) force? Would this also explain the difference in gravitational accelleration (g) at sea level in different parts of the world?

ANSWER: 
The reason is believed to be because of the earth's rotation. Think of a sphere which is rotating; if the sphere is deformable it will tend to stretch out in directions perpendicular to the axis of rotation, kind of like the forming of pizza dough. It is said that the earth is an oblate spheroid with a negative quadrupole moment rather than a prolate spheroid (like an American football) with a positive quadrupole moment. Although this has an effect on g, I strongly suspect that g is more strongly affected by local variations of the mass density of the earth.


QUESTION: 
Is light emission a contiuum/continuity? Or is it the speed of light particles that causes it to appear as such? Do these particles connect to create the continuity.

ANSWER: 
You two should read earlier answers (duality1, duality2, duality3)and all the links in those answers.


QUESTION: 
A photon has energy, yet it has zero mass. If E=MC^2, how is that possible?

ANSWER: 
I have answered this question several times before.


QUESTION: 
If an object hits another object that is at rest, is it possible for the sum of their speeds after the collision to be more than the initial speed of the original object that hit the object at rest?

ANSWER: 
Why do you ask about the sum of the speeds? This is really not a very meaningful quantity to look at. However, let me give you an example which shows that the answser to your question is yes. Suppose that a very massive object collides elastically (which means no energy is lost in the collision) with a very light object at rest; imagine a bowling ball hitting a BB. After the collision the speed of the bowling ball is almost the same as what it was when it started and the speed of the BB is almost twice the bowling ball's initial speed.


QUESTION: 
If matter has special qualites at the atom and sub partical levels, such as quantum state. And if matter has different qualities at the macro level, the stuff we interact with. Then does matter take on different properties at the jumbo level/galatic level, such as star systems and galixes. What I'm trying to ask does matter act differently when you take a jumbo view. The analogy would be us observing matter on a atom level.

ANSWER: 
Perhaps what you are asking is whether the laws of physics as we know them, particularly of gravity, are valid at extremely large distances (say across the whole galaxy of between distant galaxies. Many astrophysicists think we understand gravity well enough to say yes. My perspective is that it has not really been tested at these scales.


QUESTION: 
The door of a domestic microwave oven is usually made up of a glass panel with wire mesh embedded in it. Why microwave couldn't go through the holes on the wired mesh? What physics phenomenon is involved. Is there any simple (or empirical) relationship between the shielding effectiveness of the wire mesh with the wavelength of microwave and the dimension of the holes on the mesh ?

ANSWER: 
Shielding of electromagnetic waves can be effective if the grid spacing is small compared to the wavelength of the waves. The wavelength of microwaves is around 12 cm.


QUESTION: 
It makes sense to me that the light I see when I look at our sun is something like 8 minutes old. I do not understand how we can look back toward the big bang and see light that is some billions of years old. What was that light doing while we were travelling away from the point of the big bang?

ANSWER: 
If it makes sense to you that light from something 8 light minutes from you arrives here 8 minutes after it left, why does it not make sense that light from something a billion light years from you arrives here a billion years after it left? This is not light from the big bang but from things existing long after the big bang.


QUESTION: 
From an atomic point of view of a living cell, let's just assume that that cell has X number of atoms (huge number, of course), when the cell duplicates the new cell will have an X/2 number of atoms or the number will be the same? Does that mean that a living organisme can create more atoms?

ANSWER: 
This is really biology, not physics. A dividing cell will get the needed materials for growth from its environment but I do not really know how much it gets before and how much after the actual cell division. One thing for sure, though: the cell does not create new atoms although it will "manufacture" new molecules from the atoms in its environment.


QUESTION: 
Assume you have a steel bar of a known mass, would the mass of the bar change if the bar is strongly magnestized?

ANSWER: 
Does it take energy to magnetize the bar? Yes, of course; let's call that energy E. Therefore the mass of the bar will increase by an amount
Δm=E/c2 where c is the speed of light. This change of mass is so small that you would not be able to measure it.


QUESTION: 
A woman flying in an airplane executes a maneuver where she is upside down. Does her hair hang down toward the ground or remain naturally unchanged?

ANSWER: 
It depends entirely what maneuver is being done. If she is just flying at a constant speed horizontally but upside down, her hair will hang straight down. If she is at the top of a loop going fast enough, it will not hang down.


QUESTION: 
What is the difference between matter and energy? Are they really the same thing as expressed by E=MC^2?

ANSWER: 
Mass is simply a form of energy.


QUESTION: 
A cursory search of the answers you've given to seemingly related questions has not yielded an answer to mine: Exactly what physical grounds are there for the oft-televised claim that "gravitational waves" propagate at the "speed of light", c? (please pardon me, if I have, in fact, missed an earlier answer from you on this point.) I suppose anyone would find the expression "c = sqrt(1/(epsilon0*mu0))", which relates the electric "permittivity" and magnetic "permeability" of "space" to the speed of light, to be aesthetically pleasing. Though not necessarily intended as such, in it I find a kind-of explanation for why light, termed an "electromagnetic phenomenon", travels at a speed that depends upon constants related to electricity and magnetism in a very straight-forward way. No such tie, however, in the expression for c (for example, something involving G0, maybe) even hints to us that the propagation speed of "gravitational waves" has anything to do with that of light - or that it can even be calculated at all! Can you help with the math/history that applies to my question?

ANSWER: 
Since nobody has ever directly observed a gravitational wave, their speed has never been measured. What we know is that the speed at which a gravitational field propogates is very fast; for example, we do not need to correct for the time for the gravitational force to "travel" to an object to calculate its orbit. It is generally assumed that the speed of propogation is the speed of light but that has never been accurately measured as far as I know. In electromagnetism the speed of propogation of fields is the same as the speed of electromagnetic waves, so it reasonable to assume the same would hold for for gravity; it is, however, not proven by experiments. See my earlier answer on this topic.


QUESTION: 
Why does a milkshake suspended 4-6" off of the table, initially attached to its mixer, spray milkshake upwards of 5ft into the air when accidentally dislodged from the mixer and falls the 4-6" to the table below? This student works at a milkshake stand and is perpetually surprised by the height of the explosion given the short fall.

ANSWER: 
The answer to this question is essentially the same as that to an earlier question which asked about the splash from a diver going higher than the height from which he dived. The energy of a large mass is given to a small mass and therefore the large velocity.


QUESTION: 
Why does light travel at 186,000mps and not some other speed?

ANSWER: 
This question has been previously answered.


QUESTION: 
I am wondering how hubcaps can have an outer decorative plate that spins at a lower rate than the wheel.

ANSWER: 
The kind I have seen are free-wheeling, that is they spin independently. When you start moving forward they tend to stay fixed but the small friction in their axels makes them eventually spin up to the speed of the rest of the hubcap. But when you slow down they tend to keep spinning for a while.


QUESTION: 
My question is regarding Solar panels. The most efficient solar panels currently are only about 22% efficient. Now assuming that we could create a more efficient solar panel to, say, about 85% efficient, wouldn't taking all that heat out of the air then ultimately result in us cooling the earth down? And if that is the case then wouldn't that be a way to fight global warming?

ANSWER: 
Although I could argue a lot of technicalities, all I need to note is that there is never going to be a significant fraction of the earth covered by solar panels (think of the oceans) for there to be any effect at all on global warming, even if they were 100% efficient.


QUESTION: 
My friend and I are having a debate. He is a pilot, and he believes that when he is climbing at a constant rate, he would feel slightly heavier. Likewise, if he were decending, he would feel slightly lighter. I believe that, at a constant rate of climb or decent, he would not experience any difference, regardless of the rate.

ANSWER: 
Provided that the plane is traveling in a straight line the apparent weight is unchanged. This is the physics answer because there is no acceleration and so all the forces must add to zero and the force which the pilot's seat exerts on him must therefore be equal to his weight and this force is that by which we would judge apparent weight. In an upward accelerating elevator you feel heavier because the force of the floor up on you must be greater than your weight. But now there is a technicality we need to worry about. How is your friend actually judging how heavy he feels? If it is by the force the seat exerts on his butt then he is right because some of his weight is being held up (if climbing) by the back of the seat so, placing a scale under his butt would result in a scale reading less than his weight if he were climbing. However, no physicist would use this as a measure of apparent weight.


QUESTION: 
Often cooking recipes say to "cover and let simmer". I don't see why covering a boiling liquid would have a reducing effect on the rate of evaporation. True, vapor condenses on the cooler top of the dome of the lid and falls back in to the main reservoir but isn't the liquid to gas conversion rate determined by the energy input, and should not the gas escape (assuming the seal is not air tight) at the rate it is generated? In fact, by putting a lid on the system, aren't you concentrating the heat in the liquid and increasing the temperature in the liquid state, causing a increase in the gas production? Yet, remove the lid and the dish dries out.

ANSWER: 
Covering the pot inhibits the steam from escaping. As you note, it condenses and returns. Also, the evaporation rate will be lower because the air in contact with the surface of the liquid will have more steam than if the lid were off. Regarding your "concentrating the heat" argument, keep in mind that the recipe calls for simmering and you have to adjust the heat such that it simmers when covered. As you probably know, it takes a lower fire to simmer a covered pot than an uncovered one.


QUESTION: 
I have a metal drinking bottle with a sippy cap on the top. There is also a small air hole that lets air in when you drink. To drink from this bottle, you must turn it upside down (i.e. the cap is not a straw that goes to the bottom of the thermos). When the bottle is full of water, it is muchharder to sip from it, than when it is half full or almost empty. I assume this is because of the great difference in density between water and air...but can't figure out why the density matters. Can you explain why this is happening?

ANSWER: 
The key is not the density, it is the compressibility. I cannot picture the "small air hole that lets air in when you drink", but let's ignore that since air most likely does not go in at the rate that water goes out (or that you want it to go out). So, suppose you pull out a quarter cup of water; the air in the bottle must increase in volume by the same amount. So if there is no air in the bottle you end up with a vacuum in a quarter cup of the total volume of the bottle, very hard to create. If there is a quarter cup of air to start with, then you double the volume which is easier to do than to create a vacuum. If there is a cup of air to start, then it is easier still to increase its volume by a quarter cup. And so forth


QUESTION: 
We have a swimming pool that needs a new liner. We'd like to select a liner that helps warm up the pool water as much as possible, but we're arguing about which color will help the temperature the most: I say that a black liner will trap the sun's heat more effectively. He says that a white liner will allow the sun's rays to bounce off the bottom and thereby would be able to heat the water on the way down as well as the way back. Our pool is 8 feet deep at the deepest part.

ANSWER: 
Because the water is very transparent to visible light, this means that very little energy is left in the water by the light passing through. However, if you efficiently absorb the light with a black pool liner, the liner itself will warm up and transmit energy to the water both by conduction and by radiation of infrared energy (heat). So, black is better if heating the water is what you want.


QUESTION: 
I know these wont work, I am just curious as to why. I am no expert in mathematics, quantum mechanics, or astrophysics but I am very interested and curious about those subjects. I just had a few thoughts on faster than light travel: Why won't a beam of light be traveling faster than, well... the speed of light if it is emitted from a source already traveling at any speed. Like headlights on a car, if a car is traveling 50mph than isn't the light being omitted by the car's headlights traveling at the speed of light, plus any added speed by the car?

Suppose there is an extremely long pole (or just a long anything) in a vacuum, and it could somehow pivot around point that lets say splits the pole so that 10% is on one side of the pivot and 90% is on the other. Lets further say that on the short end of the pole (10%) there is some sort of propellant mechanism that pushes purpendicularly the pole around the pivot. Wouldn't the engine only have to go a fraction of the speed of light to make the farther end travel greater than or equal to c? like how the inside of a record spins slower than the outside, just on a massive scale.

ANSWER: 
Actually, the first question you ask, that the speed of light is independent of the source should not be puzzling because this is the way most waves behave. For example, if a car is coming toward you blowing its horn the sound waves pass you with the same speed as if the car were standing still. What is surprising is that if you are running toward the source with some speed you still measure the same speed of light as if you were standing still. So, if you move toward a lightbulb with a speed half the speed of light you measure light with the speed of light, not 1.5 times the speed of light. Now this is often "explained" by saying that this is just the way things are, this is the basic postulate of special relativity. But you do not have to do this because there is a simple reason why all observers see light moving with the same speed. I have discussed this in an earlier answer.

Your second question is an impossible situation. Suppose we just look at the last 1 inch of the pole. In order for it to accelerate you need to exert a force on it. What exerts that force? It is the stick itself, the only thing touching it. But, to accelerate it to speeds close to the speed of light would require forces so big the stick would break. To accelerate it to ths speed of light would require an invinite force. So the stick would have to be infinitely stiff (impossible) and you would have to exert an infinite force on the other end of the stick (also impossible).

In future, please submit single questions as required by the groundrules of the site.


QUESTION: 
My colleagues and I have a debate about the "speed of light" actually being the upper limit of speed beyond which no matter can travel, based on Einstein's theory E=MC^2. To help in this debate, I posed the following scenario that we are currently thinking about before we continue and would appreciate your feedback. Scenario: For the sake of simplicity, assume that we have a universe consisting of a perfect vacuum and a single electron travelling at a constant velocity V. Upon this electron sits a "tiny man" with a "pocket rocket" which he periodically fires for a brief period, say 1 second, to cause the electron to accelerate by 1 m/s^2 for the duration of the rocket firing. This is the only periodic force acting on the electron in this universe. Questions:

  1. Would the velocity V of the electron increase and remain constant at the increased value following each firing?
  2. Would the rocket have to exert the (a) same or (b) an increasing amount of force on the electron with each firing to increase its acceleration by 1 m/2^s in the vacuum? (i.e. if the rocket were to exert N Newtons of force, would each firing still need to exert N or N+x Newtons where x is increasing?)
  3. Assuming the answer to 2 is that the same force of N Newtons is needed, what would change in the universe, based on Einstein's theory, that would result in 0 m/s^2 acceleration upon firing of the rocket while the speed of the electron is equivalent to, or just below, the speed of light?
  4. Assuming the answer to 2 is that N+x Newtons of force is needed, (a) what is the reason for this, and (b) why would it not be possible for the electron to exceed the speed of light in this universe?

