home
the physicist
groundrules
answers
old answers
donate

FAQ

Questions and Answers

Here is a history of questions and answers processed by "Ask the Physicist!". If you like my answer, please consider making a donation to help support this service.

If there is a link to a previously answered question, be patient. Since the files containing the older answers are rather large, it takes some time (maybe as much as 15 seconds or so) to find the appropriate bookmark.


QUESTION: 
This might take a little time to explain. I know that the speed of light is such that when we look at stars, distant galaxies and other stellar phenomena, we are looking at them as they were (since it took light time to reach us). Here lies my problem. If physicists and cosmologists state that the universe is expanding due to the red-shifted light from galaxies, and the further away a glaxay is, the faster it is traveling, then that red-shifted light being observed took a long time to reach us. So, we are seeing a red-shift from, say, 1000 years ago. Could it be that now galaxies are moving towards us, just that the blue-shifted light has not reached us yet? What am I missing? Every text I read, every show I watch, everyone is certain the universe is expanding.

ANSWER: 
First, your time scale is way off
—we're talking billions of years, not thousands. You are right, it is certainly possible that distant objects could now be moving toward us and we won't find out for billions of years. But, we are able to see a huge range of distances into the cosmos and all systematics, near and far, indicate that everything is moving away. In recent years evidence has been found that the distant objects are actually speeding up (the source of which is called "dark energy"), providing even more evidence that we do not expect anything to be turning around.

QUESTION: 
Is a low or high current solar charger/battery system (same voltage) more efficient to charge small electrical devices (iphone etc) please?

ANSWER: 
Well, since power is current times voltage, higher current is higher power
—charges faster.

QUESTION: 
if electricity is made up of millions / billions of electrons and has a tiny amount of mass. Is it affected by G force when it goes from zero to the speed of light and back to zero, eg when its switch a light on and off?

ANSWER: 
Indeed, electrons do not go at the speed of light in a wire. They go very slowly, maybe on the order of 1 mm/hour! In any case, the force of gravity on an electron is negligible under almost all possible conditions.

QUESTION: 
I am not wishing to make a weapon its purely scientific curiosity. If you had a laser beam and placed it behind a magnifying lens of some sort such as a telescope, you could magnify the lasers strength. Now my question is if you were to put another magnifying lens in place to magnify the current beam would it magnify it even stronger? What I am asking is if your laser is 1kW and by placing the magnifying lens or telescope makes your laser go to 2kW. Adding another lens or telescope would make it 4kW?

ANSWER: 
You are confusing power with intensity. You can use a lens to increase the intensity (energy/second/square meter) but not the power (energy/second). Hence, you cannot take 1 kW and increase it, you can only decrease the area over which it hits.

QUESTION: 
If matter and anti-matter annihilate each other when they come into contact, how is Positron Emission Tomography possible ?

ANSWER: 
Because PET detects the products of the annihilation (two 512 keV gamma rays).

QUESTION: 
How do you have Kinetic Energy at the top of a projectile? I thought the velocity in the y direction was 0, and even if we consider the x direction, wouldn't the kinetic energy at the top of the projectile equal the kinetic energy when the projectile initially set into motion?

ANSWER: 
Anything which is moving has kinetic energy. Part of the the kinetic energy has been converted to potential energy, but not all of it. The energy conservation is expressed as
˝mvx2mvy2=mghmvx2 at the top (y=h).

QUESTION: 
What l want to know is can you see something before the sound without it being faster than the speed of sound? For example, all my life l've noticed that l will hear the jet plane over here and see the jet plane there. l do hear and see it at the same moment in time but the sound and the sight are not in the same place. l hope what l just wrote makes sense, haha. The current argument raised is that this is not possible unless the thing is traveling faster than the speed of sound. However, the speed of sound is like 1000 miles an hour or something, right??? l'm not sure on that but l'm pretty sure it's faster than most commercial jets fly. So l have a sneaking suspicion that what's going on here is some kind of matter of semantics and/or we are both right and both wrong. Or we are talking about different things, l don't know.

ANSWER: 
The answer is very simple: the speed of sound (about 330 m/s) is enormously smaller than the speed of light (300,000,000 m/s).

QUESTION: 
As the stars die, will the sun die too?

ANSWER: 
All stars will eventually die because they will run out of energy. The sun may last a few billion more years but life on earth will be unsustainable in about one billion years because the sun will become too hot.

QUESTION: 
My question is: why does light bend when it slows down? I'm currently a physics student at UC Santa Barbara (2nd year) and so I know that light has a finite speed, and it slows down when it enters another medium, and if it enters another medium at an angle other than perpendicular it bends (refracts). I know about angle of incidence and all that, I just don't understand why light has to bend because it slows down? As I understand it light can be thought as a quanta of energy, an infinitesimal wave packet (maybe that's where I'm wrong?) So why does it need to bend just because it is slowing down in an angled medium? I've often heard the analogy of a car: if it hits glass angled downward from left to right then the upper left tire hits the glass first and slows down while the right wheel remains faster and therefore bends it "upwards". But light packets don't have a discernible "thickness" where one part hits before another (I don't think!) so that analogy breaks down for me! Sorry for rambling, I'm just confused.

ANSWER: 
Refraction is geometric optics and thinking of photons is definitely not the way to best understand it. You should think in terms of waves
wave fronts and rays. I found a nice description (based on Huygen's principle) on the web. Refer to the picture on the right.
  • Wavefront 1 reaches A.
  • Wavefront from A starts to spread out.
  • When incident wavefront reaches B, secondary wavelet from A has travelled a shorter distance to reach D.
  • It gives a new wavefront 2.
  • As a result the wave path bends towards the normal.

QUESTION: 
What is the word for the amount of upward velocity necessary for an object to continue moving upward against gravity after it leaves your hand. I believe there is a word that distinguishes between simply applying the force necessary to move an object against gravity vs. accelerating that object so that it will continue moving against gravity on its own.

ANSWER: 
I am not sure what you are asking because you have many misconceptions. You may be asking the term which describes the minimum velocity something must have to completely escape from the earth. This is called the escape velocity. Escape velocity from the surface of the earth is about 25,000 mi/hr. For comparison, the required speed to put a satellite in a low orbit (like the shuttle) is about 18,000 mi/hr.

QUESTION: 
What is the proper term for the opposite of antimatter? "Regular matter"? "Normal matter"? Just "matter"? Is anitmatter a form of matter, or are "regular matter" and antimatter both forms of a larger concept? And, if they are, what is that concept called? Is it also called "matter"?

ANSWER: 
This is just semantics. As far as I know, there is no formal definition of the word matter but it very well could include antimatter. Here is what Wikipedia has to say: "
in practice there is no single correct scientific meaning; each field uses the term in different and often incompatible ways. A common way of defining matter is as anything that has mass and occupies volume."

QUESTION: 
What part of a U-235 or Pu-239 atom is converted into energy during nuclear fission? Proton? Neutron? or something else?

ANSWER: 
The energy comes from mass, you know, E=mc2. When a heavy nucleus breaks into two pieces, the masses of the pieces are smaller than the mass of the original nucleus; that is mainly where the energy comes from. After the fission occurs, numerous neutrons are ejected from the smaller nuclei and these have kinetic energy which comes, again, from mass. The nuclei turn out to be unstable (they have too many neutrons) and so they beta decay, converting neutrons into electrons and protons in the nuclei; again, the energy released comes from the mass decreases which occur. In the end, if you measure the mass of everything that is left, it is less than everything you started with and that missing mass is where all the energy came from. You might like to learn a bit more at an earlier answer.

QUESTION: 
Do spirits have any mass ?

ANSWER: 
The density of alcohol is 0.79 gm/mL.

QUESTION: 
I have another question for you sir. This is a simpler one, perhaps laughably simple to you, but I haven't taken physics in school yet. (It's next on the list). Anyway, why is the distance traveled by an object dropped from rest equal to half of the 9.8 m/s/s it's traveling at in the first second? Also, why does it's distance then increase four fold to match the velocity in the second second, and then why is the distance traveled by the object much larger than its velocity after the second second?

ANSWER: 
This is very basic kinematics. 9.8 m/s/s is not the rate at which it is traveling, it is the rate at which it is changing its speed, its acceleration. At the end of one second after being dropped the speed is 9.8 m/s but, if you think about it, it started from rest and so its average speed over that first second must have been less than 9.8 m/s. It turns out (I will leave it to you to learn kinematics when your time for physics arrives or sooner if you want to dig it out for yourself) that the average speed is 4.9 m/s over that first second. In general, x=
˝gt2 where x is distance fallen from rest, g=9.8 m/s2, and t is the time fallen from rest. So you can see why the distance increases quadratically with time.

QUESTION: 
Are colors associated with sound? Specifically, do visible colors correlate with the musical scale... red=DO; orange=RA; yellow=MI; green=FA; blue=SO; indigo=LA; violet=TI

ANSWER: 
Sound and light are two completely different kinds of physical phenomena and there is no intrinsic relation between any colors and pitches. You may define a corespondence but it has no meaning in physics. Your correlated pairs are at least both in the order of increasing frequencies, red and do are low frequencies and violet and ti are high frequencies.

QUESTION: 
I'm wondering about buoyant force. It seems to me that a particular liquid would exert a particular buoyant force, making the buoyant force a constant much the same way gravity is a constant force. An object's density would then determine the effect the buoyant force would have on the object. If an object is less dense than the liquid it will float - the buoyant force is having a great effect on the object. On the other hand, if the object is more dense than the liquid the buoyant force will have less of an effect. Am I thinking about this correctly? I would really appreciate an answer as soon as possible because I'm teaching a teacher workshop and I don't want to give teachers misconceptions.

ANSWER: 
You are talking about a classic physics problem, perhaps the oldest known physics. The basic principle was discovered by Archimedes more than 2000 years ago. Simply stated, Archimedes' principle is that the buoyant force is equal to the weight of the fluid which the object displaces. So the buoyant force certainly depends on the density of the fluid. So your conclusions follow: an object whose weight is greater than the weight of the fluid it displaces will sink and the converse situation implies it will rise to the surface where it will settle to where it displaces a volume of fluid equal to its weight.

QUESTION: 
I am interested in building a device that might collect electrical energy from using a solenoid generator that would be push the magnet by being connected to the inside of the tire. The solenoid generator would be in-line on the rim where when the tire comes in contact with the road surface it would come toward the rim, then when the tire leaves thr road surface it would be returned with force to the original position. DOES THIS SOUND LIKE IT MIGHT COLLECT SOME USEFUL ENERGY IN=ORDER TO HELP RECHARGE A BATTERY IN A ELECTRIC VEHICLE ??

ANSWER: 
It sounds to me like you want to replace the shock absorbers with electromagnetic dampers. What a shock absorber does is damp out the vertical motion of the wheels by converting the energy to heat. What you want to do is convert that energy instead into electrical energy. Although the idea is sound, I believe that the amount of energy you would harvest would be smaller than the energy you would lose by having to carry those heavy permanent magnets along. Already hybrid cars harvest the kinetic energy of the car by electromagnetic braking. Think about it: conventional brakes get very hot in use indicating that there is significant energy there to harvest whereas I suspect shock absorbers really do not heat up that much.

QUESTION: 
If I pointed an infrared-based remote control at a some mirror that are aligned so the beam can hit it several times before being recieved by a television or similar device, would the device be able to recognize the command sent to it, or would it think that nothing useful happened? Under what situations might these results be obtained?

ANSWER: 
I have certainly bounced the signal from a remote to the TV. How many times you might bounce it around would depend on the nature of the reflecting surfaces and the power of the remote.

QUESTION: 
We know that the gratest persentage of an atom is vacuum. We also know that soundwaves do not travel through the vacuum. However sound travels through matter which, though, is mainly vacuum. How can this happen?

ANSWER: 
Sound waves have wavelengths of a few centimeters to a few meters. Atoms are of a size roughly 10-10 meters and their structure is totally invisible to sound. Anyhow, the atom is not mostly a vacuum, but is filled up with electrons. The atom should not be thought of as tiny electrons orbiting the nucleus but as a "cloud" of electric charge surrounding the nucleus. You might think about the space between atoms in a gas as being "vacuum" but since the distance betweens atoms in a gas is on the order of 10-7 m, this too is invisible to sound waves.

QUESTION: 
I am reading a book "Power Hungry" by Robert Bryce. He takes a position that wind and solar energy are basically scams because they are unreliable. Society demands that power is availbale immediately, the moment it is needed. The nature of wind and solar is that it is unreliable - we cannot predict with 100 per cent certainty how hard the wind will blow or whether the sun will shine. More power may be generated from these sources when it is not needed and less energy may be generated when it is needed. Therefore, natural gas and coal-fired generators are still needed for backup and these generators must be constantly running because it takes time to bring them on line. Wind and solar are a waste because the gas and goal generators must run anyway. He gives other reasons but this is the main reason. My question is whether it is possible to store the energy generated from wind and solar. If we could build up reserves of this energy, it seems we could use it the same way as coal and gas is used - just switch it on when it is needed. Does this concept violate the laws of thermodynamics?

