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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 T_r along the radius R is (see first picture). Similarly, the centripetal acceleration is not V²/R but V_t²/R where V_t 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 f_r which is along R (see third picture). So, your teacher is wrong because the velocity V is not perpendicular to the centripetal force T_r-f_r (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 (-T_tR). 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 v₀cosθ. If someone on the ground measures it, the horizontal component is (v₀+10)cosθ.

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 matal 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 ½Iω² 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: 
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 19^th 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 Feynmen 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 Schrödinger'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 k_eZe²/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 W_T. I will call the bottom book B and it has a weight W_B. 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 F_TB. Book T exerts a force down on book B and I will call that F_BT. 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 F_TB must be equal in magnitude to W_T and pointing upward because of N1. Now, focus your attention on book B. There are three forces on book B, its weight W_B, the force from the table F, and the force from book T F_BT. These three must add up to zero because of N1. But we do not know F_BT. But wait, aha! We know that F_BT is the force which T exerts on B and F_TB is the force which B exerts on T, so N3 tells us that these must be equal and opposite. Therefore the magnitude of F_BT must be equal in magnitude to W_T and point down. Putting it all together and using N1, the magnitude of F must be W_T+W_B 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=rω, 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 λ_glass=λ_air(v_glass/v_air). Since v_glass<v_air , 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(2πR/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 T_lab=½mv²=½MV². So, clearly, unequal masses with equal lab energies have unequal velocities. Now, the center of mass energy is given by E_cm=½μv² or E'_cm=½μV² where μ=Mm/(M+m) is the reduced mass. Since the v≠V, it follows that E_cm≠E'_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 x_n=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 v≈Ft/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.)

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-W_A where W is your true weight and W_A is your apparent weight. Hence, W_A=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 E=½mv²+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: 
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 (R_e) is usually used to characterize what the velocity dependence is. If R_e<1 the force is mainly proportional to the speed v whereas if R_e>1000 the force will be quadratic in v, that is proportional to v². Taking a lead sphere with diameter 1 micron, I reckon that R_e 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 R_e<<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 outside observer. 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: 
   

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 -mgΔy 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.