ANSWER: 
To really understand this you should read my earlier answer on relativistic momentum. The crux is that the correct form of Newton's second law is that force equals the time rate of change of linear momentum and linear momentum is no longer mass times velocity but rather p=γmu where γ=1/√[1-(u/c)2], u is the speed, m is the mass, c is the speed of light, and p is the momentum. So, you see as u approaches c the momentum becomes infinite so the force it takes to change it approaches infinity. Although acceleration is not a very useful quantity in relativity, you can write Newton's second law in a form that sort of looks familiar if you are used to F=ma; it becomes F3ma. You can see why a is not very useful because the force depends on both a and u. So, clearly, to have an acceleration equal to one, F will have to get bigger as u gets bigger and F will approach infinity as u approaches c. And, at very high speeds, it is not like you can use a big force and apply it for one second, say, because as soon as a nanosecond has elapsed a much larger force will be required to have an acceleration of 1 m/s2. So you cannot periodically apply a force which results in the desired acceleration in one second bursts, that is the force would have to be increased as you apply it. Answers to your specific questions:

  1. Yes, if you applied a force which would give an acceleration of 1 m/s2 at the beginning of the 1 s, the speed would increase and remain constant until the next burst. However, as you approach c the final speed would not increase by 1 m/s with each burst and really close to c the increase in speed would be undectably small. However, the energy of the electron would increase appreciably even though its speed would not. (I have always thought that high-energy accelerators are not well named since very little acceleration goes on.)
  2. I think this is answered above; F would have to increase with the speed.
  3. There is no answer since the same force is not the answer to 2.
  4. Again, the discussion above answers this.

There are other ways to show that the speed cannot exceed or equal c. Usually physicists show their students that the energy required to accelerate an object to c is infinite.


QUESTION: 
People are claiming they can run a car more efficiently by getting its engine to break down water into hydrogen and oxygen, and burning the hydrogen to produce...water! Wouldn't this break the first law of thermodynamics if it were true, and make perpetual motion possible? Is there any other way an increased efficiency could be got down this route?

ANSWER: 
You are right, it takes energy to dissociate the molecule, the same amount of energy you would get by putting it back together. The best you could do is break even and, of course, with real machinery you could never get out as much work from the engine as you had to invest to make the hydrogen.


QUESTION: 
If a person was standing still and for example a jet passed him at 1000 mph and at the same instant a weapon was fired in the opposite direction travelling at 1000 mph, would the person standing still see what appeared to be a stationary object in front of him? Also if he reached up to grab the bullet would it cause damage because the bullet would still have energy behind it?

ANSWER: 
I presume it is someone in the jet who fires the weapon. The answer is that the observer on the ground would see the bullet falling straight down (starting from rest at the time it left the weapon). The only damage would be that the bullet would be hot from its firing and could burn whoever caught it. Kinetic energy is not a conserrved quantity here, that is the bullet starts with zero kinetic energy to the observer on the ground but a high kinetic energy to the observer on the jet.


QUESTION: 
the speed of light in vacuum, C, is approximately 3 x 10^8 meters per second. In some materials, such as inside a diamond or a length of coaxial radio transmission cable, C is said to be less than 3 x 10^8 m/sec (velocity factor). If C is slower in those materials where do the photons get the energy to return to C(vacuum) once they exit the medium? Is it actually that C does not change and that the photons are refracted and simply take more time to travel through the medium? I sometimes teach radio antenna theory and this is what I've always told my students, not that C becomes less. A subtle difference but can you comment?

ANSWER: 
The energy of a photon is not determnied by its speed but by its frequency. The frequency is the same in and out of the medium so, in the medium, the wavelength is shorter but the energy is the same.


QUESTION: 
When particles are made to collide in "atom smashers" and in the CERN machine, when it is placed in service, and physicists start "looking" at the particles that result from those collisions does the time dialation effect come into effect such that the resultant particles exist long enough to be observed?

ANSWER: 
Yes. In fact, this effect has been observed even before accelerators. Cosmic rays create particles in the upper atmosphere which would not reach the earth were it not for time dilation.


QUESTION: 
You are on your way to Mars in a spaceship, moving at a high speed relative to the Earth. (a) Would you notice a difference in your pulse rate? (b) Would you observe the pulse rate of a friend back on Earth to be slower or faster?

ANSWER: 
This sounds suspiciously like homework to me. I will answer it since it is pretty elementary. You would not notice any difference in the rate which clocks (like your heart) ran in the ship with you. Any other frame of reference (like the earth) would have clocks which you would determine to be running more slowly. Note that this is different from saying that they "look" like they are going slower. In fact the earth clock would look like it was going slower on your way out and look like it was going faster on your way home but, in both cases, would actually be going slower. To understand this, see my earlier discussion on the twin paradox.


QUESTION: 
I'm trying to understand light wavelength, frequency and amplitude *physically* in addition to their depictions on graphs and in formulas. I got an A in high school physics years ago but felt then that I didn't really understand much as my head incessantly filled with questions as the class progressed. I spit the formulas back on demand on tests but had little confidence I actually understood what the heck was going on. It recently occurred to me that depicting light as a wave drawn as a graph may have confused me. Such depictions seem to describe light as physically following a path just like the graph -- i.e., a graph's large amplitude represents an actual fat beam inside of which particles go up and down, defining the beam. I recently started wondering if light wavelength, frequency and amplitude are really measurements of pulsating light particles' detectable energy, as if the light particles are glowing or even turning off and on, rather than descriptions of the shape of a light particle's physical path.
I. Do light amplitude, frequency and wavelength describe the following physical properties of light?
(A) is light AMPLITUDE the difference between pulses (glows; energy emissions?) of a light particle's dimmest and highest detected energy? # if so, then amplitude does not describe the max height of an actual wavy physical path of light.
(B) is light FREQUENCY how fast a light particle pulsates (glows) from dimmest to brightest or off to on? # frequency therefore does not describe the rate that a light particle travels up and down.
(C) is light WAVELENGTH the distance a light particle travels between one pulse ("glow")? # wavelength therefore does not describe the length of a "strand" of a light wave between it furthest points from a straight line.
II. Is it possible to understand what the heck light is really PHYSICALLY doing without understanding advanced physics?

ANSWER: 
You have numerous misconceptions here. I would recommend that you read the Wikepedia article on electromagnetic radiation. Let me give you just a few basics. If you are unfamiliar with the concept of fields, then there is no possibility that you can understand what light "is". However, if you got an A in a physics course, there is a good chance that you understand what a field is.

  1. The picture to the right shows what is "waving" in light. There are time varying electric and magnetic fields as shown (the arrows in the sine-wave shaped envelopes). This whole configuration moves to the right with a speed c=3x108 m/s. The relative magnitudes of the electric and magnetic fields are fixed; they have been drawn to have equal magnitudes here to make the picture easy to look at but they are, in fact, not even measured in the same units so it does not really matter what the relative magnitudes are in a picture like this. The amplitude is simply a measure of how large the maximum electric field is. The intensity of a wave of light, the rate at which it transmits energy, is proportional to the square of the maximum electric field, that is a wave with twice the amplitude will have four times the brightness.
  2. Also shown in the picture is the wavelength of the wave (usually denoted as λ) which, you can see, is the distance between two successive maxima of the electric field. The red end of the visible spectrum has longer wavelengths and the violet end has shorter wavelengths.
  3. Suppose you are standing next to the wave as it is zipping past. Then the frequency (usually denoted as f) is the rate at which maxima of electric field pass you. The frequency is measured in inverse seconds, for example 500 maxima per second. The inverse second is often denoted as a Hertz, 1 s-1=1 Hz. Related to the frequency is the period (usually denoted as T) of the wave, the time it takes for one wave to pass you. If you think about it for a bit you should realize that the two are the reciprocals of each other, that is T=1/f.
  4. Finally you should realize that the frequency and wavelengths are not independent since the wave moves with a specific speed c. The relation among the three you can probably figure out by thinking sbout it a little: fλ=c.

Please note that your question violates the site groundrule requiring "single, concise, well-focused questions". In future questions please try to keep it more concise.


QUESTION: 
Particle accelerators are said to reproduce the early conditions of the universe. At the same time the energies of these collision are said to be no greater than that of cosmic rays hitting the upper atmosphere. Can we say then that the early conditions of the universe are constantly reproduced in our own upper atmosphere? Do you think it would be feasible in the future to just build a detector in the upper atmosphere in the future? Can't we just point a very sensitive telescope toward our skies and learn from these collisions?

ANSWER: 
Yes, some cosmic rays have extraordinarily large energies. But to study the physics you need two things: since the events you want to study are very rare, you need a very intense beam of particles to hope to see anything of interest in your lifetime; accelerators give large numbers of particles per second striking a very small area, cosmic rays do not. Second, one needs control over the conditions. Cosmic rays can have a broad spectrum of energies, an unpredictable location where they will hit, and an uncertainty as to what they will interact with if anything at all; accelerators put the particles where you want them, with the energy you want them to have, and on specifically what you want them to hit.


QUESTION: 
what is the differences between the meaning of charge and matter densities of a nucleus?

ANSWER: 
The charge density distribution is a description of how electric charge is distributed in the nucleus. The matter density distribution is a description of how the mass of the nucleus is distributed. These turn out to have roughly the same shapes for nuclei, that is the radii are about the same, both are more or less constant inside the surface, and the surface is reasonably well defined, that is the distributions drop to zero in a distance small compared to the radii. This need not be the case because the charge is determined by the protons whereas the mass is determined by both the protons and neutrons. There is evidence that, in some nuclei, the mass distribution is slightly larger than the charge distribution; the mass outside the charge distribution is sometimes called a "neutron halo".


QUESTION: 
My question is about the Wave-Particle Duality of light.By certain theories such as Newton's Corpuscular theory and experiments such as Thomas Young's 'Double Slit' experiment,it is proved tha light has Wave nature.But in accordance with Einstein's Photo-electric effect,etc,light has particle nature.How can a single entity have two forms?Or even if it does so,in which case is it wave and in which case is it a particle?

QUESTION: 
I thought that because of wave/particle duality of light, a photon could be thought of as a wave also, but if not let me rephrase my question. In the electromagnetic spectrum, is amplitude of single wave or wave stream measured in same way that wavelength is, as indeed amplitude is suppose to represent the right angle measurement (height and depth) of the wave and should be just as valid a measurement as the wavelength itself. Is there a measurement for this dimension of the wave and is it constant regardless of frequency or wavelength?

ANSWER: 
You two should read earlier answers (duality1, duality2, duality3)and all the links in those answers.


QUESTION: 
a question regards conservation of angular momentum and principles. I stand on a frictionless turntable holding a shaft-bicycle wheel arrangement with the shaft in the vertical (Y axis) position. Initially, there is no motion. I reach up and start the wheel rotating in the clockwise direction. The conservation law then requires that the turntable & to begin to rotate in the counterclockwise direction. This must be case so that the total angular momentum = 0, as it was initially. With system's motion as above, I now rotate the shaft 90 degrees to the horizontal(X axis) position. What idoes this now do the system's rotation, since there was no motion along the X axis either initially or when I started the wheel's motion. Does my counterclockwise rotation continue or stop? If still rotating, then it has kinetic energy.

ANSWER: 
Because there is a gravitational field and you are standing on the turntable (which can exert torques on you), you are not an isolated system and angular momentum is not conserved. You need to imagine yourself doing this experiment in empty space. Your body's angular velocity will end up being opposite the wheel's just like the first part of the experiment. But now you will be rotating about an axis roughly through your stomach and parallel to your shoulders instead of one through the length of your body.

FOLLOWUP QUESTION: 
I agree with the answer to how I'd rotate in empty space. Would the rotation about the turntable still continue? With regard to the first part of the original question in gravity field, would the shaft/wheel when held in the horizontal do anything to the ccw rotation that I & the turntable had before the 90 degree rotation of the shaft/wheel? This the part of the principle that is not clear to me.

ANSWER: 
To actually present the theory here is too tedious and, I believe the exact answer may depend on how quickly you make the transition. However, we can study the question experimentally. Have a look at http://www.physics.umd.edu/lecdem/services/demos/demosd3/d3-05.mpg which reproduces your conditions (zero total angular momentum). Although he does not stop at 900, note that as he is passing through 900 he still has rotation at that point; of course, the transition from rotating to not rotating would have to be continuous. Since angular momentum is obviously not conserved, an external torque must have been applied to the system (you, table, and wheel); this can only come from the friction between the table and the floor I believe.


QUESTION: 
Magnetism is a great wonder of mine. From what I know, magnetism works by the arrangement of the metal molecules which creates some sort of electrical field, Correct me if I'm wrong. My question is in regards to credit cards, hotel cards; any magnetized card really. How does each card possess a unique magetnic identity? My guess is that each card has a distinguishable arrangement of the metal molecules direction.. I am really just pulling this off my head. But if this were the case, how does the computer that clears a card and reprograms it with its code work?