ANSWER: 
The storage device is called a battery. This is already how home installations work, using wind or solar to charge a battery. And when the batteries are fully charged, send the excess onto the grid. Another storage possibility (less efficient, I suppose) is that energy may be used to pump water up to a reservoir and then let it fall back to drive turbines when power is needed. Yet another storage device is to use the heat from solar to melt salt; the molten salt is stored in insulated containers and used to later heat water to run steam turbines. To say that solar and wind are "scams" is idiotic. It is true that for a long time we will still need conventional power plants, not because of storage issues but because the amount of demand cannot be met overnight by alternative sources.

QUESTION: 
I was wondering, what are estimated or known deviations of the masses and radii for the 3 basic particles (protons, neutrons and electrons ) making up the atoms ? Can we expect atoms from one edge of the universe to match atoms plucked from the opposite reaches ? Is it possible that some atoms would be scaled up or down by multiple factors with respect to the lot we are familiar with ?

ANSWER: 
Masses are extremely accurately known. Regarding the sizes, they are not simply well-defined particles like a marble or bowling ball, they have fuzzy edges. But you can define a size (for example, where charge density drops to
˝ the maximum value). The size of an electron is unknown and maybe unmeasurable. The sizes of protons and neutrons are on the order of 10-15 m. There is no evidence whatever that atoms at the extreme edges of the known universe are different. Some physicists speculate that the fundamental constants of physics may change over time (hence distant regions would have different physics) but there is no good evidence for this being the case.

QUESTION: 
Can I move the Earth myself by jumping? Can a single atom move the Earth?

ANSWER: 
When you jump, the earth exerts a force on you and you exert an equal and opposite force on the earth; that is Newton's third law. Hence the earth accelerates away from you and you accelerate away from the earth. But, your mass is small and the earths is huge, so your acceleration is modest and observable and the earth's is tiny and so small as to be unobservable. But, after you leave the ground you and the earth exert attractive forces on each other (gravity) and you fall back together so the net effect has been not to "move" the earth. Of course, a single atom would really have a negligible effect.

QUESTION: 
Electromagnetic physics asserts that when a charge is accelerated, in the medium of a copper wire (antenna), a disturbance in the electric field results (a "kink" in the E field lines develops), this "disturbance" is associated with propagating E and B fields. Does the summation of all photon energy comprise the energy of the E and B fields, and, if not, to what degree does photon energy comprise the energy found in either the E or B fields?

ANSWER: 
There are electric and magnetic fields which are associated with the charge but are not radiation fields. Therefore, there is less energy in photons than in all the electric and magnetic fields.

QUESTION: 
I have read your ground rules, particularly the rule that bars asking about traveling faster than light. If the question I'm about to ask is in violation of this rule, please tell me. My question is this: If you could travel "at" the speed of light, and you shined a flashlight in front of you, what would happen to the beam? Would you be unable to see it, or would it behave normally?

ANSWER: 
You have not violated the groundrules because I neglected to specify at the speed of light; I have fixed that now! As you can see from a previous answer, it is not possible for anything with mass to go as fast as the speed of light. However, if you happened to have a speed of 99.9999999999% the speed of light, you would observe the flashlight beam to be moving away from you at the speed of light. Again, see an earlier answer.

QUESTION: 
I was told in my electromagnetics class by my professor that a RF wave is a "photon." A photon appears to be defined as a quantity of measureable energy. I then read, that a microwave oven permits "photons" of light to pass through a metal mesh but not microwaves." Can you please distinguish between a light photon and a microwave (define)? How can a mesh be designed to block microwaves?

ANSWER: 
Electromagnetic waves, it turns out, are both waves and streams of particles called photons. The photon is the smallest amount of energy possible for a wave of a given frequency. For most practical applications there is no need to talk about photons at all, just considering the wave properties is sufficient. There is no need to think about microwave photons when trying to understand why the waves do not penetrate the mesh enclosing the oven. Light waves have wavelengths typically hundreds of nanometers (very short). The wavelength of the microwaves is several inches. The holes are much bigger than the light wavelengths and much smaller than the microwaves.

QUESTION: 
I am perplexed at something a subsititute teacher in physics class told me. He said, rockets rely on Newton's Third Law (action/reaction) to leave Earth's gravitional force (they have to reach a speed called 'cosmic speed' I think). But I remember him saying rocket engines don't work in space. Is this true? Does that mean we rely on Inertia for space travel? Then, how would we avoid the gravity of other objects to stay on path? I remember you answering a question with 'influential gravity' to 'steer' through space; please explain this concept a little more. Back on rocket engines, how do we return from the moon then?

ANSWER: 
It is scary how ignorant teachers can sometimes be. It is absolute nonsense to say rockets do not work in space.

QUESTION: 
Why do neutrons decay with a half-life of 10 minutes when free, but do not decay when they are attached to other protons in nuclei or when they occur in bulk in neutron stars? Does the presence of other particles have a stabilizing effect on the neutrons?

ANSWER: 
Yes, the presence of other particles in nuclear matter does have a "stabilizing effect" on neutrons. But it is most easily understood in terms of energetics. A free neutron has a certain mass and has the "option" to decay into a proton, a neutrino, and an electron; the mass of a neutron is larger than the summed masses of its decay products, so energy is released and it can decay without any "help". A free proton, on the other hand, also has the "option" to decay into a neutron, a neutrino, and a positron; the mass of a proton, though, is smaller than the summed masses of its decay products, so energy must be added to it for it to "decay" and it is therefore stable. These processes are called beta decay. In a stable nucleus, the mass of the nucleus is smaller than the sum of all its part because it is bound. The net result is that the mass of a neutron is effectively reduced in nuclear matter, thereby rendering it stable.

QUESTION: 
I understand that the moon being tidally locked always presents the same face to the earth, and to do so must rotate about its axis in the same time as it takes to complete a single orbit. Now if you have a ball on a string which you spin around in a horizontal circle above your head, the ball presents the same face to the centre of the circle. I am certain that the ball also rotates about its axis to achieve this, but an associate says it isn't rotating about its axis otherwise it would wind up the string. Can you say which of us is right and explain why?

ANSWER: 
You are right. The reason is that the string is also rotating with the same period around the axis about which the ball rotates. For example, imagine a very tall vertical pole sticking up from the earth; you wouldn't say the earth was not rotating on its axis just because the pole moved along with us.

QUESTION: 
obviously it isnt possible,but lets say for talking sake that it was... what would happen if something could travel faster than the speed of light? would there be a light version of the sonic boom?would the object pass you before you could actually see it?

ANSWER: 
Although I normally refuse to answer "faster than light" questions, this one I will address because there are actually "sonic boom" (maybe we should call it optic boom) phenomena associated with superluminal speeds. What a particle cannot do is travel faster than the speed of light in a vacuum; but in a material medium (say glass or water) light travels more slowly and so a particle in water can go faster than the speed of light in water. When this happens a shock wave (which is what a sonic boom is) is created and the resulting effect is that there is a glow emanating from the water. This radiation is called
Čerenkov radiation. For example, the water around some reactors has very fast neutrons traveling through it and glows blue.

QUESTION: 
How come we are able to see shadows of objects? Say there's a shadow on the wall, why can see the shadow of something, say an ornament when there is lack of light?

ANSWER: 
Normally, a shadow blocks light from one source but there are other sources of light illuminating this place (where the shadow is) which are not blocked.

QUESTION: 
I know about most of the aspects of nuclear fusion (for example: Where it occurs, requirements), but what I do not understand is: Just because 2 light atomic nuclei fuse together to make a heavier atomic nuclei, why does this release a large amount of energy? I know with nuclear fission, you seperate the protons in the nucleus which releases energy due to the energy stored by keeping the protons attracted intially. But why does fusing nuclei release energy? Shouldn't it only store the energy?

ANSWER: 
See an earlier answer.

QUESTION: 
what is rate of change of acceleration?

ANSWER: 
It is called jerk.

QUESTION: 
Suppose a frictionless cube is placed at rest on a frictionless billiard table. If the cube is struck by a cue stick normal to one face of the cube, but not on a line that passes through the center of mass, in what direction will the cube move? Certainly the impulse J is normal to the face of the cube. But it can be decomposed into a component directed toward the cube's center of mass, and another component at right angles to that direction. Whether one uses this decomposition, or the original impulse, to calculate the angular momentum acquired by the cube, the answer obtained for that quantity is the same. But the motion of the cube's center of mass is not the same. So what's the answer?

ANSWER: 
Let's assume the force is in the same horizontal plane as the center of mass of the cube; then you do not have to worry about tipping it over or similar rotations which might not be perpendicular to the table. You also must assume the impulse is delivered in a very short time so that the cube doesn't start rotating as you are pushing on it; I assume that is what you mean. Newton's second law still holds, the momentum after the collision must be in the direction of the impulse and have magnitude equal to the magnitude of the impulse
p=Ft. The angular momentum acquired by the cube about its center of mass is L=Γt where the torque is Γ=rxF and r is the vector from the center of mass to the point where the force is applied. I do not understand your statement that resolving the force into two components results in a different motion of the center of mass; calling any two components of the force F1 and F2, then p'=F2t+F2t=(F2+F2)∆t=Ft=p, the same as before.

QUESTION: 
If photons have mass (E=mc^2) and a large mass curves its path, then 1) By newtons 3rd law will it attract the larger mass towards itself? 2) Does that mean that it can exert gravitational influence on mass and even on photons themselves? 3) If so,by using laser, can one create a black-hole by squeezing enough photons in a tiny space?

ANSWER: 
Photons do not have mass. Because it has energy, it does warp space/time, but the "reaction force" on a large mass nearby would be unmeasurably small. I have no idea how your black hole laser is supposed to work.

QUESTION: 
I always thought of temperature as essentially a measure of how fast atoms were moving. I understand that when the volume of a gas is decreased, there is a coresponding rise in temperature. Why is this ? The atoms in a not moving through space any more quickly (momentum is not increased, is it ?). They are, however, bumping into each other more frequently. Is temprature the frequency of these interactions ? I so, does an isolated atom (thought experiment), which is not interacting, have a temprature ?

ANSWER: 
Your first statement is correct for a gas. For a solid, it is more complicated since the average energy per atom includes potential energy as well as kinetic energy because the atoms interact with each other. You should read my earlier answer on heat and temperature. When you compress a gas you do work on it. If no heat leaks out or in, the energy you add (by doing work) ends up as increased kinetic energy of the gas molecules
—they go faster. Think of compressing the gas by moving a piston. If the piston does not move, a molecule colliding with it rebounds with the same speed that it came in. If the piston is moving into the gas, the collisions with the piston cause the molecules to move faster after colliding (like when you hit a ball with a moving bat, speeding it up). The frequency of collisions among molecules is of little importance; an adequate model of a gas ignores collisions between molecules. Finally, temperature is a statistical concept and a single atom does not have a temperature.

QUESTION: 
If two objects are similiar in mass, will they move toward each other or not move at all?

ANSWER: 
I do not really understand this question. What is true is that two objects with mass exert gravitational forces on each other. However, how they will move depends on what other forces they experience,where they are relative to each other, and what their masses are. In empty space, where there are no other significant forces, they will accelerate towards each other. Since you stipulate that their masses are similar, you can calculate the acceleration. Call the mass of each M; then the force each experiences is F=GM2/r2 where G=6.67x10-11 Nm2/kg2 and r is the distance between their centers. Since the acceleration of each will be a=F/M, a=6.67x10-11M/r2. Unless M/r2 is very large, the acceleration will be very small.

QUESTION: 
if i was traveling at 2000 ft per sec and shot an arrow the opposite direction also at 2000 ft per sec would the arrow travel or would it stay still?

ANSWER: 
Relative to a stationary observer (who sees your speed as 200 fps), the arrow would be at rest.

QUESTION: 
Since atoms are mostly empty space, why dont we fall through a floor we stand on?

ANSWER: 
It seems I have answered this question a hundred times. See FAQ page.

QUESTION: 
My question is simple.When Albert Einstein postulated that there was no gravity in the absolute sense but that the whole universe was made up of a 3 to 4 dimensional sheet called space time where when any sought of matter lied on it could warp the shape of the celestial sheet hence giving objects near it(the warped region) experience some form of force that in Newtonian physics was called gravity.However,I stand corrected but when Einstein was thinking of this he must have been thinking of it in reference to conditions on earth where the object smaller than the larger object on the sheet would roll toward the larger object due to a downward force,however this is not the case in space.I propose that in empty space time in non downward force pull region like space the smaller object would continual moving despite the warp in space time sheet. what do you think

ANSWER: 
The picture of a warped rubber sheet is merely a "cartoon" to illustrate the qualitative idea of how mass deforms space-time in general relativity. It is not meant to be a rigorous and literal picture. You should read my earlier posts on general relativity linked to from the FAQ page.