ANSWER: 
A piece of ferromagnetic material (usually iron) can be magnetized, that is a piece of iron can become a magnet. Then you can detect the presence of that magnet with the force it exerts on another magnet. The strip on a credit card works just like magnetic tape used to work. It is made of very many tiny particles of magnetizable material (like a powder); then a pattern of magnetization (like a barcode of magnetization) can be put on the strip and that pattern can be read by something sensitive to magnetic fields. In a tape recorder the magnetism is created by the record head and it is read by the read head.


QUESTION: 
I have a question regarding the flow of water from a fountain. How can the shape of the flow straight upward from a fountain be explained? Specifically, the stream goes straight up and then seems to widen as it gets higher, forming a Y-shape. Why does the water not flow upward in a perfectly straight column?

ANSWER: 
You assume that all water comes out the spout with a speed straight up. But, in fact, most "pieces" of the emergent water have small components of their velocities which are horizontal. Then, over a large vertical distance the stream will spread. Also there is a Bernoulli effect where the pressure is lower at the surface of the fast-moving water.


QUESTION: 
If refrigerants and coolants such as CFC by-products are molecules that are complex chains, I assume they are heavier than air and cannot possibly drift to the ozone layer to threaten it. Can CFC's drift to many km.'s altitude to the ozone layer?

ANSWER: 
You are assuming that somehow the atmosphere layers itself by weights of the gasses. But this is not the case and all the gasses in the air difuse into each other. Otherwise, you would find all the oxygen below all the nitrogen and above all the carbon dioxide, etc. In fact, ozone is heavier than usual oxygen, so how did it get so high up? CFCs can and do diffuse to high altitudes.


QUESTION: 
This is a bit silly but... If I were travelling on a spaceship, at a constant velocity, say 20% of the speed of light. if I had a live video stream, which I'd be broadcasting at the speed of light, and running on a treadmill at a contstant pace, what would that look like to someone wathing the video cam on earth? Would there be a doppler effect? would it look like i was slowing down? or would there simply be a buffer delay in the frames? I'm talking w. each passing month, say, until I was 5 yrs away (1 light year)

ANSWER: 
This question is best answered using my earlier explanation of the twin paradox. In that example I used 80% the speed of light for the spaceship to make the results come out nicely. Here your "video" would correspond to sending out one light pulse each year but the idea is the same. Note that the observer on the earth receives one of these pulses every three years. So, in the case of your video, the observer on the earth would see the action slowed down by a factor of 3 (much less if you used your 20% number). Regarding the doppler effect, there would be a doppler shift of the carrier signal but you would just make a correction for that or simply tune your receiver to receive the shifted frequency.


QUESTION: 
I have read descriptions of how light travels- dually manifested in either wave patterns or particles of photons. I have also read about the speed of light and how long it takes light to travel from one point to another. My questions are: Why does light travel and what gives it the impetus to move? If it is made up of photons, what initiates the movement- say, from the Sun to the Earth?

ANSWER: 
This question sort of like saying why is 2 plus 3 equal to 5? It is the nature of electromagnetic radiation to move with the speed of light. It is created by accelerating charges like, for example, oscillating electrons in a radio transmitting antenna. You have to put energy into the accelerating charges and, if you like, you can think as that energy input as your "impetus". But that doesn't mean that if you didn't put energy in the radio wave that it would just sit there; rather, it would not exist. For visible light the source is typically an atom which essentially behaves like an antenna when emitting a photon; here the atom drops from an excited state to a lower state, thereby providing the energy to the photon, but this does not impel it to move since it must move by its very nature.


QUESTION: 
A supersonic jet fighter is travelling at Mach I. He fires his gun at an enemy fighter. My question is, are the speed of the rounds the same as the speed of the same rounds fired while the jet is standing still on the ground? Put another way, are the speed of the rounds the speed of the muzzle velocity + the speed of the aircraft? Or, is the speed of the rounds ONLY the muzzle velocity speed?

ANSWER: 
Ignoring air resistance (which is really not ignorable) the speed of the rounds relative to the ground is the speed of the jet plus the muzzle velocity of the gun.


QUESTION: 
I am just wondering which set of equations is used when determining the de Broglie wavelenght of a 10MeV neutron?

ANSWER: 
You just happened to be wondering, eh? Kinda sounds like homework to me. You need to know the de Broglie equation,
λ=h/p, and the relation between energy and momentum, E2=p2c2+m2c4. Of course, you also need to be careful of units, probably you would want to convert MeV to Joules and work in SI units.


QUESTION: 
Just recently, I've fallen in love with physics. I'm in Iraq right now waiting to come home and start college, maybe to pursue a physics degree. However, I can't seem to make sense of a very basic concept: if everything is made of tiny particles, why do things feel solid, why can't I just put my hand through a desk or something? I'm sorry to ask such a basic question, thanks for your time.

ANSWER: 
I think that the answer to an earlier question essentially answers your question.


QUESTION: 
Is there any possibility of a kinetically powered automobile which stores energy via a dynamo-type generator and transfers it to the engine? Since there are four rotating wheels to most consumer vehicles, the energy could be collected from at least four points. I'm guessing an electric motor could start the vehicle, and be used intermittently during traffic stops, with the kinetic energy propelling a light-weight auto on stretches road that don't require frequent stops, like highways. For those of us living in the Midwest flat-land country of the US, this would be a great alternative to fuel-consuming vehicles. What obstacles would make this impossible?

ANSWER: 
Yes, you could take some of the kinetic energy and use it to charge a battery, but you would slow down because if you take and store that kinetic energy it is gone! This is important: you are not going to get something for nothing! In fact, that is one of the cool things about hybrid cars
—when they brake the kinetic energy of the car is used to charge the batteries, not thrown away as heat as most cars do. But you can't use the kinetic energy without  losing it.


QUESTION: 
In Newton rings experiment interference occured between which two reflected rays and why only those two rays ?

ANSWER: 
between the curved surface of the lens and the flat section on which it rests. Other pairs of surfaces are unimportant because the distances between them is so large that they cannot be sufficiently flat to have constant distances between them.


QUESTION: 
My question relates to time and acceleration. I know any acceleration through space causes time within that particular system to slow, my question is what is the exact mechanism that happens? I'm aware that a specific measurement of time is an increase in entropy within that particular system. So, i was wondering if acceleration through space, specificly at rates near the speed of light, causes entropy within that closed system to decrease? causing it to appear as if time has slowed relative to other frames of reference? i was just curious, i'm sure there are other ways to measure time but i'm familiar with only that one way. thank you for your consideration of this question.

ANSWER: 
I do not think you mean acceleration, you mean velocity. Time in a moving system runs slower than time in a stationary system. This is just a consequence of special relativity which is based on the premise that the speed of light is the same for all observers. It has nothing to do with entropy. There is no "mechanism" since it is simply a property of space-time itself.


QUESTION: 
Does a fan in a room actually make the room hotter because it is putting out energy in the form of wind which converts into heat energy? Or do fans actually work to cool rooms down?

ANSWER: 
Yes, the net effect of a fan on a completely closed room would be to heat up the air. However, most of this heating comes from the fan itself heating up (primarily the heating of the electric motor) not from the small kinetic energy given the air molecules in the "wind" since these drift velocities are generally small compared to the average speed of molecules in the air. Of course, if the room is not closed, just the circulation of air can help with the overall comfort of the room and a fan can help cool people by speeding up evaporation of perspiration.


QUESTION: 
Bandsaws and belt sanders both have a flat blade or belt that runs around the surface of two wheels. If the wheel rim is flat, the flat belt tends to walk off one side of the rim or the other. To avoid this problem and keep the belt tracking in the center of the rim, the rim is crowned. That is, the center of the rim has a slightly larger radius than the edges. (on a bandsaw the crown is about 5-8 degrees on either side of the ridge. As the wheels spin, the blade will track right on the ridge. My question is about the forces involved that keep the blade centered. My intuition is the the blade, rather than tracking on the high spot, would tend to "fall off" the ridge and move toward the edge, but just the opposite happens. Can you clarify how I should think about this?

ANSWER


QUESTION: 
Where does the energy that electrons use to rotate around the atomic nucleus come from?

ANSWER: 
An electron does not "use" energy to rotate around the nucleus any more than the moon uses energy to rotate around the earth. Once in a stable orbit it goes around forever without any energy input.


QUESTION: 
Is it possible to harvest gravities force in order to make clean natural energy?

ANSWER: 
Sure. It's called hydroelectric power. Here the water at a high altitude has a large potential energy and when it falls its energy decreases so it must give us some energy. I do not know what you mean by harvesting the force. It is not the force you use but the energy something has by virtue of that force.


QUESTION: 
Why, when a person jumps into a pool from a given height does the water "splash" higher then the initial height that the person jumped from? Assuming they jumped with a more vertical trajectory then horizontal.

ANSWER: 
I guess I would reply "why not?" On what are you basing your expectation? Lets look at a greatly oversimplified situation from the perspective of energy conservation. Suppose that no energy were lost when the person hits the water (of course, not correct but an extreme case). And suppose that the entire splash is one blob of water 1/100 the mass of the person. Now, the person starts out with an energy of MgH where M is his mass and H is the height from which he jumps. Let's take M=100 kg, H=3 m, and g about 10 m/s2, so the energy is 3000 Joules. The blob will end with the same energy, so 3000=(100/100)x10xh where h is the height to which the blob goes which you can see will be 300 m! Now, to make it more realistic, assume that the blob gets only 5% of the person's energy instead of all of it. Then you would find h=15 m.


QUESTION: 
Is there a way to show how the earth is rotating (i.e., locate the poles and determine the rotation rate) without using external references like the sun and the stars?

ANSWER: 
One can easily see the effects of rotation in numerous ways. One is to observe the Coriolis force which alters the flight of projectiles over long distances; in order to accurately aim long-distance artillery you must make corrections for the earth's rotation. This effect is also what causes weather patterns to be circulatory. It would probably be difficult to extract good-accuracy measurments of pole positions and rotation rate from this. Another way is to observe and measure the motion of a pendulum (Foucault pendulum) which would give accurate determinations of properties of the rotation.


QUESTION: 
How fast does a baseball travel (mph) when a batter hits a pitch. Is it faster if the pitcher is throwing the ball faster. Does the speed of the pitch determain the speed the ball travels after being hit.

ANSWER: 
See an earlier answer.


QUESTION: 
what would happen if i were bombarded with 10^24 neutrinos? How do i calculate the energy of that?

ANSWER: 
Practically nothing would happen since neutrinos are very non interactive and most would go through having no effect. The energy left by those which did interact would depend on the energy they had before they hit you.


QUESTION: 
What is a measure of the average enery of random motion of particles of matter?

ANSWER: 
The average energy of a molecule is proportional to the temperature. If you double the temperature, you double the average energy of a molecule. The temperature must be in absolute units for this to work, that is room temperature is about 3000C since absolute zero is about -2730C.


QUESTION: 
in the equation e=mc2, what is the unit of measure for the energy? Is it, for example, horse-power? or kilo-tons of TNT?

ANSWER: 
Like all equations of physics, the units of the answer are determined by the units used in the equation. The most common units used are m in kilograms and c in meters/second. Then energy is in joules. Incidentally, horsepower is not a unit of energy but a unit of power, energy per unit time; the most common unit of power used by physicists is the watt which is a joule/second.

FOLLOWUP QUESTION: 
A followup question if you don't mind -- just to check my numbers: If the speed of light squared is roughly 900,000,000 m/s then a 1 kilogram block of butter should yield 900,000,000 joules of energy - right? One website said that a 100 watt light bulb requires 8,640,000 joules to burn for 24 hours. Can I conclude that if we could completely convert a block of butter into energy, the a block of butter should power a 100 watt light bulb for about 104 days?

ANSWER: 
Your calculation of c2 is wrong, c2=(3 x 108)2=9 x 1016 m2/s2, so the energy content of the block of butter is 9 x 1016 J. This would run your lightbulb for about 1010 days, about 27 million years!


QUESTION: 
Could you explain why is it that a sample of solid matter when cut does not recombine the way liquids and gasses do?

ANSWER: 
Because in a solid all the atoms are fixed to a specific location whereas in a gas or liquid the atoms move around.


QUESTION: 
Since protons are positive and electrons are negative, what keeps an atom from collapsing in on itself?

ANSWER: 
The simplest answer is the simplest model of the atom, the planetary model suggested by Niels Bohr. Essentially the electron orbits around the proton like the moon orbits around the earth. You do not ask why the moon does not crash into the earth due to the attractive force between the two.


QUESTION: 
Why do icecubes sometimes stick to your skin?

ANSWER: 
Mositure on the surface of your finger freezes and then welds to the ice cube.


QUESTION: 
The problem of radioactive waste is the half life duration of high radioactive atoms. What affects the half life of an atom? I know science breakthroughs are rapid at times. I just want to know the breakthrough on controling the half life of an atom so far, because i know there might not be an answer to my question

ANSWER: 
The half life of a nucleus is determined by the nuclear wave function, that mathematical function which contains information about the nucleus. Without changing the nucleus itself, it is not possible to change its wave function. It is possible to change the nucleus (for example by adding a neutron or bombarding with a proton) thereby chaninging the wave function, and such methods have been considered to achieve what you want to do. However, given the low probablility of having the desired interaction occur, it is considered to be not practical or economically unfeasible.


QUESTION: 
O.K. Energy and mass are equivalent Gravity radiates a significant amount of energy.Is mass lost from a radiating body,if not why?