QUESTION: 
could steam be below 100 degree Celsius.

ANSWER: 
Yes, at pressures less than atmospheric pressure. See the phase diagram of water.

QUESTION: 
Why is the speed of light the speed that it is? What is stopping it from being even faster?

ANSWER: 
See earlier answers.

QUESTION: 
I have read some things online about why ice and salt are mixed together to make ice cream. I'm not really understanding how a salt and ice mixture is colder than ice itself as is being said online. I understand the concept of how the freezing point of water and salt is lower than water itself. However, I do not see how adding salt to ice makes the solution colder. Ice freezes at 32 degrees, but ice in a freezer for example can reach a temperature as low as the freezer is set, correct? So, it seems to me that adding salt to this ice will make a brine because of the molecule interactions and the temperature will be very low (less than 32 degrees) because the ice in the beginning was much colder than 32 degrees, not because the salt/ice mixture is colder than ice alone.

ANSWER: 
Yes, ice can be colder than 320; but, when put it in an ice cream maker it starts to melt and the melted water will be about 320. So, after a while, you have an ice-water mixture at about 320 and that is what the ice cream sees. Adding salt lowers the freezing point of the mixture (the brine) and hence the ice-brine mixture is below 320. You can lower the freezing point of the brine by as much as about 300.

QUESTION: 
If I shoot a laser at a hard drive platter on one side. Put a pin hole in the platter where it hits so the phtons can pass. Can I set up a Retroreflector at such a distance that the photon will return to the platter, when spun at 15,000 rpm, will pass back through the same hole except the hole has rotated 180 degrees? Or.... The photon having no time dimension or dimension in the direction of travel, always hits the platter 180 degrees from the start regardless of rotation? Or something I didn't thik of?

ANSWER: 
The hole will let through a very short pulse of light. If the distance to the reflector is L, the pulse will take a time t=2L/c to return to the plane of the platter. If the hole is there, the pulse can pass back through. Hence, in the time t the platter must rotate through 3600, 7200, 10800, etc., some integer multiple of 3600.

QUESTION: 
If atoms, and hence the matter made up of attoms, are 99.9999999999999 percent empty space, why isn't everything invisible?

ANSWER: 
Because there are many electrons with which the light (which is composed of electric and magnetic fields) interacts strongly. So, light is absorbed, reflected, and/or bent when it interacts with matter.

QUESTION: 
Can the radiation from CAT scans penetrate walls? I had my pocketbook in a room adjacent to the room where my CAT scan was performed. I had a metal water container in my pocketbook. Is it possible for the radiation to have penetrated the wall and permanently ruined my container/water bottle making it permanently radioactive?

ANSWER: 
CT scans are nothing more than glorified x-rays. I am sure that the walls are probably shielded to keep the x-rays in the room where the scan is done. But that is beside the point because x-rays do not make anything radioactive, permanently or temporarily. If your purse and water bottle had been in the room with you they would not have become radioactive. The only reason for shielding is to protect people; excessive exposure to x-rays can cause damage to living tissue. That is why technicians leave the room when an x-ray machine is on. For the patient, who is there for a one-shot deal, it is no reason for concern, but for personnel around all day every day, it is.

QUESTION: 
I'm trying to settle an office debate. We are debating on how an electron travels through a conductor. Lets assume we are taking about DC current through a copper wire. I think that the electrons are passed from atom to atom on the last (4th) energy level. Can you tell me if this is accurate, if not can you explain to some detail how it works?

ANSWER: 
In a good conductor, the conduction electrons (those which, in an isolated atom, would be called the valence electrons and are outermost) are almost perfectly free to move around. The atoms behave differently in the bulk than singly. In fact, a very good model of a conductor is simply that the electrons move around freely like gas in a box. Now, when a potential difference (voltage) is applied across the conductor, an electric field is established which causes the electrons to experience a force in the opposite direction from the field (since they are negative). If they were really free, they would simply accelerate from one end to the other. However, as soon as one electron starts speeding up along the field, it bumps into an atom and stops, then accelerates again, etc., etc. The net effect is for the conduction electrons to have, when all are averaged over, a net drift velocity in the direction opposite the field direction. For normal household currents this velocity is very small, smaller than 1 mm/s. It is really not accurate to say the electrons "jump" from atom to atom; rather the atoms and electrons suffer collisions.

QUESTION: 
I have played with special relativity and think I have a feel for it. But there is one example I find confounding. The thought experiment where a man in on a train and a man is on the platform of the station and lightening strikes far ends of the platform equal distance from both. It is easy to see how the man on the train if in relative motion (either he or platform moving) will not see the two lightning bolts as simultaneous. But wouldnt there be a doppler effect which would clue him that he is approaching one end and receding from the other? Thus he could calculate back and have the two bolts hitting at the same instant? I imagine a very large platform, like millions of light years in length, thus replicating the distances of galaxies. We say a relativistic doppler effect tells us about galaxies and their motion. If we can use this with galaxies couldnt the same be done with the lightning bolts?

ANSWER: 
You are missing the point. Yes, he could use the doppler shift to determine that he was in motion with respect to the lightening bolts, but that does not alter the fact that they are simultaneous in one frame and not in another. You take the age-old stance that there is something special about one frame and that they just "appear" to not be simultaneous in other frames. In fact, even without doing any fancy doppler measurements, you could take any two events not simultaneous to you and find a frame of reference in which they were simultaneous. There is nothing special about the frame in which two events happen to be simultaneous. The thing you have to accept in relativity is that strange things are, they don't appear to be; moving clocks run slow, moving sticks are shorter, simultananeity is not absolute.

QUESTION: 
I live 3/4 of a mile from a motorway. Some days I can hear the traffic - other days I can't The wind only needs to be 5mph and it seems to "deflect" or "absorb" the noise vibrations Sound travels at 500+mph. How come a 5mph wind can "blow away" something that travels at 500mph?

ANSWER: 
Wind will likely not be a big factor in sound suppression. More likely, a change in humidity can change the amount by which the air absorbs the sound wave energy. Another possibility is that there could be temperature gradients; if there were layers of different temperature air, this could cause the sound to be deflected, either down to the ground or up over your house. This is analogous to how mirages are formed except the deflected waves there are light.

QUESTION: 
Last week I set up a demonstration of Brownian Motion whiich worked fine and showed movement of the correct magnitude. It is a dilute suspension of 1 micrometer diameter polystyrene beads in distilled water. I came back to look at the suspension a day later with the same x400 microscope and found most of the beads had settled out but perhaps 1% were still doing the drunkard's walk. No surprise yet. What did strke me as odd was that a couple of beads wandered across rapidly spinning round each other. A bit too fast to count so perhaps around 3 revs per second. They did separate momentatrily by a few micrometers and slow down (at which time I wondered if I could see three beads) but were soon close together again and continued their frenzied dance until I lost them out of focus. I would have expected such spinning motion to be dissipated by viscous forces very quickly. There were other pairs of beads visible just being joggled around. How could this spinning be sustained?

ANSWER: 
This is only a guess. I agree with you that if two or more were bound (maybe by electrostatics?), any orbital motion should quickly damp out. What I think you are seeing are vortices in which beads can become trapped. Vortices are much more common in nature than most people appreciate.

QUESTION: 
In physics class, we learnt that 'it is important for life on Earth that the Sun is in fact, a second generation star'. We were told it was important "because elements higher then helium on the periodic table would not be present in either the sun or any of the planets. These heavier elements are necessary for life on Earth". My question to my teacher was: Just because this elements are 'made' (correct term?) in the Sun, how does it relate to life on Earth? Were they 'transported' to Earth? I thought elements were already present on Earth after the Big Bang. My teacher actually requested me to ask you

ANSWER: 
When the sun and solar system were formed, there was debris from earlier stars from which the planets were formed. Although the sun is continually forming heavier elements (up to iron), that is not where the planets' heavy elements came from. See earlier answers.

QUESTION: 
I've only had 1 class in physics, but that was about 5 years ago. I am trying to make cheese and I find that I need some physics information. My question is regarding force/pressure. How does pressure/force work when exerted over a certain area? In order to make some hard cheeses, you need to apply, say, 50 lbs of pressure to press it. Now, if the diameter of the cheese is, say, 5 inches, and you apply a direct downward force of 50 pounds per square inch on the press plate, would applying that same amount of pressure to a cheese with a diameter of 9 inches be the same amount of force needed to press the cheese? If you are using a 50 lb spring, compressing it the whole way would apply 50 pounds of pressure. Would it matter if this pressure is exerted on a 5 in diameter plate as opposed to a 9 in diameter plate? If the pressure is distributed evenly throughout the plate, does the size of the plate make any difference on how hard the cheese is being pressed? Here is a more specific example: the cheese weights 5 kilos and has a volume of 5 liters and the surface to be pressed is 23 cm in dimeter. Would compressing a 50 lb spring on this surface have the same effect as compressing the 50 lb spring on a cheese that weighs 2 kilos and has a top surface diameter of 12.7 cm? Sorry that this message is a little long, I'm having difficulty expressing my confusion. I guess what I am trying to ask is whether 50 pounds of pressure is always "50 pounds of pressure," irregardless of the surface area it is pressing? Are there formulas etc. that could help me solve this problem (because different cheeses have different volumes/diameters/etc.)? I'm sure this is an unusual question, but it is applicable to everyday life; I'm trying to figure out how to construct a press that will apply the right amount of pressure to different solids.

ANSWER: 
You keep saying things like "50 lbs of pressure". This is the source of your confusion. Force is measured in pounds or newtons. Pressure is force per unit area, measured as pounds/inch2 (PSI) or newtons/m2. So, 50 lb loaded on a cheese with area 50 in2 results in a pressure of 1 psi. 50 lb loaded on a cheese with area 25 in2 results in a pressure of 2 psi. I do not know what you mean by a "50 lb spring". Springs are characterized by a spring constant k=F/x where F is the force necessary to cause a compression amount x. The spring constant is measured in pounds/inch or newtons/m. For example, a spring with spring constant 50 lb/in would exert a force of 50 lb on your cheese if compressed by 1 in, a force of 100 lb if compressed by 2 in, etc.

QUESTION: 
in a beam of sunlight entring a room.we can see sometimes dust particles moving in a zig-zag movement.what is the name given to this movement?what is the cause of this?

ANSWER: 
Brownian motion caused by collisions between air molecules and dust particles.

QUESTION: 
Please settle an argument for me. I have tried to convince my wife that a compound bow cannot deliver more force to an arrow than one exerts upon it when pulling it back. My point is that regardless of the pulley system involved of the compound bow, the force imparted upon the arrow by the bow string cannot exceed the force imparted by the archer upon the bow string when pulling it back.

ANSWER: 
My understanding is that the advantage of the compound bow is something called "let-off". The bow is designed in such a way that the force you must apply at a particular pull distance first increases and then decreases. (A conventional bow has force increasing the more the bow is pulled.) This allows you to be more relaxed and have longer time to aim. But, because the force applied in the middle was large, the energy stored in the bow can still be very large. The figure shows the force as a function of pull distance. You are correct, though, the force you exert on the string at each position will be the same as the force the string exerts on the arrow at each position after release.

QUESTION: 
I am pondering the classic equation E=MC2 in head and understand its straight definition (i.e. the correlation between mass at rest and energy). I also understand the equations implications for the relativity of the speed of light for observers as well as for nuclear fission and nuclear fusion. I was wondering where, in laments terms, the speed of light squared plays in the equation. I get how the math was worked out and that when mass is lost energy is produced, but does the speed of light squared mean that mass needs to travel this fast to produce energy? I am an amateur physicist and I have been trying to wrap my head around this for a few days.

ANSWER: 
The only way that you can see where an equation comes from is to follow the derivation of that equation. The derivation is outlined in an earlier answer. And, why would the fact that c2 appears imply anything about the particle's speed?

QUESTION: 
I'm in 9th grade and so far none of my teachers, math or science, seem to be able to answer it; what keeps electrons from becoming attached to protons in an atom?

ANSWER: 
I think the correct answer to your question is that the electrons do become attached to the protons. That is what an atom is
—a bunch of protons (with neutrons) in a nucleus and a bunch of electrons "attached" to the nucleus. The attachment, though, is not a static attachment but more like little planets (electrons) orbiting a little sun (nucleus). The reason they do this instead of just sticking to the protons is that the laws of physics (quantum mechanics) forbid this system to have zero kinetic energy (being at rest). This whole picture is a little qualitative but probably good for a high school freshman.