ANSWER: 
I do not know what you mean by gravity "radiates a significant amount of energy". This is just not true. According to general relativity, gravity waves eminate from accelerating masses like accelerated electric charges radiate electromagnetic radiation. Gravity waves have never been directly observed although efforts over the past 50 years continue with improved sensitivity. Gravity waves have been indirectly observed by observing the orbital motion of two massive stars; here the energy of this binary system is systematically decreasing exactly at the rate predicted if gravitational radiation were responsible. Hulse and Taylor received a Nobel prize for this discovery. And, yes, if the energy is decreasing, so is the total mass.


QUESTION: 
What is the name of the device and what is the effect called? Description below; In a vacuum glass bulb with 4 black on one side white on the other diamond shaped blades their planes attached perpendicular to a movable shaft. When light shines on the glass bulb the blades rotate.

ANSWER: 
Crookes Radiometer.


QUESTION: 
The lines of electric or magnetic flux, as is illustrated in physics books, are often drawn as bundles of discrete lines. Is this an accurate depiction? If so, do these lines have a measurable finite width, and what is the width?

ANSWER: 
These drawings are graphical representations of the vector fields. To actually draw a field would mean that for every point in space you would have to draw a vector representing the field at that point in space; this is certainly not possible since we can't draw an infinite number of vectors and they would all overlap each other and make the whole thing just black. One way to approach this is to draw the field vectors at only a select number of points in space and use your brain to interpolate between those points. Another way is to draw lines the tangents to which are the directions of the field at each point. Another advantage of this representation is that the more tightly packed the lines are, the stronger the field is in that region. It is possible to adopt a convention so that a certain number of lines per square meter corresponds to a particular field strength, for example we could define that the number of lines coming from a 1 nC charge was 1000. However, normally these pictures are just that, pictures which help us visualize the qualitative features of the field.


QUESTION: 
I've seen the Elementary Particles chart showing 12 particles and 4 force carriers. Do the Higgs particle and the Graviton fit into this puzzle or are they part of another chart?

ANSWER: 
You may assume that there is no such thing as a graviton since there is no successful theory of quantum gravity; physicists talk about gravitons wistfully wishing them to exist. The Higgs boson is predicted by the standard model but has not been observed. When and if it is observed, it will be allowed to join the club of 16 (as a new boson) I presume.


QUESTION: 
Baseball related. I've heard conflicting answers and this question is prompted by the recent MLB homerun derby. When a player hits a pitch for a homerun does the speed of the pitch at all affect the distance? Will a fastball go further than a slower pitched ball? One camp contended yes, as although some energy was lost at contact with the bat there was still a transfer of energy from fastball to bat. The other camp said no, that the distance ball traveled had more to do with the strength of the hitter and the rotation of the ball than with the speed of the ball. Any light you can shed would be appreciated.

ANSWER: 
Let's simplify the discussion by assuming a particular hitter, because of the way he swings, can impart a set amount of impulse to the ball with his swung bat. The definition of impulse is the average force on the ball during the time it is contact with the bat times the time during which it is contact. The pertinent physics principle here is Newton's second law which may be stated as change in momentum (momentum is mass times velocity) equals impulse. In words, this would mean that since it takes more impulse to "turn around" a fastball, you are more likely to hit a homerun with a slower pitch. Here is a numerical example: Suppose the average force is 400 lb over 1/100 of a second; this would result in an impulse of 4 lb s=17.8 N s (physicists prefer metric units!) Now, the mass of a baseball is about 0.15 kg; the velocity of a fastball is about 100 mi/hr=45 m/s; the velocity of a curveball is about 80 mi/hr=36 m/s. So, the incoming momentum for a fastball is about 0.15 x (-45)=-6.75 kg m/s (the minus sign since the ball is coming into the plate but I choose positive velocity (and impulse) to be away from the plate) and the momentum after the impulse is 0.15v where v is the velocity of the batted ball. So, momentum change is 0.15v-(-6.75)=0.15v+6.75 and, setting this equal to the impulse (17.8 N s), I find v=74 m/s=166 mi/hr. If you repeat this calculation for the curveball you will find that v=84 m/s=188 mi/hr, faster. Of course, there are many subtleties of baseball which this simple physics explanation does not address and my assumption that a hitterr can impart only a certain amount of impulse is obviously a rough approximation. Also, my guess for a typical impulse may be not very realistic. Still, simple physics suggests a slower pitch can be hit farther.


QUESTION: 
Light, photons, have no mass yet general relativity proves it can be bent by mass, i.e. massive stars. Without mass how can this be? Is it the massive star distorting surronding space and the light simply follows the curve as opposed to yielding to gravity.

ANSWER: 
You are essentially right, but you should probably not say "opposed to yielding to gravity" since this distortion of space by mass is our best understanding of what gravity is. Two earlier answers (link#1 and link#2) may give you some more insight into general relativity, the theory of gravity.


QUESTION: 
I would like to know if there is a correlation between inertia and free-fall. In what ways? Considering you drop the sandbag from a hot air balloon, should I consider inertia?

ANSWER: 
I am not really sure what you are asking. Inertia is measured by the mass which something has. The acceleration of something in free fall (really free fall) as you probably know is a constant, independent of mass. In that respect, the motion of a freely falling body is independent of its inertia. If the fall is not really free, in particular if air resistance is taken into account, the motion will depend on the mass; if you are interested, there is a lengthy and technical discussion of air resistance in an earlier answer.


QUESTION: 
a fairly simple one, i think: i'm trying to figure out how much electricity my recording studio takes. it's set up in a room that i rent, and my landlord says i use "a lot". so, i'm trying to do this without having to read the watt meter for an extended period of time.. the building has a GE watt meter. i measured the amount of time it takes for the wheel to spin once with basically nothing on in the house except for the fridge and kitchen stuff... it took 41.37 seconds. the time to spin once with all my recording gear on 20.51 seconds. is this a valid way to measure? do you need more information?

ANSWER: 
Your idea of estimating the the power consumption of your equipment is ok with the following proviso. Your baseline is important and you can easily be fooled if you think just the fridge and a couple of clocks are contributing. Nearly all modern electronic equipment
—stereos, tvs, computers, and probably a good part of your equipment—have a standby mode when you turn off the power switch, mainly so that they will come on instantly when you turn the device on. Hence there is probably a lot of power you are not aware of. Also, if there is an electric hot water heater (which uses a large amount of power) and it happened to be on then your equipment really does use a lot of power. By the way, how did you make such incredibly accurate measurements of the time?!

FOLLOWUP QUESTION: 
thanks for your response. i did take into consideration the other baseline stats.. to answer your measuring question, i mounted a stopwatch next to the watt meter and videotaped it.... i suppose the 100's is probably off given the frame rate of miniDV recording is only 30 fps. what i still don't know is how to turn that measurement into "how much" electricity/ "how much money" i'm burning with my gear-

ANSWER: 
If it is not clearly marked on your meter, call up your power company and ask how many kilowatt-hours (or kilowatt-seconds or watt-seconds or joules) corresponds to one rotation of the wheel. Take that number and divide it by the time for one revolution to get the rate (watts or kilowatts) that energy is being consumed. Also ask what the cost per kilowatt-hour is. Then, with the knowledge of how many hours per month you run your equipment you can estimate your monthly expense. I will walk you through a made-up calculation:

  1. Suppose one spin is 8200 joules (a joule is a watt-second).
  2. Then if one spin takes 41 seconds, the power consumed is 8200 joules/41 seconds=200 watts.
  3. If one spin with an additional drain (you) is 21 seconds, the power now consumed is 8200/21=390 watts.
  4. Your equipment uses 190 watts=0.19 kilowatts.
  5. Suppose the cost on one kilowatt-hour is 7 cents.
  6. Then your power costs you 0.190 kilowatts x $0.07 per kilowatt-hours=$0.013 per hour.
  7. If your equipment runs 100 hours per month, the total cost is 100 hours x $0.013 per hour=$1.30

Hope this helps.


QUESTION: 
Several friends and I are divided on a basic fluid dynamics question. We have decided to go to a higher source for the answer. Here's is the idea... You have a pipe that goes in a sort of loop so that its two ends don't touch each other but do face each other. Let us say that it is a copper pipe. The pipe is filled with some fluid, and there is a piston in each end that keeps the fluid inside the pipe. The two pistons are connected to each other, and if you are picturing this correctly, it should make sense that as one piston moves into the pipe the other moves out of the pipe by the same distance. Here is the catch: The diameter at one end of the pipe is greater than at the other end and correspondingly, one piston is thicker than the other. The question is this: if you pressurize the fluid inside the pipe, will the pistons move in one direction or the other, or will they stand still. My camp argues that because one piston has a greater surface area exposed to the pressurize fluid, the force on it will be greater than the force on the other piston. Therefore the pistons will move in the direction of the smaller of the two. The other camp argues that because the pressure is equal on both pistons there will be no movement.

ANSWER: 
If you are talking about an essentially incompressible fluid like water, the pistons will not move because, since they have different diameters, to move would mean that the volume of the fluid would have to change which it cannot. If it is a compressible fluid like air, for example, then if you increase the pressure (by heating it up, say) the volume would increase which would mean that larger piston would move out, the smaller would move in. If you cooled the gas, the volume would decrease so the pistons would move the other way.


QUESTION: 
Is the Proton Stable, or does it decay over time? For instance I read it's Mean lifetime is: >1.9 x 10^29 years (stable) it confused me because of the stable in the brackets, is it saying that it is stable and that the lifetime is actually hypothetical?

ANSWER: 
Although the proton is predicted by the standard model to be stable, there are other theories which predict it to decay. It is therefore of interest to try to observe proton decay experimentally. Experiments have all had negative results. However, a negative result of an experiment usually does not mean that the sought result does not exist, it puts an lower limit on its happening. For example, the analysis of an experiment might result in being able to say that the proton is stable unless its lifetime is longer than, say, 1035 years. A later more sensitive experiment might be able to lower the limit to maybe 1030 years, and so on.


QUESTION: 
i've had this dream since i was a child. You shine light through the un-relective part of a mirror aimed and the light comes out the other side...then there's a mirror that reflects the light back upon itself but is trapped between the two mirrors. This is the only way i can descibe the 'invention' in my dream, but the end of the dream always becomes a nightmare where the trapped light builds infinitely from being trapped resulting in power that in my dream goes out of control etc. in other words in the experiment the energy engulfs me then i wake up.

ANSWER: 
Several things you can use to put your mind to rest:

  1. Only the front of a mirror is reflective, it will not reflect (appreciably) off the side toward the glass;
  2. The back side of a mirror (the surface of glass which would be against the wall, for example) is not reflective;
  3. Even if trapped light were able to bounce back and forth inside, it would be absorbed by the glass almost instantly;
  4. There isn't all that much light in visible light.

QUESTION: 
I have a strange thing occurring at our beach house in Monterey California. The house is located on the beach, approximately 2 miles from the" Moss Landing Power Plant" Every time I visit our beach house, my cell phones battery runs down in a matter of hours. (It usually stays powered for days.) I can charge my cell at the beach house, it will take a full charge but, it will not stay charged. Within hours its dead. Once I leave the beach house and return home, the battery runs normally. Any idea what might be draining the battery? Could the power plant be involved in some weird way?

ANSWER: 
I feel quite confident that the power plant has nothing to do with it. The only thing I can think of is "that lovely sea air" which can be very humid which could result in the current being drained from your battery.


QUESTION: 
When you bungee jump, you are falling under free fall until you reach the end of the elastic rope. Bunjee-jump organizers must use the weight of a jumper to calculate the length of the elastic rope to use. Why is this calculation important?

ANSWER: 
The answer is a bit complicated but it boils down to the fact that, using identical elastic ropes on a light and heavy person, the heavier person will fall farther than the light one and you do not want anybody crashing into the ground/floor! There is a detailed discussion of the physics at bungee.com.


QUESTION: 
I understand that electrons spin in an orbit around a nucleus, I don't understand what else constitutes this electron shell. What are the electrons existing 'in'?

ANSWER: 
Why do they need to exist "in" something? Do you worry about what the earth's orbit exists in? Or the moon's orbit? The Bohr model simply has the electron following a particular path around the proton/nucleus of the hydrogen which it does under the influence of the electric force it feels just like the earth goes around the sun because of the gravitational force it feels. The use of the word "shell" maybe confuses you; it does not mean there is some physical shell there.


QUESTION: 
I've never received a clear answer on why they started making household electrical appliance plugs polarized - that is one side bigger than the other so the plug only goes into an outlet one way. After all, AC is AC. I suspect it's a plot by under paid electricians. Really, whats the deal?

ANSWER: 
It is not a plot by electricians but rather a simple way of protecting users from electrical hazzards. AC is not just AC; you need to have an constantly changing potential difference across two wires but what is the potential difference of either of these wires with respect to some other potential we might define as zero? The zero reference potential is usually simply the earth, that is we drive a steel stake into the ground and define that potential to be zero. One of the two wires (called the neutral and the one with the wider blade on a plug) is connected to the ground and the other (called the hot wire and with the narrower blade) varies relative to the neutral. Note that the neutral wire, even though it is always at zero potential, will carry electric current and is therefore sometimes called the return. If a switch is connected to a an outlet it is the hot wire which is iterrupted to turn the outlet off so that you cannot get shocked if you touch the outlet. Sometimes a third wire connects everything not electrical (the outlet box, the metal box encasing your toaster, etc.) to ground so that a malfunction of the device will not expose you to the hot wire.


QUESTION: 
Since an LED seems to produce the same intensity of light using considerably less energy than an incandescent bulb, could an LED be tuned to emit infrared energy, thereby being a more efficient way to produce heat with electricity than typical resistance heating?