QUESTION: 
Why does light travel at 186,000 mps rather than some other speed?

ANSWER: 
Maxwell's equations, which are the equations describing electromagnetism, predict waves which travel with a velocity 1/
√(ε0μ0) where the two constants are determined by the strengths of electric and magnetic forces. It just so happens that this velocity is 186,000 miles per second. See earlier answers.

QUESTION: 
I was wondering how a magnetic field is generated from a capable metal and why this happens for particular metals and nothing else?

ANSWER: 
An electron is, in addition to being a tiny electric charge, a tiny magnet. In most materials the electrons north/south poles point in random directions and cancel each other out. In a few materials (called ferromagnetic) a large fraction of the electrons line up with each other making the whole object look like a magnet.

QUESTION: 
How many times does your heart beat during your lifetime?

ANSWER: 
A healthy resting heart rate is about 60 beats per minute, or once per second. There are about 2.5 billion seconds in 80 years.

QUESTION: 
This is something I have thought about for a long time. I know it may seem off the wall, but here it goes. Thinking of distance, and the fact that anything can be divided in half, how is it we ever get anywhere (walking, driving etc.). Don't we always get halfway there first? Where do we "cross over" and actually make contact or get to where we are going?

ANSWER: 
See my earlier answer on Zeno's paradox.

QUESTION: 
I read something recently about solar sails, whereby a spacecraft is given thrust by the momentum imparted by photons bouncing off a large reflective sail. My question : if a photon has imparted its momentum this way and has bounced off, it must presumably still be travelling at light speed, and rest mass by definition is still the same (zero) - so what has it lost to the sail? Presumably not kinetic energy? If this loss is represented by a lowered frequency or longer wavelength of the photon, what happens if the photon keeps bouncing off subsequent reflective surfaces? is there a lower limit to the frequency it can get to? Say a photon is introduced inside a hollow reflective sphere , does it continue to exist and bounce around forever but with frequency approaching zero?

ANSWER: 
The photon has a momentum of p=E/c where E is the energy. When it reflects from the sail its momentum changes by an amount 2E/c. Hence, the sail gains an equal amount of momentum but in the opposite direction to conserve momentum. Technically, the photon will actually change its energy because of the recoil of the sail, but because the mass of the sail is large, this is a trivial correction and, for all intents and purposes, the reflected photon has unchanged energy (color). Your second question violates groundrules in that it requires a perfectly reflective surface; in practice, the photon would be absorbed in the blink of an eye even if the surface were 99.999% reflective.

QUESTION: 
I was wondering how the Heisenberg uncertainty principle applies to photons. As I understand the principle, it states we can either know the position, or the momentum of a particle with 100% certainty, but never both. With light though it's my understanding that it always travels at C. That this is one of the principles of relativity, the speed of light is always measured as the same no matter the speed of the observer or frame of reference. Therefore I'm puzzled about how this would reconcile with the uncertainty principle. If we had a photon in an eigenstate of position wouldn't we know that its momentum was C, and thus violate the principle?

ANSWER: 
I do not know what you mean by "an eigenstate of position". The momentum of a photon is p=E/c where E is its energy. The uncertainty principle states that if the position of anything is known perfectly, its momentum is completely unknown. Hence, a photon whose position is known perfectly has an unknown momentum and, therefore, energy (color).

QUESTION: 
Which object more inertia a jet plane sitting still or a ping pong ball traveling at 100 mph?

ANSWER: 
Inertia is just another word for mass, the resistance something has to being accelerated. The jet plane has more inertia than the ping pong ball regardless of their motion. The moving ping pong ball has more linear momentum than the stationary jet plane does.

QUESTION: 
Is their a equation that relates an object's real size, its distance from the viewer, and the size it appears to be?

ANSWER: 
"
the size it appears to be" really has no meaning. We judge how large something appears to be by the angle it subtends. If the angle is small, the object looks small, if it is large, it looks large. The angle in radians is given by θ=D/R where D is the size of the object and R is the distance to it. Hence, if something is 10 times as far away it looks 10 times smaller.

QUESTION: 
I recently got a big magnet, and a small magnet. The big magnet is clearer stronger than the small magnet, yet when i use a magnetic field sensor to see how strong they are, it says they are the same strength. How can this be?

ANSWER: 
Your measurement device presumably measures magnetic flux through a small area. Although the flux near the magnets are about the same, the larger magnet has a larger area overall and hence will result in a larger force on something iron.

QUESTION: 
Could you explain why the driver of a car must keep her foot on the accelerator to maintain a constant speed and therefore why energy is needed to maintain the cars speed??

ANSWER: 
Wouldn't it be great if we could have a car which had no energy loss? Unfortunately, the world has forces which we cannot avoid which take energy away from something moving along. These fall into the category of frictional forces: a spinning wheel has friction in its bearings which will eventually cause it to stop; an object moving thought the air has air resistance which will eventually stop it as it moves along; the tires are not perfectly elastic and as they roll they are being continually deformed and undeformed and energy is lost. Without all these forces, we could accelerate up to speed and disengage the engine from the wheels and turn it off and just cruise. However, one can work hard to minimize these forces in the design of cars; making the cars aerodyamic, reducing the weight, and other tricks can minimize the energy we lose.

QUESTION: 
What force confines electrons to a negatively charged metal? I've read from many textbooks that a neutral metal confines its electrons because as soon as an electron escapes into the vacuum it leaves behind a positive charge that quickly attracts the electron back into the metal. Thus the electrons are imprisoned inside the metal forever, unless a high energy photon comes along to knock it out (or something of that nature)... Well, what about metals (say a metal sphere sitting in vacuum) that has a net negative charge?... It would seem to me that the metal "wants" the electron to leave... What I mean is, as soon as the electron leaves the metal it does not leave behind a positive charge anymore (because the metal is charged negatively). It leaves behind a negative charge that should push it away even further. What is the mysterious force that is counterbalancing this repulsive force and keeping the electron in?

ANSWER: 
Imagine an electron very close to the surface of the sphere on the outside. Its field repels electrons inside the conductor leaving a net positive charge closer to the electron than the residual negative charge. Hence, the electron outside experiences an attractive force.

QUESTION: 
As you may or may not know, traditionally boats are moored through the use of a figure 8 knot on a cleat thats attached to the boat. In other words, the system is as follows Weight (usually about 400 lbs ) sitting at the bottom of the ocean, connected to a chain held up in the water by a buoy, which is attached to a thick rope connecting the buoy to the boat. Instead of using a figure 8 knot on the cleat, I want to use a climbing carabiner rated to a max load of 25 KN but that has no rating for a constant stress. My logic is that the only relevant force is the force of the current pulling the boat away from the buoy and weight set up which would come no where near (or even within any reasonable factor of safety) the 25KN rating. Is this valid? In other words, the fact that the boat weighs upwards of 6000 lbs is irrelevant, as the carabiner is not supporting its weight, the buoyancy of the boat is, correct? Also, is it safe to assume that the thick rope wouldn't even be able to match the 25KN stress rating meaning that if something were to break, it would be the rope before the carabiner? Please let me know if I'm overlooking some crucial force of if there's any reason this shouldn't work. Obviously I'm doing this project at my own risk and am just seeking a second opinion based more on theory than practical experience. As such, I understand you can't give me a 100% answer but under ideal circumstances, would you expect that I be ok?

ANSWER: 
I do not want to get into all the numbers, but certainly 25 kN is much stronger than 400 lb. The only proviso I would add is that the mass (not weight) of the boat does matter but not because the weight needs to be held. Here is an example: assume, to make it clearer, that the boat is moored to a wall. Suppose the rope is slack and suddenly a strong wind starts the boat moving rapidly. When the rope becomes tight, the boat stops but it will not stop instantly. Rather the rope will stretch a bit so that the boat will stop in some short time t. So, during t, the boat will be accelerated by an amount a and the force F needed to provide this acceleration will be ma; F comes, of course, from the rope. If the required F>400 lb, the rope breaks. Probably a battleship will break the rope, a canoe will not. (How do you attach the rope to the carabiner?)

QUESTION: 
I was wondering how many times you'd have to double water's density to turn it into a solid (if that's possible).

ANSWER: 
You make it sound like we could just squeeze the water and reduce its volume to half; this is really hard. It is very hard to answer your question quantitatively because the compressibility of water (how much it compresses for a given pressure increase) depends on the pressure and on the temperature and the details are not, as far as I have found, well known. We can make some rough estimates, though. The compressibility (I will call it C) is about C=0.46x10-9 Pa-1 at normal pressures and temperatures (say atmospheric pressure, about 100,000 Pa and room temperature). What this means is that the volume decreases by 0.46x10-7% if you increase the pressure by 1 Pa (1 N/m2). So, if compressibility were independent of pressure (a big if), you would have to increase the pressure by 1 GPa=109 N/m2
1000 times atmospheric pressure to reduce the volume to 0.46 of its original value, about half. At around 2 GPa no liquid phase of water exists below temperatures of about 3000C and one of the exotic forms of ice would form.

QUESTION: 
What determines how much of a boat displaces water?

ANSWER: 
The weight of the boat. For the boat to float, the buoyant force, equal to the weight of the fluid displaced, must equal the weight of the boat. (Archimedes' principle)

QUESTION: 
Why does the glass of a wine glass vibrate when you rub your wet finger along the rim. i already know that it vibrates and has a natural frequency and what not but i want to know why it actually produces a sound?

ANSWER: 
When you rub your finger here is what happens. Because of static friction, as you start to pull your finger it does not move but as you pull harder you eventually "break away" and your finger starts to move. If this were all that happens, the wine glass would not "sing" because your finger would just be moving around the rim; if you push down very gently while rubbing, it will just go smoothly sliding. But, if you push down a bit harder, the finger rubbing will start to "squeek". What is happening is that your finger sticks, then slides, then sticks, then slides, over and over and at very quick intervals. To make the glass sing, you usually have to mess around a little varying both the speed and push of your finger until you are exciting the glass with a frequency equal to its resonant frequency.

QUESTION: 
I don't quite understand Newton's Third Law of Motion. It makes sense when object A and object B are exerting the same force (thus, they do not move), but what if object A exerts more force on object B so that object B moves three inches in the direction that object A is pushing? What happens to the "equal and opposite" force that object B should be exerting?

ANSWER: 
It makes no difference
—if A exerts a force on B, B exerts and equal and opposite force on A. What you are forgetting, I think, is that how B moves does not just depend on what A does. For example, if A pushes on B and B does not move, then something else must be also pushing on B.

QUESTION: 
Photons are said to be carrier particles for the electromagnetic force but how do they do so. I have read that a force can be like two people passing a ball between them where one person throws the ball and recoils and when the second person catches the ball it pushes them away. And if photons cause a repulsive force between electrically charged bodies, what wavelengths do the photons have? If you can explain a repulsive force by exchange of particles (virtual or real) how can you explain an attractive force occurring in this way?

ANSWER: 
See an earlier answer, particularly the note added at the end.

QUESTION: 
If it is given that light as no mass, how then, does it have the momentum to knock electrons out of there orbital shell? Wouldn't it be more reasonable to assume that light photons would just bounce off the electrons causing no change to them?

ANSWER: 
In the real world, an object does not need mass to have momentum. The correct definition of momentum is not mv but p=
√[(E/c)2-m2c2] where E is the energy and c is the speed of light. Hence, if m=0, p=E/c. For small velocities it may be shown that p≈mv.

QUESTION: 
does QED explain why magnets attract each other? If so, how do they?

ANSWER: 
QED is simply classical E&M with the fields quantized. Classical E&M explains why magnets attract (or repel) each other, you do not need QED.

QUESTION: 
Can you please explain how the decay times of particles such as pi mesons are measured in accelerators to show they live longer than at rest?

ANSWER: 
You set up detectors to look for the decay products, in the case of a pi meson they are a mu meson and a neutrino. You can trace the mu trajectory back to the path of the pi and hence determine where the decay occured. Knowing the speed of the pi and where it was created you can deduce the time from creation to decay.

QUESTION: 
On the myth busters, they took on a bullet versus gravity. They have a 45 caliber bullet being shot from a gun hitting the ground at the same time another bullet simply dropped from the same height hits the ground. My question is: Would changing caliber of weapon or height in which the gun is shot from change this outcome. A 22 caliber bullet will travel 2 miles approx. and leaves the chamber at 1200 ft. per sec.. I am not a gun person nor great at phsics. But I am curious enough to ask.

ANSWER: 
The vertical and horizontal motions of a projectile are independent, so, since each bullet has the same initial conditions in the vertical direction (at the given height of the gun and at rest), they will hit the ground simultaneously regardless of the caliber or the height. If the gun is not aimed perfectly horizontally, it will not work.

QUESTION: 
how do you work out the average force of water in a swimming pool at one end of the pool?

ANSWER: 
The force on an area of the bottom of a pool is equal to the weight of the water above it.

QUESTION: 
A person is sitting in a car that travels with constant velocity. He throws a ball verticall upwards. Why doe the ball fall in his hands when it returns?

QUESTION: 
why is it that when i drop a ball in a car that is going 60mph the ball doesnt fall in the place where i droped it relative to the highway?

ANSWER: 
These are essentially the same question. When the ball is in motion in the car, the car passenger sees the ball having no accelerration in the horizontal direction (because, neglecting air friction there are no forces with horizontal components) and so, since it started out with no velocity in the horizontal direction it never acquires velocity in the horizontal direction. From the point of view of an observer on the road, though, the ball starts with some horizontal velocity component (the velocity of the car) and that never changes.

QUESTION: 
do alph,beta and gama comes from same element? why do we find all three in many radioactive substances?

ANSWER: 
Gamma radiation can come from any nucleus; it is the electromagnetic radiation emitted when an excited nucleus deexcites (much like atoms emit visible light). Beta radiation is emitted from a nucleus which has either too many or too few neutrons to be stable and consists of a beta particle (either an electron or a positron) and a neutrino. Alpha radiation is a nucleus spitting out a helium nucleus (two neutrons and two protons) and is usually seen in very heavy unstable nuclei; it may be thought of as very asymmetric fission, the nucleus splitting into two nuclei. Some alpha emitters are also beta emitters.

QUESTION: 
I understand from Einstein, that a moving object experiences less time passing relative to a stationery object. Would this also happen to an object moving in a circle? More specifically, would a rotating object experience this effect at different rates across it's diameter?

ANSWER: 
It is incorrect to say that the object "experiences" less time; the experience is that time just marches on like usual but his clock runs slow if viewed from another frame. The case you ask about is complicated because it is an accelerating frame of reference. Where the acceleration is larger (farther out on the rotating object), clocks run more slowly. This is called gravitational time dilation (because of the equivalence principle), it is not the usual time dilation you speak of.

QUESTION: 
I have been doing some calculations concerning interference/diffraction, and I saw that i could do some shortstepping and get a decent approximation using a little bit different equation than I was supplied with. I am talking about a laser beam in a Single-slit diffraction where the laser beam goes through a diffraction grating and is splitted into couple lines. Using the equation dsin(phi) = n*(gamma). I found that using an equation n*(gamma) * l / D = d (where gamma is wavelength, l = distance from the grating to the wall, D = distance from the center max to min/or between maxes and d = width of the grating)) Provided an excellent approximation, was easier and without the need to calculate the phi.  My question is: Why is this a good approximation, and why cant I find it anywhere.. Is it because the original equation is simple enough.

ANSWER: 
You seem to have discovered the small angle approximation. For very small angles you may write that sin
φ≈tanφφ. For your case, tanφ=(D/l)≈sinφ.

QUESTION: 
I put my cell phone in our microwave oven and closed the microwave oven's door. I then called that phone with another cell phone and it rang inside the microwave oven. Does this mean that since the calling signal was able to be picked up by the phone in the closed microwave oven, that microwaves could also be leaking out of the oven while it is being used in the cooking mode??

ANSWER: 
The frequency used by your cell phone is not the same as the frequency used by the microwave. The oven is designed to shield the frequency of microwaves used in the oven. Of course, it is not a perfect shield and tiny amounts may get out.

QUESTION: 
How did scientists come to the 1st and 2nd law of thermodynamics and why are they so certain that they cannot be violated?

ANSWER: 
The first law is simply conservation of energy. There is nothing mysterious there. The second law essentially forbids things which do not violate energy conservation but are never observed to happen. For example, when a hot body and a cold body are put in contact with other, we never see the hot body get hotter and the cold body get colder.

QUESTION: 
Is "Dark Matter" the proof for the existence of an ether? I have read articles stating that the Michealson-Morley experiment was flawed and that the existence of an "ether" was therefor not unproven.

ANSWER: 
As far as I am concerned, dark matter does not exist until there is a definitive measurement of it. There is evidence that the majority of the mass in the universe is invisible. However, that evidence is based on the assumption that our current theory of gravity is correct. I am a member of a minority who believe that this evidence is likely a result of our not understanding gravity as well as we do. I have been heard to say that maybe dark matter is the 21st century's
ćther; I know, that is heretical, but it is defensible that a hypothesized something does not exist if it is not observed. Also, the MM experiment is not as pivotal as many physics texts would have you believe. Einstein would have created special relativity with or without it. And relativity is such a successful theory that it is a moot point whether such a thing as the ćther exists. It certainly does not exist in the context that it is the medium with respect to which light travels. Finally, to answer your question, even if dark matter exists, it has nothing to do with the ćther.

QUESTION: 
why the tyers of a car moving with high speed looks to rotate with reverse direction of rotatio

ANSWER: 
I assume you mean on a movie or TV. This question has been previously answered.

QUESTION: 
In Physic classes we are taught a stationary electron generates an electric field. Also a moving electron generates an magnetic field. Does a moving electron also generate an electric field (as well as the magnetic field)? For example if a stationary electron is placed near a wire carrying an electric current, does it experience an electric force pushing it away from the wire?

ANSWER: 
The moving electrons do have an electric field. But the wire is overall neutral, that is there is a positive ion in that wire for every electron and so the net electric field is zero.

QUESTION: 
I am listening to Stephen Hawking say that heavy object slow down time, like a slow space in a river. Then using the pyramids as an example, he said that people looking out from the pyramid would see things traveling faster (time moving faster). Just to let you know I do know that this is just an example taken to the extreme and no one would really appear to be traveling faster. Ok here is my question, why isn't it called speed rather than time. Are speed and time the same in this case?

ANSWER: 
In physics there are three fundamental concepts: length (L), time (T), and mass (M). All other physical concepts can be expressed in terms of these three fundamentals. For example, speed is L/T (e.g. miles per hour), force is ML/T2 (e.g. a kilogram meter/second squared is a Newton which is about a quarter pound), energy is ML2/T2 (e.g. a kilogram meter squared/second squared is a Joule, a million of which are about 3.6 kilowatt-hours). So we always talk about how M, L, and T are affected by the ideas of relativity and everything else, like speed, follows.

QUESTION: 
How is it that light, but nothing else, can travel at the speed of light? What allows it, and it alone, to travel at that speed?

ANSWER: 
Actually, anything with zero mass will travel at (and only at) the speed of light. It just happens that light is the only massless particle we know of right now. Until recently we thought neutrinos were massless, but they have been shown to have mass. A particle called the graviton, hypothesized to be the quantum of gravity, might have no mass if ever found. Technically, the reason is that any particle with any mass would have to be given an inifinite amount of energy to accelerate it to the speed of light.

QUESTION: 
my class and i were discussing since light waves are part of the electromagnetic spectrum as are radio waves, then could light waves be converted to sound just as radio waves are? a concert with ROYGBIV?

ANSWER: 
When you listen to music on the radio, you are not listening to the frequency of the radio waves. A trick called modulation allows one to use radio waves (whose frequencies are far too large to be heard by the ear) to carry a signal which has a frequency we can hear. The electronics in the radio passes the sound frequency to its amplifier and speakers. Sound waves which we can hear are in the range of about 20-20,000 cycles per second; radio waves are in the range of about 0.5-100 million cycles per second. The figure to the right shows an example of what is called amplitude modulation (AM). There is also something called frequency modulation (FM) which is similar but the modulating wave changes the frequency of the carrier instead of its amplitude; it's a little trickier to visualize. You can modulate light waves, but it is not as convenient and you could not use them for long-distance transmission. But then, you wouldn't be "hearing red", for example, but whatever you were modulating it with.

QUESTION: 
I am having a physics mental breakdown. My physics textbook (and experience studying physics) is contradicting popular belief and I would like to run this by an expert. When it comes to magnets, opposites attract. North poles are attracted to South poles and South poles are attracted to North poles. So when the North pole on my compass points towards the Geographical North Pole, that implies that the Geographical North Pole is really close to the Magnetic South Pole, correct? Similarly, the South pole on my compass pointing to the Earth's Geographical South Pole implies that the Geographical South Pole is really close to the Magnetic North Pole. Additionally, magnetic field lines are drawn from the North pole to the South pole, so on the Earth they are drawn from (near) the Geographical South Pole to (near) the Geographical North Pole. In summary, I have been researching a few educational websites and most are saying different (and contradicting) things. I believe that my physics book is correct, but other physics sites have labeled the Magnetic North Pole near the Geographical North Pole and the Magnetic South Pole near the Geographical South Pole. Which is correct?

ANSWER: 
This is not really worth having a breakdown about! The north pole of a compass needle points in a generally northward direction. It is therefore pointing toward a magnetic south pole. The geographic pole near the point toward which a compass points is what everybody calls the north pole. Geographers tend to call this point the magnetic north pole but, if examined closely, magnetic field lines go into it, not out. It would be just too confusing to everybody to call it the magnetic south pole. See the picture at the right.

QUESTION: 
I have a question regarding the uncertainty principle. Whether expressed in the position-momentum form or the energy-time form, I have seen the right-hand side of the inequality given in two slightly different ways. The first is "greater than or approximately equal to h-bar" and the second is "greater than or approximately equal to h-bar over two". In the second case, that would of course be h over 4*pi instead of h over 2*pi. The two different forms differ by a factor of 2. Could you please shed some light on this?

ANSWER: 
This question often comes up because, I believe, scientific statements are generally expected to be precise and unambiguous. But, this is a principle, not a law; it is not an equation, rather an inequality and an approximate one at that. The root of the problem is that the notion of "uncertainty" itself is a qualitative one. A precise quantative definition depends on the specific situation. For example, what is the shape of the wave packet which describes the position of a particular particle? Is it gaussian, square, maybe half a sine wave? And, establishing that, what is to be called uncertainty for that situation? Better, I think, to appreciate the principle as a philosophical idea
—there are some things in nature that you simply cannot know with arbitrary precision. The statements you see give you an idea of the order-of-magnitude of how well some things can be known.

QUESTION: 
My question is about fluid dynamics. What is the relationship between the drag coeffient and the reynolds number? Based on my research I've been able to establish that for low reynolds numbers (which is what I'm interested in) the relationship is inversely proportional and defined as: Cd=24/Re

ANSWER: 
This took some research on my part! The Reynold's number is Re=
ρvL/μ where ρ is fluid density, v is speed of an object relative to the fluid, L is a length characteristic of the situation, and μ is the viscosity of the fluid. Drag coefficient is CD= 2FD/(ρv2A) where A is the area presented normal to the flow and FD is the drag force. To relate these, we must choose a specific example, typically a sphere at relatively low velocity for which the drag force may be written as FD=6πμRv and the characteristic length is the diameter, L=2R. Putting in that A=πR2, simple algebra leads to ReCD=24. I learned this all on Wikepedia!

QUESTION: 
Does (or can) light, or any EM radiation, interact with other EM radiation, of any wavelength? Simply asked: Can a beam of light hit another beam of light and if so, what happens?

ANSWER: 
EM radiation is time varying electric and magnetic fields. If you add all the electric and magnetic fields at a point in space at a particular time you find the net effect of multiple beams of light. This is called the superposition principle.

QUESTION: 
From what I can gather, Einstein used the equivalence principle to formulate the idea that gravity and acceleration might be the same. This led to the idea that gravity is "simply" a depression in space-time caused by an objects mass. Could the same concept be used to explain the attraction between the proton and the electron? And maybe between opposite poles of a magnet? Perhaps these "attractions" are also due to depressions in space-time caused by an objects mass. Maybe in this concept lies the "unified field theory" that Einstein was searching for?

ANSWER: 
There are at least two reasons I can think of, right off the top of my head, why "depressions in space time" are not the explanation of electrostatic (or magnetic) forces. First, mass and charge have nothing to do with each other; for example, a proton and a positron have identical charges and masses about a factor of 2000 different. Second, there are two kinds of charges and therefore both attractive and repulsive forces whereas there is only one kind of mass and only attractive gravitation. Einstein did, indeed, spend his entire life after about 1918 trying to incorporate electromagnetism into the theory of general relativity, to no avail.

QUESTION: 
I came across the fact that the average human being is made up of 7.0 x 10^27 atoms. In my effort to understand this number I calculated what this volume would look like if the atoms were the size of bb’s. I was astounded by the implications. If my calculations are correct, this quantity of bb’s would not only cover the contiguous United States, but they would be stacked 40,837 miles high. I’ve recalulated this numerous times and still can’t believe it. Looking for someone to check out my answer. Here are two givens: There are 6 bb’s per lineal inch. There are 3,119,884.69 square miles in the contiguous states

ANSWER: 
It is good that you have been impressed by how big a number this is but be prepared to be shocked further because your numbers are way too small. I find, if I say it takes 36 BBs to cover one square inch, that it would take about 4.5x1017 to cover the US (using your number for area), about one ten billionth of the number of atoms in your body. I find that if I stacked up 7x1027 BBs, each 1/6 of an inch in diameter, the stack would be about 2x1022 miles high, way bigger than your number. Wait a minute! I now think you mean that you will stack many layers on top of the US covering layer. I figure that it would take about 7x1027/4.5x1017=1.6x1010 layers or 9.3x1010 inches or 1.5 million miles. So, your number is about 40 times too small. To see another example of how small atoms are, see an earlier answer.