ANSWER: 
Actually, the first LED invented, in the 1950s, was infrared. A common use for an IR LED is as the source in a remote control for your TV, stereo, etc. LEDs are invariably low-power devices (typically milliwatts) and therefore not suitable as heating devices (kilowatts). Also, what does efficient mean regarding a light source? We normally think of an incandescent bulb as inefficient because a large fraction of the consumed energy goes into heat instead of light. A flourescent bulb is more efficient and an LED more efficient still. But, from the perspective of a heat source, the incandescent bulb is the most efficient! Resistive heating is, in fact, an very efficient source of heat since nearly all the energy consumed goes to the desired purpose.


QUESTION: 
If a car hits you traveling at half a mile per hour the effect would be to gradually and gently nudge you out of its way. At 10 mph the same car would certainly grab your attention, and at 35 mph you could expect to pay a visit to the hospital. At 80 mph you'll be taking a trip to the morgue. Any moving object can destroy life given sufficient velocity ... or can it? Surely a baseball flying at 3000 feet/sec or a feather at 10,000 miles/sec would kill an organism, but what about a speeding speck of dust? A helium nucleus at 99.99% c? Or an electron at ??? Can single small objects kill an organism if their velocity approaches arbitrarily close to light speed? Or is there a limit at which the size of an object will prevent it from killing even if it was speeding along at say 99.9999999% c? I was watching a program about space travel and the danger posed by cosmic rays. It prompted me to wonder if it was at all possible for an astronaut to be killed by just a few impacts as opposed to a bath of radiation ... or even a single impact at ridiculous speeds.

ANSWER: 
To estimate the effect of the collision you need to estimate the force the object would exert on you during the collision. Suppose your mass is 100 kg and your collsion with the car lasts 0.1 s and the car does not slow down significantly because of its collision with you. Then the force is the rate of change of momentum which will be 1000v where v is the speed of the car (and your speed after the collision). So, if the speed is 0.5 mi/hr=0.2 m/s the force on you during the collision would be about 200 N, about 45 lb. However, at 80 mi/hr the force would be about 7200 lb. Now, let's look at an electron (mass about 10-30 kg) with a speed of 0.999999999c. Here, the most force it could exert on you would be if it were to come to a stop in your body thereby transferring all its momentum to you. Its momentum would be about 10-9 kg m/s and I estimate it would take about 10-8 s to stop so the force would be about 0.1 N, about 0.02 lb. In the real world, it would pass right on through your body and transfer far less momentum to you. The harm which charged particle radiation does to you is not because of force but because of the damage it does to cells as it passes through. You need far more than one such electron to do you any harm.


QUESTION: 
If one atom of fissile uranium fissiled right in front of you, would you see it or hear it. Would it be like a small snap, or a large bang, or would it incinerate you? Just trying to get an idea of the energy released from a single event.

ANSWER: 
The energy released in a single fission event is on the order of millions of electron volts (MeV) and 1 MeV=1.6 x 10-13 Joules. This is a tiny amount of energy and it will reside mostly in the kinetic energy of the products (the two main pieces plus a few neutrons) which you could not see or hear. The reason we think of fission as a great source of energy is that this tiny amount of energy is a very large fraction of the mass energy of the fuel, on the order of 1%, huge compared with more conventional energy sources like burning coal.


QUESTION: 
Could a craft made of materials with different resonate frequencies, use pulsating sonic emitters tuned to these frequencies to cause the craft to vibrate in a manner that would propel it in any way? maybe through water or space but i was just thinking and it hit me.

ANSWER: 
I suppose so, but here is what you can be sure of: you can not put energy into your craft which exceeds the energy you put into your "sonic emitters"
—you're not going to get something for nothing.


QUESTION: 
My understanding is that 2 objects with mass are attracted to each other and we call this gravity. Light can not escape from a black hole because its gravitational pull is too great. Does this mean that light has mass, and is it settled yet whether light is made up of particles (I think they are called photons). Also since energy and mass are interchangeable I feel like light must have at least convertible mass. The reason I am curious is because if light does have mass then would it not be subject to the rule that anything traveling at the speed of light becomes to infinitely massive? It seems a contradiction.

ANSWER: 
I often get this question, that if light is affected by gravity it must have mass. Just two answers down from yours is a brief discussion and links to earlier answers.


QUESTION: 
We recently spent the day at the local Water-Slide and this reminded me of a question that bothers me every time I have ever gone there. It seems rather counter intuitive but I would swear that the heavier the rider is, the faster they seem to go. My train of thought is this: the rider's speed is a function of the angle of the slide, the friction coefficient of the rider/slide interface and the mass of the rider. If a 200 lb. rider has only a slightly larger surface area than a 100 lb. rider( certainly not twice the area; my guess is surface area increases by about 1/8?), my thought is that this large increase in mass and only slight increase in surface area would increase the friction and cause a heavier rider to go slower than a lighter rider. My intuition is that the smaller rider should go faster (less mass=less friction) but this doesn't seem to be the case. What is really going on here?

ANSWER: 
See earlier answers.


QUESTION: 
Black holes can bend and capture light, once it has reached the event horizon right? But doesn't that mean that light has mass for gravity to act upon it? Also, does that mean light exerts a force for everything that it hits?

ANSWER: 
Just because something responds to gravity does not necessarily mean it has mass; light does not have mass. You should read (1 and 2) earlier answers for a discussion of general relativity. Light does exert a force when it hits; this is because light carries linear momentum and when it transfers momentum to something it hits (either by being absorbed or by bouncing off) it exerts a force.


QUESTION: 
Someone told me something really insane, but not having a physics background, I could not refute them properly. If an object is frozen, and be it that the atoms are still moving, why wouldn't the atoms create a kinetic energy from their movement over a long period of time and the object heat up and melt?

ANSWER: 
Suppose the frozen object is well insulated so that no heat can enter or leave. Then the temperature will remain constant (below 00C for water). The average energy per molecule will remain constant because that is what temperature measures. But, ice will not melt until it gets to 00C and so it will not melt. Moving atoms do not "create" energy, they are energy.


QUESTION: 
My question relates to the sport Airsoft ( it's like paintball). If a force of air is compressed into a cylider via a piston that pushes the air through a small air nossel and propels a .2 gram bb at 300ft/s is that the same amount of kinetic energy as 3 .4 gram bbs propelled simarly down a barrel at 237ft/s? The 3 .4g bbs are lined up touching each other. All the bbs start inside the chamber of their respective guns.Are the 3 .5g bbs considered having a center of mass and therefore greater net mass than by themselves? This is under big debate in my airsoft community right now. If at all possible could I get a force model or system drawing for this problem to show others.

ANSWER: 
The energy of something is
½mv2 where m is the mass and v is the speed. A 0.4 gram bb at 300 ft/s has an energy of 9000. (Don't worry about the units, I just did the arithmetic.) Three 0.4 gram bbs (total mass of 1.2 grams) at 273 ft/s has an energy of 44,717, nearly 5 times as much energy. (You made a mistake somewhere since you later in your question refer to 0.5 instead of 0.4 gram bbs, but this mass would have even more energy.)


QUESTION: 
Why do nuclei contain protons and neutrons? Given that neutrons 'feel' the strong force but do not repel, being uncharged, why doesn't nature build nuclei with just neutrons?

ANSWER: 
The strong interaction is not just a simple attractibe force like gravity or the Coulomb force. It is dependent on something called isospin, that quantum number which is used to distinguish neutrons from protons, among other things. It is too complicated to explain here, but no stable nucleus containing only protons or neutrons has ever been observed.


QUESTION: 
The question is about the formula for kinetic energy. KE=1/2m x v sq. A mass of 4 kg with a velocity of 2 ft./sec. I divide the mass by 2 = 2kgs. I change the velocity into m/s, 2ft/s = .615m/s. If I square this number it = .378. So now I have 2 kgs. x .378 = .756 joules. I don't understand how this could be possible. Squaring a fraction shows me that the object is going slower.

ANSWER: 
I do not understand what the problem is here. If the speed of something is v=0.1 and v2=0.01, then the speed is still 0.1 since the square of the speed is not the speed.


QUESTION: 
Is it possible that tunguska event was caused by a piece of antimatter coming down from space as some theories proposed before?

ANSWER: 
It is pretty well accepted that the cause was a small asteroid (large meteorite). If it were antimatter, how would it have gotten here? Space, you know, is not entirely devoid of material and in any long amount of time a "piece of antimatter" would erode away by annihilation.


QUESTION: 
Can the total mass of a closed system fluctuate at any given time, while the total energy remains of course constant, considering the existence of mutual conversions of matter and energy ?

ANSWER: 
Yes. Since we understand mass as just a form of energy, it may change into a different kind of energy thereby changing the mass of a system. Examples are:

  • There is an electron and a positron (an antielectron). They encounter each other and annihilate resulting in two photons. The original mass was twice the electron mass and the final mass was zero.
  • A uranium nucleus undergoes fission. Afterwards there is less mass but more kinetic energy of the fission products.

QUESTION: 
when an electron emits a photon is their recoil? if so, does this mean the electron loses mass/energy? if so, if an electron is repeatedly emitting photons won't it eventually "burn out"? If not, how does it get "replenished"? Also, if an electron is repeatedly emitting photons, what direction is the emission (i.e. is it random)? If random, has this been experimentally proven? How can this be?

ANSWER: 
Electrons do not spontaneously emit light. Usually it is an atom, changing from an excited electronic state to a lower state, which emits light and the photon's energy is supplied by the excess energy of the atom. The atom will, in fact, recoil so that the energy of the photon will be a tiny bit smaller than the energy loss of the atom because the recoiling atom will have a tiny bit of kinetic energy afterwards. An electron will emit energy if it is accelerated which is how a radio antenna works. But, as you note, the energy carried off will have to come from the electron which then will slow down and eventually run out of energy to radiate away (but this is just its kinetic energy which goes away). Therefore, if you own a radio station you have to continue feeding energy into your antenna for it to continue to send out photons.

FOLLOWUP QUESTION: 
actually, I should have been clearer. What I meant was "virtual photons".

ANSWER: 
A virtual photon (the emission of which violates energy conservation) may exist only for a very short time after which it must be reabsorbed by the original electron or absorbed by another charged particle. The total energy of the system will remain constant.


QUESTION: 
Is white a color?

ANSWER: 
This is really a question of semantics. If you are referring to color as being defined as the wavelength of the light, then, no it is not a color because white light is the mixture of all visible wavelengths. If you are talking about color as a pigment, the color of paint or the color of a shirt, then clearly white is a color.


QUESTION: 
I was wondering about computer information storage. Why is it that a magnet will erase old 3.5" floppy disks and hard drives? Does a magnet affect a CD at all? What about flash memory? Moreover, what effect would a powerful magnet have if placed next to an operating appliance with electronic circuitry?

ANSWER: 
Floppy disks and hard drives both store information by magnetizing the medium; data are read by detecting this magnetization. So, obviously, a magnet can change them. A CD has physical changes to store data, little pits are burned to affect the reflection of a laser which is used to read the disk, so magnetis have no effect on CDs. Flash memory is electronic, charged transistors, and not affected by magnets. Your last question has no answer because there are too many different types of circuitry.


QUESTION: 
Why does a fat person (me) roll down a hill faster than a thin person (Eric) when we start at the top on roughly the same bike/tires and the same coasting speed? I know about Aristotle/Newton/Galileo and what each found - That is Aristotle thought heavier(massive) objects descended more rapidly. Newton/Galileo proved Aristotle to be in error and in fact Galileo used balls on inclines to more accurately measure descent times when compared to "dropping" items. (It seems to me that "balls on inclines" is _very_ similar to bicycles rolling downhill.) Also I know that one of the Apollo missions astronauts dropped a hammer and a feather on the surface of the moon and they both hit the ground at the same time. Given this, why does a fatter/heavier cyclist go downhill faster? On a short gradual hill Eric and I tested this earlier this week and I beat him coasting downhill by easily 20-25 yards.

ANSWER: 
This is one of those real world situations which simple physics does not do very well with. See my earlier answers to similar questions.


QUESTION: 
1) is it possible to know the exact location of a single atom? 2) If we use the most powerful microscope in the world and make it 1 million times more powerful would we be able to physically and directly see what is happening at an atomic level? would we see the atoms?

ANSWER: 
It is not possible to know the exact location of anything because of the uncertainty principle. However, it is certainly possible to see individual atoms but not, as you suggest, by making a more powerful optical microscope. A conventional microscope will not work since once you get on the scale of the wavelength of the light you are using you begin to get unavoidably fuzzy images and eventually no image at all. Atoms are usually detected individually by something called th atomic force microscope.


QUESTION: 
Do the valence quarks in a proton always remain in the same proton, or do they pop in and out of existence from one particle to another.

ANSWER: 
There is, in quantum mechanics, something called the exchange interaction. If you have two identical particles, for example, in a bound system there is nothing you can do to distinguish which is which, that is, you cannot label one of them #1 and the other #2. Each will be part #1 and part #2. Consider two protons, both in the same nucleus: an exchange interaction will likely play an important roll in the description of the sysem. Next consider two protons in two different nuclei in adjacent atoms in a solid. Although technically the exchange interaction will still be present, its importance is negligible any you can go ahead and label these protons without any problem. The same is true of quarks: inside a nucleus quarks in nearby nucleons (a nucleon is a proton or a neutron) will be subject to exchange interactions. Sometimes theorists attempt to model a nucleus as a "quark-gluon" plasms where the individual nucleons lose their identities.