QUESTION: 
is there any work done when we ignite a matchstick?

ANSWER: 
Sure. You exert a force over a distance, pushing against friction. The match heats up as work is converted to thermal energy and this little bit of heat initiates a chemical reaction in the match where most of the subsequent energy comes from.

QUESTION: 
Why is it that air has the same index of refraction as empty space?

ANSWER: 
It doesn't. Empty space has n=1.0 exactly, air is about n=1.0003. These are close enough that, for most purposes, one can approximate them to be the same. As to why they are so close, it is because air is already mostly empty space.

QUESTION: 
I have read that cosmic ray particles can pass completely through the Earth. if this is true, is it possible to measure fluctuations in how consistently or rapidly they do so? The reason I ask is because, it seems that in the future we could produce a map of the Earth's substructure by reading such fluctuations like an x-ray machine. As the Earth rotated, I would imagine, it could be possible to triangulate on structures of varying densities, such as minerals, gas, or oil.

ANSWER: 
The only type of particle which will pass completely through the earth is a neutrino and neutrinos are notoriously difficult to detect. They interact with matter very, very weakly (otherwise, how could they go through the whole earth). I can imagine no practical way they could be used like an "x-ray machine".

QUESTION: 
Situation 1: A and B are moving on a paralell course, at similar velocities. Since the velocity differene between A and B is small (100M/sec) Situation 2: Particles A & B are Moving on the paralell course at similar velocities to each other, but particle C is moving at a significant fraction of the speed of light (say, .7c) relitive to A/B Does the mass of the particles change when the frame of reference changes to include particle C? A&B are still moving on the same course at the same velocity, only instead of viewing particle B from the frame of reference of particle A, we're viewing B from the frame of particle C, which has a velocity difference of .7c

ANSWER: 
A and B will measure each other's masses as virtually unchanged. A and B will both measure C's mass to be significantly larger than its rest mass. C will measure A's and B's masses to be significantly larger than their rest masses.

QUESTION: 
Laminar flow and turbulent flow as in the two ends of the spectrum having to do with Reynolds #. What is actually happening to the liquid being forced through a pipe at a high Reynolds # >4000 compared to the liquid that is flowing laminar <2000? When turbulent does the water begin to spin like a bullet spins caused by rifling? What would happen if rifling grooves were implemented inside the pipe? Would a turbulet flow become less turbulent or stay the same?

ANSWER: 
According to Wikepedia, when 2300<Re<4000, flow can be either laminar or turbulent, depending on the nature of the walls of the pipe. You are barking up the right tree with your rifling idea although I have no idea what rifling would do. My suspicion is that to keep it laminar you would want the walls very smooth. I think you will find the whole Wikepedia article very enlightening.

QUESTION: 
What keep a locomotive, with a load several times it's weight, from spinning when it starts off? How can it pull so many times it's weight?

ANSWER: 
It is the friction between the wheels and the rails. As long as the force you require is not too great, the wheels will not slip. And, the amount of load which there is does not really matter because a small force can move a large mass, only with a small acceleration. If you try to accelerate too quickly, the wheel will slip; you often see this happen in old movies where the locomotive wheels will spin some when leaving the station. The exact same thing happens with a car: if you try to accelerate too quickly, the wheels will spin.

QUESTION: 
if you put an empty soda can on a skate board, set it in motion (with out knocking over the can) and then stop it suddenly.... compared to if you put a full 2 liter soda bottle on a skateboard, set it in motion at the same speed and stop it suddenly... would there be a difference in how far the can or bottle goes (because of inertia)?? Also would it make a difference if the same force were used to set it in motion (not the same speed) assuming it would take more force to move the 2 liter on the skate board.

ANSWER: 
I am afraid you have designed an experiment which will not be very enlightening. The inertia part is that the object on the skateboard keeps going when the board stops. But how far each go will depend on the situation. Soda bottles are not good because they can tip over or roll. Let's consider a simpler situation: two cubes of the same size on the skateboard with different masses (to replace your empty and full bottles). Suppose that the boards are very slippery, frictionless. Then each cube will continue with the speed the board originally had, because of their inertia. To change the speed of something, a force must act on it. Now, suppose there is some friction. If there is, the force of friction is proportional to the mass, that is the frictional force on the heavier cube is larger. But, if there is ten times more mass, there will be ten times more friction. But, the acceleration of each cube will be the force on it divided by its mass (this is Newton's second law). Therefore, the accelerations will be the same! Each will go the same distance.

QUESTION: 
My physics teacher says if I were to wear a metal suit outside during a thunder storm and was struck by lightening, I would be safe. I think hes nuts. Is he right?

ANSWER: 
He is right. Lightning kills because electric current if forced to flow through your body on its way to the ground. if you wore a metal suit, the current would flow through the suit, not you. However, the suit might get very hot and burn your skin, so you better wear something to protect you from that. My best advice is to not expose yourself to lightning in any way.

QUESTION: 
Prisms separate light into colors. The different colors refract differently. So Snell's Law says that they must have different speeds. I have always heard that electromagnetic radiation moves the same speed in the same medium. If blue light has a different speed in glass than red light, does that mean that radio waves have a different speed than say ultraviolet waves in air?

ANSWER: 
The phenomenon you describe, velocity of electromagnetic waves being a function of wavelength, is called dispersion. All material media have some degree of dispersion; although air is a medium, it is so close to a vacuum in terms of the speed of light in it, that any dispersion would be all but unobservable.

QUESTION: 
I was recently participating in a Physical Test for a job. One of the tests was quantity of pushups, consecutively, without stopping or rest. Standard pushup postion, fully extending and locking out arms, coming down until chin touched the mat. I am in quite good shape so naturally I started at quite a rate of speed and was zooming through them when the Evaluator stopped me and said "You have to slow down, your cheating the number of pushups you can do by simply going off momentum." Does "momentum" even apply in this situation? There are 2 diffinitive start/stop postions and I can only go the distance of my arms.

ANSWER: 
This is probably more physiology than physics. As far as physics is concerned, the same amount of work is done if you get up by exerting a large force for a short time or a smaller force for a longer time. However, different demands are placed on the muscles and, since I have been told that, when working out at the gym, all motions should be done slowly to get the maximum benefit, the evaluator probably had a point.

QUESTION: 
If a space craft like the Space Shuttle or the Millenium Falcon had fast spinning turbines beneath the floor, would that generate a gravitational field sufficient to keep things down?

ANSWER: 
Why would you think that? It does not create any (at least any significant) gravity at all.

QUESTION: 
what evidence supports the contention that the strong nuclear interaction can dominate over the electrical interaction at short distances within the nucleus?

ANSWER: 
The fact that nuclei stay together?

QUESTION: 
I was wondering if explosions are symmetrical. Or, more specifically, explosions in a vacuum.

ANSWER: 
It depends entirely on how the bomb is designed. Probably very unlikely to be spherically symmetrical.

QUESTION: 
How dose a stainless steel tea pot generate the swirling, popping, and loud sound when applied to an electric radiant heat range top burner.

ANSWER: 
Swirling sounds are, I presume, just the convection in the water as it heats and moves around. Popping is likely do to unequal expansions of various parts of the teapot.

QUESTION: 
if you are in free fall (having reached terminal velocity), will you still feel the air resistance on your body? on earth,you feel your weight because of the pushback of earth. will you feel some of your weight if you feel the pushback of air on you as you fall? i a trying to get at whether it is possible to truely feel weightless during free fall?

ANSWER: 
Yes, because you are not in free fall, there is an upward force on you equal to your weight. You would feel like there was wind of about 120 mph in your face. Free fall means that there is no force on you other than your own weight. If you were in free fall in a vacuum, you would feel weightless. If you were in a freely falling elevator, you would feel weightless. If you were in an airplane which was following a particular parabolic path (Google vomit comet), you would feel weightless. If you were in a circular orbit around the earth, you would feel weightless. All the above are examples of free fall. I am pleased that you said "feel weightless" rather than "be weightless"; you are on the right track to understand.

QUESTION: 
I am researching on how it is possible for neutrinos to have mass, and I have come across flavour and mass eigenstates. I was wondering if you could enlighten me on what exactly an eigenstate is?

ANSWER: 
If you have an equation of the form
Ωfn(x)=Cnfn(x) where is some mathematical operator, Cn is some constant, and fn is a function of x, it may have many possible solutions (which is what the n subscripts represent). The functions fn are called eigenfunctions and the constants Cn are called the eigenvalues. If a particular physical system is in a state described by one of the fn, it is said to be in an eigenstate. A physical system may also be in a superposition of eigenfunctions, for example g=af1+bf2; g is a possible state of the system, but it is not an eigenstate. One of the best known examples is the energy eigenvalue equation, n=Enψn where H is called the Hamiltonian and En is the energy of the nth allowed state of the system described by the Hamiltonian H. The Wikepedia article on eigenfunctions is quite good (if you understand differrential equations).

QUESTION: 
if two cars are coming at each other at the speed of 50mph. Is it equal to one car heading at a concrete wall at 100mph?

ANSWER: 
It depends a lot on the details of the collisions, but likely the car hitting the wall would be more damaged. Consider one possible case, a perfectly inelastic collision (one where the objects are stuck together after the collision). Also, let's assume the two cars have the same mass M. Further, let's assume that both collisions occur in the same time T (from when the car first contacts until it comes to rest). And, to generalize, let us let 50 mph=V so 100 mph=2V. When the car hits the wall, its momentum (mass times velocity) changes by 2MV; when the car hits the other car, its momentum changes by MV. Finally, what is the physics? One statement of Newton's second law is that rate of change of momentum equals force. So for hitting the wall, the force the car experiences is Fwall=2MV/T; for hitting the other car, the force is Fcar=MV/T. So, the force experienced by the car is twice as big when hitting the wall. (Don't forget the assumptions I have made in reaching this conclusion.)

QUESTION: 
Why does fusion stop at iron in a stars core? Is it temperature/pressure or something to do with the structure of iron?

ANSWER: 
It is because iron is the most tightly bound nucleus and so any further fusion would require that energy be added. Heavier elements are made in supernova explosions. See an earlier answer for more detail.

QUESTION: 
Since mass is not conserved, gravitational field of the Sun for example is always changing. If its always changing, in this case decreasing, won't the potential energy of all objects around it decrease? Where does this energy go? If it is changed into energy according to special relativity, won't that transfer be a constant despite the position of such a body with changing mass in the universe? If so how do we prove that the energy is conserved despite the position of such an object in the universe.

ANSWER: 
First of all, this is a very small effect. If all the hydrogen in the sun fused into helium, the total mass would decrease by less than 1%. Second, this energy is completely converted into forms we understand, namely the kinetic energy of the products and radiation (light and other electromagnetic waves). If the mass of the sun decreased, the orbits of the planets would change slightly, but that would not violate energy conservation. I am afraid the rest of your question does not make much sense to me.

QUESTION: 
I thumbed through as much of your old answers as I could but couldn't find what I was looking for. I understand that it is accepted the light travels at a constant and that nothing ( other then theoretical particals already doing so ) can travel faster then it.. But Why? I get the ideas behind the formula but it just seems arbirtary speculation. Why is it that light has been decided as an unbreakable barrier?

ANSWER: 
So, do you know what FAQ stands for? Frequently Asked Questions. You will find on the FAQ page an entry which links to my discussions of why c is a universal constant and another explaining why nothing material can go that fast.

QUESTION: 
a question regarding the situation where one twin stays on earth and the other travels on a space ship at close to the speed of light, only to return to earth younger than her sister who stayed behind. They're moving relative to one another. from the earth bound twin's perspective, her sister is moving away at a high velocity. but from the space ship sister's perspective, isn't it her sister on earth who's moving away at a high velocity? so why doesn't the earth bound sister age more slowly? is it the acceleration from a steady state that slows time? and if everything in the universe dissappeared except for the twins, how could we know which was accelerating away from the other? does the spaceship sister accelerate from a distinct point in the fabric of space?

ANSWER: 
Well, that's why they call it the twin paradox! In fact, it is not a paradox at all and there is no question that the traveling twin is the one who ages less. See my earlier discussion of the twin paradox.