QUESTION: 
How do I determine the ideal weight of an object with a given size to achieve maximum distance when put in motion? Example: I know a baseball made of styrofoam or lead would not go very far when thrown. An official baseball weight is 5 oz. If it were 4 oz or 6 oz would it go as far of a greater distance when hit or thrown as the 5 oz baseball?

ANSWER: 
Take first the baseball and the styrofoam ball. The reason the baseball goes farther is that air friction has a larger effect on the smaller mass. In particular, the terminal velocity, that speed which an object will end up having after a long time, is proportional to the square root of the mass. Hence the greater the mass, the greater the range will be. The reason you assume a lead ball will not go as far as the baseball is because you are thinking that you cannot throw it as far; that is certainly true, but the reason is that you do not have the strength to give it the same initial velocity as the baseball. For a meaningful experiment each ball should be projected with the same initial velocity; if that is done, the lead ball will go farther than the baseball which will go farther than the styrofoam ball.


QUESTION: 
What is a point charge?

ANSWER: 
An object which has a net electric charge but absolutely zero size in space.


QUESTION: 
If a man (60 kg) can run very fast then, can he run over the water surface without sinking (like a pond or river)? What is the (minimum) speed required? I thought it has something to do with viscosity and possibly the density of liquid. This means that you can walk freely over a very highly viscous &dense liquid but you require light speed to run over Superfliud helium II. Can it be described by an appropriate formula of the form- Min. Speed=f(Viscosity,density)

ANSWER: 
This is not possible unless you mean simply skipping like a stone. And, the vertical extent of the body would probably result in toppling over after a skip or two.


QUESTION: 
Does an object extremely far away from any other object has inertia in space?

ANSWER: 
Inertia is the property inherent of objects which cause them to resist accelerating when pushed on by a force. It has nothing to do with the environment. The object has no weight if in empty space.


QUESTION: 
When an object is at rest it means that is NOT under the influence of any force in nature? Talking at the macroscopic level. Because at the atomic level there are forces that take place in the object.

ANSWER: 
An object at rest or moving with constant velocity experiences zero net force from all forces which are acting on it; this is Newton's first law. All internal forces (like forces among atoms inside the object) add up to zero. For example, the force which the 321st atom exerts on the 45,395th atom is equal and opposite to the force which the 45,395th atom exerts on the 321st atom; this is Newton's third law.


QUESTION: 
What formula would I use to determine how many pounds of "force" it would take to push the bottom of a 6 fool long cylindar that weighs 512 pounds, that is attached at the top to a pivot point. The situation: Two men are replacing the bottom pin on a hydrolic cylindar. The cylindar weighs 512 pounds and they must push it 24 inches to aligh the end up with the hole that the pin fits into. We felt it would be less than 150 pounds, a co- worker said it would be half the weight.

ANSWER: 
I will assume that the push on the bottom will be horizontal. Then one needs to sum the torques about the pivot and set that to zero (when the cylinder bottom is displaced 2 feet). The magnitude of the torque due to the weight (I assume also that the center of gravity is at the center of the cylinder) is 3x512x(1/3) foot-pounds; the magnitude of the torque due to the force F is 6xFx(0.943) and so F=90.5 pounds. If the force is instead applied perpendicular to the cylinder the force would be 85.3 pounds. If you pushed up on the bottom vertically, the force would be half the weight.


QUESTION: 
I was at Water World today and I have a question about water slides. This may sound funny but i'm really curious. Who would go faster on a water slide, someone smaller or who weighs less versus a person who weighs more, etc.

ANSWER: 
This is one of those real world situations which simple physics does not do very well with. See my earlier answers to similar questions.


QUESTION: 
I have not training or education in physics and am trying to understand the 1st and 2nd law of wind power as they relate to sailing. The first law I've come across states that as the velocity doubles the power available increses by cube. The second law talks about power on the object only increases by square everytime the wind doubles. I'm trying to understand the "so what of these laws". I tried explaining it that if the wind increased from 10mph to 11mph the actual "force" or "strain" put on the sails, rigging, boat, etc, would be 33% or increase by cube. But the disagreement is that the second law is applicable, which is that the power or force on the sails, boat, rigging would only increase by square. Can you explain which is right? And if it only increases by square, how does the first law apply to sailing?

ANSWER: 
I have never heard of the laws of wind power. Basically what you are asking is what is the force exerted on an object by a wind with a certain speed (or, equivalently, the force on that object moving with a certain speed through still air). This force depends mainly on two things (assuming we talk about air and don't try to get too general by including other gases or liquids), the geometry of the object and the speed. So, clearly, a big sail will have a bigger force than a little sail, for example. The way it depends on the speed is not simple. At very low speeds the force depends linearly on v, that is, doubling the speed will double the force. At high speeds the dependence tends to be quadric, doubling the speed will quadruple the force. What high speeds and low speeds are depends on a lot o things, but most wind speeds a sailor is likely to encounter are in the quadratic range. I have never heard of a situation where the dominant term in the force was a cubic term.

FOLLOWUP QUESTION: 
Here is my question in the most simple terms: As wind speed doubles, does the force excerted by the wind on my boat, sails and rigging, assuming I don't reduce sail area, and the wind is hitting my boat at the same angle, increase by a factor of 4 (square) or 8 (cube) ? The "laws" I was refering to are pasted just below. I just don't understand how to apply them in the above question. Thanks. There are two main laws in physics concerning wind power. The first one states that the power available in the wind is proportional to the cube of the wind speed. This means if the wind speed doubles, the power available increases by the factor of eight. Even small increases in wind speed yield major gains in power. An increase in wind speed from 10 to 11 MPH results in a 33% increase in the power of the wind. The second law states that the power available to an object is proportional to the square of the diameter of the object. In other words; If you double the diameter of the object by making the object twice as long, you increase the power by the factor of four.

ANSWER: 
I did not read your first question carefully enough because I read your question as asking about the force on the sail (you did state it that way) but in fact you quote the "laws" as being the power and power is different from force (power is usually measured in watts and force in pounds). Power is the rate at which the wind delivers energy to your sail and one way to calculate power is the product of the force times the velocity. As I argued in my frist answer, force is approximately proportional to v2 and so Fv is proportional to v3. The force on the sail is proportional to its area, hence the second "law" since doubling the diameter quadruples the area of a circle; my own preference would be to state the second "law" as power is proportional to area. The simple answer to your question is that wind speed doubles then force quadruples; keep in mind that this is strictly true only if the boat is at rest, otherwise it will see a slower wind speed.


QUESTION: 
do you think the string theory explains gravity

ANSWER: 
Actually, string theory explains nothing. Further, gravity is already well understood in terms of general relativity.


QUESTION: 
if the speed of light is the top speed in the universe then how can you square it such as in the equation E=MC2?

ANSWER: 
Because the square of a speed is not a speed. The speed of light is not the biggest number in the universe, just the biggest speed.


QUESTION: 
If an object is moving towards a mirror,plane or spherical,with a velocity v,with what velocity will the image move-and why?

ANSWER: 
If it is a plane mirror, the answer is pretty obvious, v' (velocity of the image) is -v (velocity of object). If it is a spherical mirror of focal length f it is a bit more complicated. Start with the mirror equation which is 1/f=1/p+1/p' where p is the position of object and p' is the position of the image. Then, realizing that the velocities are the derivatives of the positions, v=dp/dt and v'=dp'/dt, you may show that v'=-v[f/(p-f)]2, so the velocity of the image depends both on the velocity of the object and where the object is. Note that if it is a plane mirror f=
∞ so v'=-v as noted at the beginning of the answer.


QUESTION: 
Since acceleration * time=velocity, I calculated that a person in a rocket(with lots of fuel) constantly accelerating at 1G for 360 days (both measures are with respect to rockets 'Inertial frame of reference') should reach the speed of light (with respect to earth's inertial frame of reference). But this is obviously wrong as per relativity, since light speed cannot be reached. So what would be the rockets speed after 360 days on such constant acceleration? Any formula?

ANSWER: 
Suppose that you are accelerating a 1 kg object. The required force to cause an acceleration g=9.8 m/s2 is 9.8 N. However, the theory of special relativity shows that as an object goes faster and faster its mass increases so a larger and larger force is required to cause an acceleration of g. If you were to go all the way to the speed of light (not possible) an infinite force would be required. The graph shows the force required for very high speeds.

 


QUESTION: 
the law of conservation of energy states that the total energy in an isolated system will not change. Let's say there exists an amount of matter in a theoretical perfect vacuum in which there is the absolute absence of all radiation. when any two molecules in this sample of matter collide, unless this collision is perfectly elastic (a scenario which i've assume to be impossible), upon collision, there would be a loss of kinetic energy of both particles and, since heat energy is no more than a measurement of average molecular kinetic energy, a loss in total heat energy as well. where would this energy have gone?

ANSWER: 
In fact, your assumption that elastic scattering is impossible is wrong. All collisions will be completely elastic unless the kinetic energy of some atoms is larger than the minimum energy required to excite such an atom (likely only for extremely hot gases). Atoms are not like macroscopic balls which do not have perfectly elastic collisions because of frictional type forces. And, if there is an inelastic collision between two atoms, one or both become excited and then they deexcite by radiating photons which have the energy lost initially in the collision; energy is always conserved in an isolated system.


QUESTION: 
Why do mirrors reverse reflections along a single axis instead of two axes? In case I have not been clear: why does writing reflected in a vertically-mounted mirror not appear upside down as well as backward? (or why in a horizontally-mounted mirror reflections do not appear backward as well as upside down?)

ANSWER: 
If you think about it, a mirror does not really "reverse" the image at all
—the image of your left hand is exactly opposite your left hand, and similarly your right hand, your head, your feet, etc. It is simply that if you imagine another you on the other side of the mirror, right hands and left hands appear to be reversed. If you hold up a picture of the letter N it will be reversed because the right will be left and the left right so it will look like И. But, even though the head/feet images are directly opposite your head/feet, up/down is not reversed like left/right is. The mathematical reasoning is a little subtle: if you imagine yourself on the other side of the mirror you imagine a 1800 rotation about a vertical axis which exchanges left and right but not up and down.

FOLLOWUP QUESTION: 
Regarding mirror reflections and, "The mathematical reasoning is a little subtle: if you imagine yourself on the other side of the mirror you imagine a 180(degree) rotation about a vertical axis which exchanges left and right but not up and down." My question is: Why is ONLY "left and right" exchanged, why are BOTH "left and right" AND ALSO "up and down" NOT exchanged? Isn't this asymmetric? Moving a mirror to lie horizontally simply moves the axis to also lie horizontally, the asymmetry remains. Only ONE exchange of TWO seemingly-symmetric conditions: LEFT-RIGHT and also UP-DOWN. This has been puzzling me for years. Have asked physicist friends, none have responded or said more than they don't know. I read a tiny bit about this years ago but don't know enough to research the source of that elusive bit that I really did not understand.

ANSWER: 
It all depends on the axis about which you rotate. If you were to rotate about a horizontal axis you would exchange up/down but not left/right. Clearly, when you look in a mirror you see your head on top so to imagine yourself over there you imagine a rotation around a vertical axis to get yourself facing the other way. But when you do that you find that this is not what you are really seeing; because if you had a glove on your left hand it would be on the right hand of the "other you".


QUESTION: 
In terms of Force = M*A, does mass trump acceleration? Example.....if a 2 ton car traveling at 60mph hits a stationary car, will it have the same (or approximate) effect as a 4 ton car traveling at 30mph? Please note these numbers are variable.

ANSWER: 
I am afraid you have things very muddled here. First, acceleration is not the same as velocity, so your telling me what the speeds are really does not tell me anything about Newton's second law. Acceleration is the rate of change of velocity. Second, F=ma means that if an object of mass m experiences a force F then that object will have an acceleration a; you are trying to find out what effect the car has on some other mass (you, for example). You have established a scenario where the linear momentum (the mass times the velocity) of each car is the same, so let us start there. What you want is to find the force each car exerts on something it hits. But there is not enough information. Here is one possible set of circumstances: suppose that each car comes to rest, after hitting a wall, in a half second. Since ma can be written as the rate of change of momentum, the force to stop each car (with momentum 120 ton mi/hr) will be 240 ton mi/hr/s. These are kind of weird units but I do not think it is worth making conversions to make my point. Each car requires the same force to bring it to rest in the same time as the other. Therefore, because of Newton's third law, each car will exert the same force on the wall which stopped it.


QUESTION: 
Heat is the movement of molecules /atoms and different elements have different melting and boiling points. Melting I assume is the breakdown of the crystal lattice and boiling is enough molecules/atoms obtaining enough energy to achieve the vapour state. Therefore I would have thought that different elements would have different temperatures at which their atoms were motionless (absolute zero) , because different elements would I assume have different interactions . A Gold atom would be considerably larger than a hydrogen atom but both are apparently motionless at absolute zero and there I am stuck !

ANSWER: 
No material may ever have T=0 K, however there is a temperature called absolute zero and it does not depend on the material. Temperature is a measure of average kinetic energy per particle and if all particles were to be at rest you would have zero kinetic energy regardless if you had electrons or bowling balls. (By the way, a gold atom is not considerably larger than a hydrogen atom; atoms do not get bigger just by adding electrons because you also add protons to the nucleus so the force of attraction the electrons experience in gold is much bigger than in hydrogen and this has the tendency to make heavier atoms smaller. This, however, is irrelevant for your question about absolute zero.)


QUESTION: 
What would an observer see if two objects are moving toward each other at half the speed of light? Would there be a collision, or would classical physics change the outcome???

ANSWER: 
I am afraid I do not understand your question. You would see the two objects approaching each other and colliding. If the "observer" were one of the two objects, he would see the other approaching him with a speed less than the speed of light but would suffer the collision.

FOLLOWUP QUESTION: 
But wouldn't the two objects be approaching each other at the speed of light, theoretically speaking? Just like if I were running 5 m/s toward a ball coming at me at 5m/s, we would be approaching at 10 m/s.

ANSWER: 
No, your intuition is wrong here; that is what is really interesting about the theory of special relativity. If the speed of the objects are not very small compared to the speed of light then the usual kinematic relations turn out to be incorrect. In fact, in this case, each would see the other approaching with 80% the speed of light. The actual formula is that the speed of approach seen by the moving observers is (u+v)/(1+uv/c2) where u and v are the two speeds and c is the speed of light. In the two cases you state, you have (5+5)/(1+25/(3 x 108)2) which is, for all intents and purposes, 10 and (0.5c+0.5c)/(1+0.25c2/c2)=0.8c.


QUESTION: 
There's a layman debate going on. Can angular momentum be exchanged/transferred with linear momemtum? My example is a bullet fired off center into a rotatable spherical elastic blob (kinda like the ballistic pendulum, but designed to spin). Doesn't the bullet initially have only linear momentum, mv, then zero linear momentum after sticking in the blob. And the blob now has angular momentum (picked up from the bullet) when it started with none?

ANSWER: 
Suppose the bullet approaches such that, if it did not stick, it would pass a distance b from the axis about which your blob can spin. In this case it carries mvb units of angular momentum relative to the chosen axis. When the collision takes place the axel will exert a force on the system; since there is an external force, the linear momentum will not be conserved so I will not worry at all about linear momentum (mv) before the collision. However, the axel exerts no torque and so angular momentum will be conserved. After the collision the angular momentum of the blob plus bullet will be numerically equal to mvb.


QUESTION: 
How are physicists able to create heavy elements (such as the recently-discovered 118) in the lab, when enormous amounts of energy are required to make helium by fusing hydrogen?

ANSWER: 
The energy is not really enormous, it is only relatively enormous. Because helium has a very high binding energy, when fusion occurs the amount of energy released is on the order of 1% of the mass energy of the hydrogen fuel used in the reaction. Still, the amount of energy released in a single fusion reaction is on the order of a million electron volts and 1 MeV=1.6 x 10-13 joules. For comparison, your kinetic energy when you are walking is about 200 joules.


QUESTION: 
what is 900 g/m square to pound/ foot square?

ANSWER: 
1 kg/m2=0.2048 lb/ft2, so 1 g/m2=0.0002048 lb/ft2. A very handy little utility for doing conversions is CONVERT.


QUESTION: 
My students ask if a metal in the photoelectric effect ever runs out of electrons.

ANSWER: 
Normally the metal is part of an electrical circuit and as the electrons flow from it (photocurrent) they are replaced by the resulting current in the circuit. Essentially ejected electrons are captured and flow back to the metal. If you just have a piece of metal isolated in space the electrons would leave and the metal would get more and more positively charged. The result of this is that it would get harder and harder to remove electrons from it. However, in normal circumstances the rate of ejection of electrons would result in a relatively trivial change in the number of electrons in the metal because there are such an unbelievably large number of conduction electrons in a metal (on the order of 1023).


QUESTION: 
A friend of mine had this doubt- If a long metal rod of length 'L' is heated by increasing 1 degree centigrade and if it increases by kL, its total length is now (1+k)L. Call this length 'L1'. Then, what if its temperature is increased by another degree? Is it L1+kL=(1+2k)L OR (1+k)L1=(1+k)(1+k)L=(1+2k+k^2)L? I said it is plainly (1+2k)L citing a physics text book. But he reasoned that many text books give 1+2k because k^2 is negligible. I said that physicist in that case would have called it "exponential expansion of solids" and not "Linear". We are undecided as to what the answer is.

ANSWER: 
The way length contraction is normally framed is to write
ΔL=kΔT, that is L=L0+k(T-T0), where L0 is the length at T0 and L is the length at T. In the real world this is only an approximation because k is a function of T. It is, however, an excellent approximation because the functional dependence is very small if ΔT is not too large. So clearly you are right, that is the 2 comes from ΔT, not from the quadratic approximation. The quadratic approximation comes into play when one tries to figure out how much an area expands. If we assume that k is small then it is easy to show that ΔA=2kΔT where A=wh where w and h are the width and height of a rectangular area, both of which expand like kΔT; here a quadratic approximation has been applied.


QUESTION: 
Given a sturdy motor with a string attached spinning a ball around in a circle with constant velocity, does the string stay in the same plane at all times, or will the ball follow another path either visible or too small to see?

ANSWER: 
(First I should say that the ball has constant speed, not constant velocity since velocity is a vector and the direction of the velocity is always changing.) It all depends on a number of things. In the ideal world of introductory physics we usually say: ignore air friction, ignore the weight of the string, assume that at the beginning of the problem the string is straight and the ball is moving in a circle with constant speed. Address that first. The string cannot be horizontal no matter how fast the ball is moving, so even in the ideal case the string will define a cone, not a plane. Now, all real strings have weight so it will not be in a straight line and the surface it defines will be a sort of pinched cone because of the sag due to the weight of the string. If air friction is taken into account all kinds of things could happen like turbulence around the string which could cause it to vibrate in a chaotic way. Even without gravity or air friction the conditions at the beginning of the problem have to be just right such that the string does not have any vibrational motion (think of a rotating guitar string).


QUESTION: 
Is there a relationship between metabolism and the passage of time. When young ,time passes quickly, as we age, it passes much slower, yet young and old occupy the same time period,,Is time an illusion. Is time a human concept.

ANSWER: 
Time is defined in a way totally independent of any living thing. How it seems to any individual is not relevant to a physicist. For example, when a physicist says that clocks run more slowly in a moving spaceship than here on earth we do not mean that it seems or appears to do so, it does.


QUESTION: 
Petrol is more flammable than diesel so why does a petrol engine need a spark plug?

ANSWER: 
The diesel engine operates differently than the internal combustion gasoline engine. In both engines the air-fuel mixture is compressed rapidly to a small volume. The result is that the temperature rises. In the diesel engine the temperature reaches the combustion temperature of the fuel and it spontaneously ignites but the combustion temperature is not reached in the gasoline engine.


QUESTION: 
I have always wondered if you have a container that is a perfect insulator, it's at room temperature when you seal it will the temperature eventually drop to absolute zero or would the temperature never change?

ANSWER: 
It would stay exactly at room temperature. If the temperature decreased this would violate energy conservation, that is where did the energy go?


QUESTION: 
I know that the gravitational force between two objects is inversely proportional to the square of the distance between the objects' centers of mass. Do magnetic fields behave in a similar manner? I think that I read somewhere that magnetic fields are inversely proportional to the cube of the distance to the surface (or something similar), but I can't remember where.

ANSWER: 
If there were such a thing as a magnetic monopole (like an isolated N or S pole) it would have a 1/r2 dependence for its field. You can verify this by looking at the field near a N pole a distance small compared to the distance to the location of the S pole. However, N and S poles always appear together in nature and when you get far away from a dipole (this happens with an electric dipole too, a + and a - charge separated by some small distance) the field is seen to fall off as 1/r3 as you have read.


QUESTION: 
What exactly determines the cutting ability of a laser?

ANSWER: 
Power density, that is the energy per unit area of the beam. For example a laser with a power of 500 watts and spot size of 1 mm diameter (6.4 x 108 W/m2) would have more "cutting ability" than one with a power of 100 watts and 0.5 mm diameter (5.1 x 107 W/m2).


QUESTION: 
Radiometric dating is used to date how old a rock is - ie how long since it was last molten, by comparing the quantity of an isotope, to the quantity of it's decay product. What I do not understand is how is it known how much of both were present when the rock was formed? If the half life is 1000 years, then after 1000 years, 1gm of isotope becomes a half gm, and we have a half gm of decay product. Was there no decay product present when the rock formed? Were the rock components not solid at some previous time and were decaying then hence the molten rock would include decay products from then?

ANSWER: 
There are many kinds of radiometric dating and the details and assumptions are different for each. Let me use the best-know dating method, radioactive carbon. There is an unstable isotope of carbon, 14C, which has a half life of about 6000 years. When the earth was first formed, about 4.5 billion years ago, if there had been any 14C it would have been long gone by now; however, if you analyze something formed yesterday with carbon in it you will find 14C in it. What is happening is that the earth is being bombarded with cosmic rays which collide with nitrogen, 14N, in the atmosphere and create 14C. Hence, if you wait 3000 years the 14C content in the thing formed yesterday will have only half the expected amount of 14C in it. The method relies on the assumption that the intensity of cosmic rays has not changed very much over a period of several thousand years.


QUESTION: 
Energy balance question: A physicist is standing on a pier along which a freighter is moving at 10 mph. A mass is sitting on the deck. Its kinetic energy is easily calculated by the physicist. Another physicist on the freighter gets a different view, but no one is confused by the result. Then the mass is given a velocity of 10mph opposite to the motion of the freighter, so that the dockside physicist sees it come to apparent rest. Both physicists calculate the same amount of energy required to accomplish this task. But suppose that the mass had been accelerated in the opposite direction. The dockside physicist now sees a velocity of 2v, which will mean a new KE that is 4 times the original. The physicist on board sees only 1v. How do they account for the difference in energy required to impart the velocity?

ANSWER: 
The short answer is that the two observers, on pier and on boat, see a different amount of work done on the mass so it gains a different amount of energy for each observer. Now the details. As seen on the boat, some force F acts over a distance s to increase the speed to v. So the work is equal to the increase in kinetic energy: W=Fs=
½mv2. Now, as seen from the pier, the force acts not over a distance s but a distance s+vt (t is the time the force acted) so the work done is W'=F(s+vt). So we need to know how long F acted for the speed to get to v on the boat; the acceleration is a=F/m so v=at, that is, t=mv/F and so W'=F(s+mv2/F)=(3/2)mv2. Since the mass, as seen from the pier started with ½mv2 of kinetic energy, the force increases the energy to mv2m(2v)2. Energy is not something seen to be the same for all observers.


QUESTION: 
let two bodies each of mass=mkg and having opposite but equal charges =Q coloumbs revolve around their common CG in circular orbits of radius r metres. since these are charged and acclerating, they will both emit electromagnetic waves and loose energy, fall towards each other and revolve in a smaller orbit. since centrifugal force(=m*w^2 *r) has to be equal to attraction (= constant/r^2 ), and the combined angular momentum =2m*w*r^2, it is easy to show that angular momentum=constant *r^(0.5). now how can angular momentum reduce? is the remaining angular momentum carried away by the emitted photons or do the bodies start to spin? in that case how do they get the torque to cause them to spin?

ANSWER: 
Since you are clearly talking about a macroscopic system, let's not talk about photons but rather about electromagnetic radiation. Electromagnetic radiation carries both energy and angular momentum. So neither angular momentum nor energy are conserved as the system radiates.


QUESTION: 
What happens if I jump vertically on a moving bus? Will I move back or will I stay in the same spot? And Why

ANSWER: 
Physics is an experimental science and this would be an extremely easy experiment for you to try. Even more impressive would be to jump straight up in an airliner which has a speed of about 600 mi/hr which is almost 900 ft/s. So, if the plane were to move forward and you not, if you were in the air for 0.2 s you would land about 180 feet behind where you jumped, obvious nonsense! The reason you come back to where you jumped from is that you and the plane both have the same forward velocity when you jump and it does not just disappear when you are not in contact with the plane, you keep moving forward with that same speed, just like the plane. An observer on the ground would see you as moving in a parabolic path as you went by. Another example is to look at WWII movies of bombers dropping bombs: the bombs all are strung out (approximately) directly below the airplane. All this assumes that the airplane (or bus) is traveling in a straight line with constant speed.


QUESTION: 
If you have a container thats 5 feet cubed that's filled up with water to the top (5 feet high). What is the pressure and force at the bottom of the cube? Is there a force acting on the sides of the cube? If so, what would the force equal to?

ANSWER: 
The density of water is about 62.4 lb/ft3 so the weight of the water is 7800 lb; the bottom must hold this weight up so the force on the bottom must be 7800 lb. The pressure on the bottom is 7800/52=312 lb/ft2. The sides certainly feel a force or else the water would just stay there if we took the sides away. It is more complicated to compute this because the pressure in the fluid gets higher as you go deeper in the water. It turns out that the force on each side is about 3900 lb. (Note that these are gauge pressures, that is pressure above atmospheric pressure which is about 2116 lb/ft2; technically all my answers should be scaled up accordingly. Gauge pressure is what you read when you measure tire pressures, typically about 30 lb/in2.)


QUESTION: 
How is the law of conservation of energy still in effect when something is at terminal velocity?

ANSWER: 
First, state the law: The total energy in an isolated system will not change. If you choose the falling object as the system, it is not isolated and so energy is not conserved. The earth does work on it increasing its energy and the air does negative work on it decreasing energy. If you include the earth in the system, the kinetic and potential energy will remain constant in the absence of any air resistance. If you include the air in your system, then as the air takes energy away from the falling object the air must gain that amount of energy which would show up as the air heating up some.


QUESTION: 
What happens to a narrow beam of light that passes through a single concave mirror? What would happen the beam of light were to enter the focal point of the concave mirror before being reflected?

ANSWER: 
Only a few rays are simple to draw. But any single ray is easy to follow because the angle of incidence of the ray (relative to the normal to the curved surface) must equal the angle of reflection. A ray passing through the focus will exit parallel to the optic axis.