QUESTION: 
The other day I was thinking about energy...and this led to a little wonder! When we drop an object we hear a sound - this is generally considered as kinetic energy transforming into sound energy, correct? Well, what I was wondering was why does it become sound energy? What in the history of the universe/physics determined that kinetic energy will become sound energy when there is a collision? Why doesn't it become heat energy for example, or light energy?

ANSWER: 
When two objects collide, kinetic energy is not conserved, that is, it does not remain constant. However, energy as a whole in an isolated system must be conserved, lost kinetic energy must show up somewhere. Where it shows up depends on the nature of the system but it can go to heat, sound (as you note), or, in rare cases, light. But, just because you hear it does not mean that sound is the only mechanism going on. In fact, even if the sound is very loud, the energy carried away by sound is usually a very small fraction of the total kinetic energy loss. In nearly all collisions of macroscopic objects, most of the energy is carried away as heat.

QUESTION: 
What is the phase of Earth one would see if viewing from the Moon during New Moon

ANSWER: 
The moon and sun would be on the same side of the earth, so I guess you would, from the moon, see a "full earth".

QUESTION: 
If the history of the universe since the Big Bang could be condensed in one year (365 Earth days), when did humans evolve on our planet?

ANSWER: 
Taking the age of the universe to be about 14 billion years and the age of "modern" humans to be about 2 million years, I find about an hour and a quarter.

QUESTION: 
in what ways can scientists test for the presence of each of the different components of the electromagnetic spetrum?

ANSWER: 
This is a too broad question since there are so many types of electromagnetic waves to, in a brief answer, list all ways to detect them. For example you could detect visible light with your eyes but your eyes cannot see radio waves. You can detect radio waves with a radio but your radio cannot detect gamma rays. You can detect gamma rays with a scintillation detector, but a scintillation detector is not sensitive to microwaves. And on and on and on
!

QUESTION: 
Frictional force always acts opposite to the motion to the body then how come it provides the necessary centripetal force when car is moving in a circular road shouldn't it be acting opposite to the motion of the tier i.e. along the circumference or tangentially ?

ANSWER: 
You have to be really careful with such a rule. In fact, friction does not have to act opposite the body. If an object is sliding, friction is always opposite the motion. But think of a box just sitting on an incline, it has no motion but friction acts up the incline. If you try to push it up the incline but do not succeed, it may be sitting still with the friction acting down the incline. So, static friction can act in any direction. This is what happens with a car rounding a corner
—the wheels are not slipping and so it is static friction. So you see, you need to focus on the surfaces where the friction is acting, in this case the tires and the road, and here they are at rest with respect to each other. Another similar example is accelerating forward in your car; what is the force which propels you? It is, in fact, friction between the tires and the road which is a force that points forward in the direction the car is moving. But the direction the car is moving does not matter because friction is not between the car and the road, it is between the tires and the road.

QUESTION: 
I'm having trouble understanding the flow of AC current. If the current is generated by a rotating magnetic field, and switches direction each wave cycle, wouldn't that simply jiggle the electrons back and forth? I.e, push the electrons a tiny distance in one direction on the positive curve of the sine wave, and pull the electrons back the same distance in the opposite direction on the negative curve of the sine wave? Whence comes the directionality of the flow?

ANSWER: 
Imagine a light bulb. Does it matter which way current flows through it? It works fine with DC current in either direction. The moving electrons interact with the filament and heat it up regardless of which way they are going. So, the fact that the electrons "jiggle" back and forth is irrelevant.

QUESTION: 
why do you use a ladder to go across a frozen lake?

ANSWER: 
Because the ladder spreads out the force of your weight over a large area. What determines whether the ice will break is not just the force on it but over what area you exert it, that is, pressure (which is force per unit area) is what matters.

QUESTION: 
At what altitude does the moon's gravity over power the earth's? How far out in space do you have to be before you start falling to the moon and not the earth?

ANSWER: 
You need to know that the gravitational force due to an object is proportional to its mass and inversely proportional to the square of the distance to it. Using this it is straightforward to show r=0.9R where R is the distance between the centers of the earth and the moon and r is the distance from the center of the earth to the point of zero gravity. R ranges over about 3.6-4.1x108 m over the course of a month.

QUESTION: 
Since the gravitational attraction between two objects varies with their mass at given distance per the Universal Gravitational Constant, why then do two objects with different masses fall at the same rate as described in the Pisa experiment? Or do they? Is not the combined gravitational force of the heavier object and the gravitational force of the Earth greater than the forces of the lighter object and the Earth? And if the force is greater why wouldn't the two objects both accelerate toward each other faster and come into contact quicker than the lighter object and the Earth?

ANSWER: 
The acceleration is proportional to the force on the mass (weight) divided by the mass, and the force on the mass (weight) is proportional to mass. Therefore, the acceleration is independent of mass. See an earlier answer for more detail.

QUESTION: 
Does a heavier object go down a ramp faster than a lighter one? Assume all other variables stay the same.

ANSWER: 
This link will link to several previous answers to this question.

QUESTION: 
If the sun was almost instantly coverted into pure energy, (e.g) the jet from a supermasive black hole the size of the sun was to hit the sun changing all the mass into energy in the time it takes for light to go from one side of the sun to the other. Would the time it took before the earth noticed there was no gravitational pull from the sun be almost instantaneous, or would it take ~ 9mins (Time light takes to reach the earth from the sun) (e.g) does gravity move faster than light speed.

ANSWER: 
See earlier answser.

QUESTION: 
I've just seen some videos on youtube where people demonstrate how water, unlike other materials, actually expands when frozen. This fact is shown, for example, by keeping water inside a pipe with both ends tightly closed, and then freezing the water inside. The pipe, even if made of steel, explodes. I was wondering what would happen if you were to perform a hypothetical experiment where a really, really, really strong pipe (or some other container) is used. Would the expanding water still perform enough force to break the container, no matter how strong it is, or it would not freeze? Or maybe freeze but without expanding the usual ~9%?

ANSWER: 
You can always make a container strong enough to perform the kind of experiment you propose. Normally, a phase diagram for a material (like water) is a picture of pressure as a function of temperature keeping volume constant. To the right is a phase diagram of water. Note that the line between liquid and solid has a very large negative slope so that as you cool the water, it just gets higher and higher pressure never actually solildifying. But this graph only goes up to a pressure of about 2 atmospheres; we cannot see what happens if we keep going colder just below zero. Water is very complicated at extremely high pressures, so I recommend that you look at this Wikepedia article to see what happens at very high pressures. It looks to me like when the pressure gets near 109 Pa (1 Pa=1 N/m2, and 1 atm
105 Pa, so 109 Pa104 atm) liquid is no longer possible and one of the exotic forms of ice will form.

 


QUESTION: 
i am having trouble finding information on the compressibility of air. If i was to compress air under 100psi - in a 1 cubic inch container, how much distance would the air ultimately be compressed ? ...in other words, how much smaller than a cubic inch would the air become as it is squeezed ? ...also, i do not understand why tools that run on compressed air are rated for Cubic Feet per Minute, while air tanks are rated by Pounds per Square Inch, when square inch refers to surface area as opposed to the three dimensional quality of cubic inches. When the term PSI is used for air tanks, does it mean that is the pressure exerted on the surface area of the inside of the tank ? ...if so, is there a formula for converting the PSI of a tank into Pounds per Cubic Inch ? ...a 1-Gallon tank (1728 cubic inches) under 100psi = How many lbs. of pressure on each cubic inch of air in the tank ?

ANSWER: 
PSI stands for pounds per square inch. It is a force per unit area, so when you ask "How many lbs. of pressure on each cubic inch
", it has no meaning, nor do your questions asking about "pounds per cubic inch". In a tank of gas, the pressure is (almost) the same everywhere; that is, if you move a tiny pressure gauge around inside, it will always read exactly the same. I say "almost" because the gas itself has weight and, technically, the pressure increases as you go lower in the tank; the effect is not nearly as important as for a liquid and, for what you seem to be interested in, of no importance. When a tank of gas has a certain pressure, say 100 PSI as you are interested in, this is normally the gauge pressure, the pressure above atmospheric pressure. If you open the valve of the tank to let gas escape, it will keep going out until the pressure inside is the same as the pressure outside—atmospheric pressure which is 14.7 PSI. So, 100 PSI means the pressure inside the tank is 114.7 PSI. Now, to ask about changes incurred by changing pressure, it is best to know the ideal gas law: PV=NRT; here, P is pressure, V is volume, T is temperature, R is just a constant depending on the units you are working with, and N is a measure of the amount of gas. It sounds like you are interested in situations where N and T are kept constant, in which case the product PV would be constant; if you double pressure, you must halve volume. In your specific case, starting with a 1.0 in3 container at 0 PSI gauge pressure (actually 14.7 PSI) and increasing the pressure to 100 PSI gauge pressure (actually 114.7 PSI), the compressed volume would be (14.7/114.7)x1.0 in3=0.128 in3. The cubic feet per minute part of your question would be relevant to the ability to maintain the pressure if gas were being removed. Again, look at the ideal gas law but now the volume and temperature stay constant but N is changing (gas is being removed), so P/N must remain constant. If you do nothing, just attach your tool to a tank, as you remove gas you must decrease pressure; if you remove half the gas, the pressure must go down by half also (use absolute, not gauge pressures here). So what the cubic feet per minute rating  tells you is how fast you must replace the gas in the tank to maintain constant pressure, presumably done with a pump.

QUESTION: 
I was wondering how you can calculate the diameter of a drop of water which falls from an eye dropper.

ANSWER: 
I did not know this, but there is a measure of volume called a drop whose volume is 0.06485 cm3. Knowing the volume of a sphere, 4
πr3/3, you can show that r=0.25 cm, or d=0.5 cm. This actually sounds pretty big for a drop coming from an eyedropper, so here is another way: drop the drops into a teaspoon measure, counting them, until it is full. A teaspoon has a volume of about 4.93 cm3. So, if there are N drops in a teaspoon, the volume of a drop is (4.93/N) cm3, and from there you can calculate the diameter as I did above. Incidentally, there are about 76 official drops in a teaspoon.

QUESTION: 
why does water in canteen stay cooler if the cloth jacket surrounding the canteen is kept moist

ANSWER: 
One of the most efficient means of cooling something is evaporative cooling. To evaporate water it takes a large amount of energy. So, the cover of the canteen extracts heat from its environment resulting in its becoming cool as the evaporated water carries away the heat. A simple experiment you can do is to wet a rag and then swing it around a few times briskly. You will find the rag gets quite cold, even on a hot day.

QUESTION: 
This question may sound trivial, but please bear with me. Assume a person is standing on Earth and is going to be hit by a very large object. Let's say there was a large object (with a large mass) and it started accelerating towards you from a foot away at a rate of about 1 foot/second. The mass of the object is such that the gravitational force between the person and the huge object is negligible compared to the gravitational force of Earth. If the acceleration is extremely small, will the huge mass of the object coming in contact with the person be enough to send the person flying?

ANSWER: 
The acceleration is irrelevant, it is the speed at the instant it hits you that matters. "Send the person flying" is not a very quantitative statement. Let's just talk about your speed after the collision. Let your mass be 100 kg (about 220 lb) and you are hit by a freight train with a hugely bigger mass. Two extremes:
  1. Perfectly inelastic, you "stick" to the locomotive so your speed is equal to the speed of the train, v. The collision will happen in a relatively short time, call it t. Your average acceleration is then v/t so the force you experience is mv/t. If v=0.1 m/s and the time of collision is 0.1 s, you would experience a force of about 100 N, about 22 lb. No big deal. But, as v gets bigger or t gets smaller the force gets bigger.

  2. The other extreme is a perfectly elastic collision where you are moving on your own after the collision (and have done so conserving energy). In this case your final speed will be about twice the speed of the train, 2v. So the force over the collision time will be about twice as large.

The bottom line of my answer is that if you are hit by a big thing going very slowly you will probably not get hurt badly.


QUESTION: 
I've often read that an object falling toward a black hole or neutron star would be stretched along the direction of its path. I can see that gravity would attract the nearest parts of the object more strongly, but I don't see why that would stretch the object. It seems like a lot of unnecessary (and uncompensated) work.

ANSWER: 
You have put your finger on the stretching aspect: since the force is so huge, the difference in force between two points separated by only a few centimeters is very large resulting in the tendency to stretch. This is called the tidal force.

QUESTION: 
For a while now I been trying to find out all that i could about the universal speed limit, the speed of light, and a recent metaphor has me slightly intrigued. say a train can travel extremely near to the speed of light, say 99.99999% close, but what would happen if there were another train, moving in the opposite direction, along a parallel track, wouldn't the relative speeds be breaking the speed of light?

ANSWER: 
I have answered this question and others just like it many times before.

QUESTION: 
Would we weigh more if the earth's spin on its axis slowed down, due to a reduction in centrifugal force against a consistent graviational force?