QUESTION: 
In newton second law you have(f=ma) but if you where to have to magnets repellng each other but in a stationary position then wont you get force without acceleration?

ANSWER: 
The f in f=ma is the net force on the object of mass m. In your example, each magnet must be held down; suppose each is held in place with a screw. Then each magnet exerts a force on the other but each screw exerts an opposite force so that the net force is zero.


QUESTION: 
What type of force would an asteroid have to produce during an impact on earth in order to have a drastic change on the earth’s orbital path? I use this estimated impact effect result calculater online(http://www.lpl.arizona.edu/impacteffects/) for an answer but as soon as I enter a higher diameter or velocity for a theoretical hit, I get an Earth that is completely wiped out.. leaving only an asteroid belt behind. Obviously this is all theoretical so I do not expect exact calculations.

ANSWER: 
There is a very simple answer to your question: you must exert a force which is comparable to the only other force on the earth, namely the force exerted by the sun. You would also have to exert the force long enough that the impulse delivered (the integral of Fdt) is comparable to the earth's linear momentum in its orbit. It is really the impulse that matters, that is you could exert a small force over a very long time and alter the orbit.


QUESTION: 
what is time travel?How is it possible?

ANSWER: 
"Time travel" refers to the idea that we might do something which would result in our "visiting" a different time than that which we would experience if we did nothing. For example, imagine that you suddenly could cause yourself to be present at an event which will happen in 200 years from today, something which you could not normally do since you would be dead by then. Or suppose that you are able to be present at an event which happened 200 years earlier than today, again something seemingly impossible since you did not even exist then. The known laws of physics state that going forward in time is possible but going backwards is not.


QUESTION: 
There is something I do not understand about Mehcnaicl energy. My text book states that Mechanical energy is the sum of K (kinetic energy) and U (potential enery), while the definition of mechanical energy states that mechanical energy is energy that depends on the position OR motion of the masses. If Mechanical energy can only be potential or mechanical, how can it be the sum of both? Is ALL potential energy converted into kinetic energy when the object is in motion or is there some potential energy left?

ANSWER: 
You are taking your second statement to literally; the use of the word "or" does not imply exclusivity, that is, either or both may contribute to total mechanical energy. And, no, all potential energy need not be converted to kinetic energy although it could be.


QUESTION: 
if the sun is emitting enormous amounts of mass into the heliosphere, (bear with me because this is in fact a physics question) then will over time the orbit of the earth change since gravity and mass are proportional..?

ANSWER: 
What may be enormous amounts of mass are very small fractions of the total solar mass. The effect of mass change on orbits of planets are negligible over millions of years.


QUESTION: 
Can I find the required force, or acceleration if I only have known distance, and mass? So f=ma, and all I know is say, m=1g, and say, distance=1 meter. I don't know how long it takes to thrown. So i don't know delta t. I want to calculate a, & hence F. Is there a=f(m,d)?

ANSWER: 
No, you cannot find the acceleration by knowing only the distance traveled; you also need two other quantities, the final and initial velocities or the initial velocity and the time elapsed are two possibilities. Also, of course, you must know that the acceleration is uniform (constant) over the time interval.


QUESTION: 
Does the electrolysis of an electrolyte give out a magnetic field? Would this be detectable?

ANSWER: 
Since electric currents flow during electrolysis, magnetic fields are made. These fields, while small, would be detectable.


QUESTION: 
I was wondering what makes a substance transparent? I know that free electrons send the photons back, but then I do not quite understand how air can be see through - the air molecules share and exchange electrons - right? And what make metals non-transparent?

ANSWER: 
The air molecules rarely collide and they certainly do not share and exchange electrons. The answer to why materials are transparent can be found in an earlier answer.


QUESTION: 
I have a question about impulse thrust. If an object weighing 106kg sits stationary on the ground was to suddenly experience an upward thrust 1200N for 110ms then off 55ms, and it where to continue the cycle of 1200N for 110ms and off for 55ms for several seconds, would the impulse frequency be enough to lift it off the ground and keep it off the ground? If its not what amount of force would be needed if the time of 110ms could not be changed? For some reason I keep thinking that because the upward force will be greater than Gravitional force, and the overall time ON within a second (1/(.110+.55)x0.11=0.66ms) means that it'll clear the ground. But I suspect I could be wrong.

ANSWER: 
The acceleration when the force is on is given by a1=F/m=(1200-9.8x106)/106=1.52 m/s2. The acceleration when the force is off is simply a2=-9.8 m/s2. The time intervals are 0.11 s and 0.055 s. The operative kinematic equations are those of uniform acceleration: y=y0+v0t+
½at2 and v=v0+at where the 0 subscripts indicate the value (of y or v) at the beginning of the time interval t. At the end of the first time interval, I calculate v=0.167 m/s and y=0.0092 m; it is moving up and is above the launch level. At the end of the second time interval, I calculate v=-0.372 m/s and y=0.00356 m; it is now moving down but is still above the launch level. At the end of the third time interval, I calculate v=-0.205 m/s and y=-0.028 m; it is moving down and is below the launch level, i.e. it has crashed!


QUESTION: 
I was wondering about the chance of two balls bouncing around a room colliding with each other? Would this chance increase if one of the balls was stationary?

ANSWER: 
The center of each ball must be in the same small volume, v, at a given time; that volume is the volume of a sphere of radius twice the radius of either ball. The probability of finding one ball in that volume is v/V where V is the volume of the whole room (assuming the balls are randomly moving around the volume of the room). If one of the balls is at rest, it is at the center of v so the probability of its being in the volume is 1 (100%). So the probability of a collision is v/V. If both balls are moving, the probability of a collision is (v/V)2, much smaller. For example, if v/V=0.001 then the probability of a collision is 0.000001 if both are moving.


QUESTION: 
Can we nullify(or atleast minimise to some extent) the effect of one of the poles of N-S magnet such that we can observe the independent effects of the other pole(as if we have a unipolar magnet)?

ANSWER: 
You cannot nullify a pole completely, but you can approximate a monopole by having a long bar magnet and making measurements a distance from one pole at locations much closer to the pole than the length of the bar. The resulting field will look much like the electric field of a point charge, radially out (or in) and having a 1/r2 dependence.


QUESTION: 
My brother and I recently watched an episode of mythbusters in which the mythbusters had to find the terminal velocity of an object. To do this they used a tube and some air nozzles, they also had a pitot tube to measure air speed. My brother says that the mythbusters cannot find terminal velocity this way because the air nozzles would create a high pressure pillow and throw the terminal velocity reading out of whack.

ANSWER: 
I do not have enough information to answer your question definitively. But, whenever making a measurement it is necessary that the measurement not change that which is being measured. My guess is that, since this was on a reputable TV show that some effort was made to insure that the measurement did not appreciably affect the measured quantity. Technically, any measurement of anything will alter the measured quantity; the trick to good science is to minimize the effect.


QUESTION: 
A swing in our backyard was made from two separate ropes hanging down and attached to a seat. i note that if the swing is twisted once (so the ropes cross) it then twists further and appears to reach a stable state with multiple twists. If left untwisted it does not spontaneously make the first twist. I assume each twist raises the seat and hence the gravitational potential energy, so why does it make multiple twists? My guess is it is something to do with the coil in the rope. The two ropes are oriented with the same "handedness" of coiling. Does uncoiling the rope somehow let the seat go lower despite extra twists?

ANSWER: 
If a rope is twisted from its equilibrium status it acts like a spring, that is it trys to restore the rope to its equilibrium condition; this is often referred to as a torsion pendulum if, say, a steel rod is attached to the center of a disk and allowed to oscillate after giving it a twist. In your case there are two ropes attached to opposite ends of the seat and, if both are twisted with the same sense, they will each try to twist the seat in the same direction and seat wants to twist until the ropes are untwisted. But, thinking about the geometry, the only way the swing can twist is for it also to rise up and if the energy stored in the twists is insufficient to increase the potential energy that much, it will not be able to untwist on its own. So you intervene and give it that first twist. Now, if the ropes are still twisted but also crossed, it can continue untwisting because after the first turn it need not raise the seat up very far as each new revolution is added.


QUESTION: 
This is about distance, height of eye, and how far one can see from one location on earth to another location on earth. Currently, the tallest building in Philadelphia, PA is Comcast Center at about 1000 feet. The tallest building in New York, NY is the Empire State Building at 1,250 feet. There is 83 miles between the cities. There are other factors involved, of course, such as sea level of both cities and what not. But just given the basic facts above, would one be able to see the top of either building from the other building on a perfectly clear day using something like a telescope?

ANSWER: 
It is easiest to calculate the heights of two identical buildings in each city. I calculate that to see the tops they would have to be about 1140 ft high. Since the Philadelphia building is a little less than this and the NYC building is a little more than this, observers on the tops of these buildings would be just about able to see each other.


QUESTION: 
Scene: A stationary spacecraft in deep space far from any gravitational forces (planets, stars, etc). If it's engine(s) suddenly went to full power, would crewmen/loose objects/etc. experience the effects of acceleration (g forces), or would they accelerate at the same rate as the spacecraft? If they do experience g's, would they be equal to those experienced at ground level on Earth for the same level of acceleration?

ANSWER: 
Objects in the spacecraft would have to accelerate at the same rate and would therefore have to feel an appropriate force accelerating them; this force would be the ship itself or the seat they were sitting in, etc. It is just the same as when you sit in an accelerating car
—you feel you are being pushed into the back of the seat but what is really happening is that the seat is pushing forward on you to provide your acceleration. Since the human body is not designed to endure accelerations much in excess of g, this is a compelling reason why space travel will never be possible for single generations of humans. This is one of the reasons sci fi movies where a spaceship accelerates from rest to near light speed in the blink of an eye are fiction, not science.


QUESTION: 
I've always wondered why exactly and how does swinging your legs keep a swing in motion?

ANSWER: 
It all has to do with raising and lowering your center of gravity relative to the seat of the swing. For details, here is a good web site.


QUESTION: 
In the famous Michelson-Morley experiment, the existence of the ether medium was ruled out. The experiment was done ON EARTH. Does it mean that the ether medium was assumed to have penetrated into the atmosphere? If yes, why was its existence not verified in other, physiology related experiments? If no, how much can we rely upon the results of the experiment?

ANSWER: 
The ether was postulated to exist because, at this time it was incomprehensible that waves could travel without there being a medium to support them (sound in air, surf on water, vibrations on strings, etc.) and so light must have have some medium through which to travel since it is surely a wave. Since light clearly propogates in the vicinity of the earth so it follows that there must be ether here.


QUESTION: 
As the earth rotates around the large Sun, is accelerates as it approaches the sun with in the ellipse and then is jettison out to the wider part of the loop. Well the physics seems simple for the first few passes. But why is it that the earth, or any other planet for that matter doesn't eventually spin tighter and tighter circles and eventually get absorbed or crash into the Star (sun)? Partial answer resolved: since I am not an idiot and I have given this some thought, I did reason to the point that the answer must have something to do with the fact that we are spinning in the opposite direction on our axis compared to what one might expect. That is, we are churning outwards from the sun on our axis rather than turning towards it. I assume that this is in part the reason we do not crash into the sun after a brief number of rotations. However, The follow-up question why are we spinning this way begs asking. (how is that I am begging / plea-ing in the original question). I am not sure this answer is known, or if the answer is Newtonian, which it seems like it should be. Maybe there is a more human scale example of this kind of relationship, to view from, but I don't know of one.

ANSWER: 
The reason is way easier than you think. First of all, what makes you think that the orbit would spiral in? It is all a completely well-solved problem and is know in physics as the Kepler problem. The mathematics are probably more than you want to plow through, but it turns out that for a 1/r2 force which gravity is (that is, the force is 4 times smaller when the distance to the sun is 2 times larger), the orbit is stable and closed. The shapes of possible orbits are conic sections (circles, ellipses, parabolae, or hyperbolae). You can get the basic results of the Kepler problem by reading about Kepler’s three laws which he deduced empirically from data snd Newton later showed to follow from Newton’s laws and the universal law of gravity.


QUESTION: ;
can a quarter go through a wooden table?

ANSWER: 
The short answer is no, this could never happen even if every atom in the quarter fell exactly halfway between atoms in the table. The long answer is given in an answer I gave to a similar question long ago.


QUESTION: 
I know that transmitting a sound, radio or light wave signal towards a remote object that is moving away from or towards the observer results in the Doppler effect; in which the returned signal frequency is shifted slightly. However if a light signal is AM modulated with an audio frequency will that audio signal also be shifted by the Doppler effect? If so how do I calculate the amount of shift given the carrier frequency is light at 1nm. wavelength and the audio signal is a 2000 Hz. sine wave.

ANSWER: 
You do not need to know the carrier frequency. Just assume you have electromagnetic waves of frequency 2000 Hz and velocity 3 x 108 m/s and compute the doppler shift.


QUESTION: 
Is energy convertible to mass (Just like mass is converted into energy in nuclear reactions)?

ANSWER: 
Yes. It happens all the time in high-energy particle accelerators where new particles are created. The most well-known example is called pair production where energetic photons (electromagnetic radiation) create electron-positron pairs.


QUESTION: 
I have two objects which are the same shape but one weighs more than the other. When I accelerate them at sea level with equal amounts of force over identical periods of time, they do not achieve the same speed, and they decelerate at different rates. Now I take object "A", the lighter of the two, and use the same force to accelerate it as before, but I do this at a higher altitude, say 5,000 or even 10,000 feet above sea level. It moves faster due to less air density, and so arrives at its destination slightly quicker than it did at the lower altitudes. Can I use the data from object "A" to reliably predict the difference in time of flight for object "B"?

A