ANSWER: 
First, as I always insist on saying, your weight is the force the earth exerts on you and it does not change because of any motion you might have. That said, your apparent weight does depend on your motion. For example, we say an astronaut in orbit is "weightless", but this is wrong; the earth is still exerting the same force on her but that force is providing the centripetal acceleration holding her in orbit so she seems to be weightless. If the earth is spinning (as it, of course, is) your apparent weight is less; this effect is biggest at the equator and absent at the poles. It can be easily calculated that, at the equator, the reduction of your apparent weight about 0.3%, rather small.

QUESTION: 
A lottery machine uses blowing air to keep 2000 Ping-Pong balls bouncing around inside a 1.0m * 1.0m *1.0m box. The diameter of a Ping-Pong ball is 3.0 cm. What is the mean free path between collisions?

ANSWER: 
For a nice discussion of how to calculate mean free path, see the Hyperphysics website. Using the equation derived there, 1/(
πd2nV), I find about 18 cm.

QUESTION: 
Is gravitational force between the earth and sun, the same at all times of the year?

ANSWER: 
No, the earth's orbit is not circular and so the distance is different at different times of year.

QUESTION: 
Do electromagnetic waves make magnetic fields?

ANSWER: 
Electromagnetic waves are magnetic (and electric) fields. See my earlier discussion.

QUESTION: 
Can virtual particle pairs (such as electron-positron) annihilate each other on contact?

ANSWER: 
They can and do. That is what they must do, in fact, to obey the laws of physics. When they come into existence, they violate conservation of energy because where there was nothing there is now something. According to the Heisenberg uncertainty principle, it is ok to violate energy conservation as long as it is done for a sufficiently short time, namely
ΔEΔtħ where ΔE is the amount by which energy conservation is violated (the mass energy of the particles, 2mc2 in this case), Δt is the time the pair exists before annihilating, and ħ is Planck's constant divided by 2π.

QUESTION: 
Current electricity is defined as the "flow" of electrons from one place to another. Would a "flow" of protons from one place to another have similar properties? I'm wondering, if I had a source of protons, could I construct a battery with them? I know that protons are not just floating around free like electrons. And I know that electrons are indivisible, while protons are composed of quarks, and electrons have very little mass compared to protons. But still... would moving positive charges along a conductor produce results similar to those we get when negative charges move along a conductor?

ANSWER: 
Any moving charges constitute an electric current. The effects of a current of electrons in a wire could be reproduced precisely with a group of moving protons (in principle, since it would be very hard to do). It is just that it is so much easier to do with electrons since they exist naturally in conductors in an almost free state making them very easy to move around and manipulate.

QUESTION: 
How about a simple question about Dark matter? Since it is affected by gravity but doesn't interact with matter, wouldn't there be a bunch of it at the center of the earth?

ANSWER: 
First, my usual refrain: as far as I am concerned, there is no such thing as dark matter until it is actually observed experimentally. I do not consider the anomalies suggesting too little observed mass in the universe to be "proof" of the existence of dark matter. My understanding is, again I am not an astrophysicist, that the workaround for the dark matter not accumulating around and in massive objects like the earth is that it is "hot", that is it is moving around very fast so it does not get bound.

QUESTION: 
how can it be that the resolution of the light being emitted from a object doesn't decrease with distance? ie if light is say 1mm2 in size and an object is emitting light particles and has a circumference of 360mm(thinking 2 Dimensionally) 1mm from it surface the resolution would be 1 light particle per 1mm2 of space, but between 1mm and 2 mm from the surface it would be 1 light particle per 2.(something)mm of space and so on. therefore by the time light particles being emitted from this object are lets say a billion lights years away....wouldn't it stand that the density of light particles being observed would be something very close to zero? ie; why do we see anything coming from a star on the other end of the known universe, or mayby this would explain why space appears mostly empty beyond what we observe, eventually we can't detect light particles coming from distant objects because the resolution (ie number of particles per given amount of space) becomes too low?

ANSWER: 
It is not the resolution which is changed by distance. But you are right that the farther away you are the more difficult it is to form an image because of the loss of intensity over distance. Suppose that a particular object we wish to see has an intensity of only one photon per second per square meter at our distance. Obviously, if we wait ten seconds we will only get 10 photons into a telescope 1 m2 in area and a very grainy image will be seen. On the other hand, if we wait ten years we will get a perfectly good image. The other way to improve things is to collect more photons by having a bigger telescope; that is why the best research instruments are very big.

QUESTION: 
It's my understanding that gravity is the bending of space fabric. Earth weighs down the fabric, the moon falls in. What force or forces keeps the moon from sinking straight into Earth?

ANSWER: 
Think of a big bowl. Take a marble and give it a push around near the rim of the bowl. It will, if given just the right amount of speed, roll around without falling to the bottom, right?

QUESTION: 
So splitting an atom causes an explosion. Would splitting other particles do the same thing? What's so unique about the atom? Is the energy in a nuclear explosion released from the atom, that is, how does splitting the atom cause all the observable chaos (mushroom cloud)?

ANSWER: 
See my earlier answer on fission and fusion.

QUESTION: 
What would be the amount of energy needed to disable Earth's magnetic field?

ANSWER: 
I don't find an estimate with a quick search and it would be extremely tedious to do because the field is not uniform. It would equal the energy stored in the field. The energy density near the surface is about 40 J/m3. So if you estimate the amount of energy over the whole earth up to an altitude of 1000 m, I find about 2x1019 J. This would probably be within a few orders magnitude of the total energy.

QUESTION: 
After constructing tables and graphs of E=mc^2, E/m=c^2 and (E/m)^1/2= c, I found that although E/m of course always gave the same value, it was not possible to destroy the whole mass whilst converting it to energy for the photon. Any rest mass value could be used, so long as it was very small ie in the magnitude of sub atomic mass, but destroying the whole mass caused the photon particle to accelerate to infinite speeds. The only way to resolve the problem was to assume that the rest mass was not destroyed in the transition from particle to photon, but that the mass just somehow gave up all of its energy without actually disappearing. This would imply that the mass was in a special degenerate state, ie very compact and at absolute zero temperature, I have not yet ascertained the magnitude of the mass, but hope to do so somehow after further and much more advanced physics study. I no that this seems to go against the accepted laws of physics and relativity, but it's the only way I can make sense of it at this point in time. Question is: Am I way off the mark here, and have a made some bad assumptions.

ANSWER: 
This question did not have a valid email, a violation of my rules, but I could not resist dealing with it because it is, as the questioner suggests himself, "way off the mark." I completely do not understand the second, third, and fourth sentences of this question; none of it makes physics sense. A complete explanation may be found in an earlier answer of mine, linked to on the FAQ page.

QUESTION: 
Why can't they make Power stations powered by their own electricity? You'd only need a device to start it and then it'd generate electricity with it's own power supply. I realise the Power station would need to be bigger than coal or hydro-electric power stations but couldn't it work?

ANSWER: 
The first law of thermodynamics can be expressed as "you can't get something for nothing" and the second law can be expressed "you can't even break even". Think about it: the power station is providing energy to itself and to you; where does this energy come from? Maybe you could imagine the power station providing only energy to itself so that it uses exactly as much as it produces; that would not violate the first law which would say that energy being used was equal to energy produced. The second law forbids any machine which is 100% efficient, even if we were able to avoid mundane problems like friction, heat loss, etc.

QUESTION: 
I am curious about electromagnetic radiation and electric charge; do these two things have anything to do with each other? How are they linked and/or separable? I have done some searching and have yet to find a clear explanation of this issue.

ANSWER: 
Electric charges cause electric fields. Electric charges in motion also cause magnetic fields. Electromagnetic radiation is a very special combination of electric and magnetic fields which propogates through space with a speed of 3x108 m/s. When I say "very special" I mean that not every combination of electric and magnetic fields is electromagnetic radiation; for example, imagine a charged particle moving with constant speed in a straight line
—there is no radiation, just the associated electric and magnetic fields. However, if you make the charge accelerate the result is electromagnetic radiation. One simple example is to simply imagine a charge attatched to a spring and oscillating back and forth; it radiates electromagnetic waves. All electromagnetic waves have their source, in one way or another, in accelerating electric charges. Radio waves have their origin in antennas in which we make electric currents flow back and forth. Light has its origin in transitions in atoms where an electron moves from one orbit to another. You might want to look at my earlier discussion of electromagnetic waves.

QUESTION: 
If I'm driving on the freeway and a fly flies into my car through an open window, what happens to it? Does the rear window catch up to it or can the fly instantly marry with the new environment? (because the air trapped inside the vehicle can exert force on the fly, right?)

ANSWER: 
Nothing in nature happens "instantly". I assume the fly flies straight in with a velocity (as seen by an outside observer) perpendicular to the car's velocity. From the fly's perspective, it enters a wind going the speed of the car and so he is accelerated in the direction of the car's velocity. From your perspective, he enters with a component of his velocity which is toward the rear window; but, you see that component decreasing steadily (he has acceleration toward the front of the car) and he may or may not hit the back window depending on how fast the car is going.

QUESTION: 
What is the difference between tangential and radial acceleration mathematically in a non uniform circular motion? Are they equal to dv/dt?

ANSWER: 
Any vector can be resolved into components. For a particle moving in two dimensions on a curved path, it turns out to be particularly useful to choose the component directions to be perpendicular to (radial) and tangent to (tangential) the path of the particle at that instant. The radial component of acceleration is the rate of change of the direction of the velocity vector and the tangential component is the rate of change of speed. If the path is a circle, the radial is the centripetal component and the tangential is the "speeding up or slowing down" component. The acceleration is dv/dt (be sure to note that v is a vector) and dv/dt=r0v2/R+t0dv/dt. where r0 and t0 are unit vectors in the radial and tangential directions, respectively.

QUESTION: 
Various online postings about subatomic particles describe angular momentum as an intrinsic property of the particle rather than a force imposed on the particle or generated in the particle by rotating mass. In other words, the subatomic particles seem to possess properties of quantized angular momentum not due to axial quantized rotation of the particle, but by simply having that property independent of rotation, as is intuitively seen as necessary in classical conceptions of angular momentum. My question is, what empirical experiments, if any, rather than just theoretical considerations to avoid paradoxes, have actually proved beyond doubt that subatomic particles do not in fact have rotation inducing their angular momentum? It seems kind of like physicists are imposing an interpretation here rather than actually pointing to experiments that prove the impossibility of subatomic particle rotation, in order to arbitrarily preserve non-violation of relativity's dictum that nothing can go faster than the speed of light, since to produce the observed angular momentum, the quantized rotation would require the surface of the particle to exceed c. Is this conclusion an experimentally proven fact (if so, reference?), or, a convenient interpretation?

ANSWER: 
There is no question that an isolated elementary particle has an intrinsic angular momentum (spin). The most common way to verify this experimentally (and how spin was discovered in the first place) is to observe the associated magnetic moment. If you think about a charge distribution which has intrinsic angular momentum, there must be associated current distributions which will make the particle look like a little magnet. For an electron, for example, which is known to have a spin angular momentum of
˝, it is found that the magnetic moment can point only up or down and therefore the spin must indeed be ˝. A much more thorough testing can be done through the study of atomic structure. The total angular momentum of an atom can be measured and hundreds of atoms and their excited states can only be understood if the spins of the electrons are included; for some very fine details, the spin of the nucleus must also be added in with the net angular momentum of the electrons. You should always be wary of, as you are trying to do, of imposing classical ideas on intrinsically quantum systems. Angular momentum need not be associated with rotating masses as you want to do. One final thing: when the Dirac equation is solved for an electron, it turns out that the orbital angular momentum L of the electron is not a constant of the motion, rather the appropriate quantity is L+˝, that is spin automatically is incorporated in relativistic quantum mechanics.

QUESTION: 
For a planet of uniform density, how would the magnitude of the gravitational field halfway to the center compare with the field at the surface?

ANSWER: 
For a spherically symmetric mass distribution like you describe, the only mass which exerts a force is that mass inside where you are measuring, so only the mass of the earth inside R/2 is effective, R being the radius of the earth. Since the mass of a sphere is proportional to R3, 1/8 of the mass of the earth is what is acting. The gravitational force is also inversely proportional to the distance squared, (R/2)2. Therefore, the field halfway is (1/8)/(1/4)=1/2 as strong as at the surface. In general the field is proportional to (r3/r2)=r, that is it increases linearly as you go out.

QUESTION: 
I was watching a recent sci trek episode that spoke of time travel as being true using the basic principle that time moves slower the faster you go. They proved this using an atomic clock. So my question is does that mean if I had fallen asleep on a jet liner going around the world, at the same time someone fell asleep in the city I left from, would I have had more sleep than the other person? I know off the wall but just poking around with some ideas.

ANSWER: 
You are posing a question which is essentially a variation on the twin paradox. You should read my earlier answer on the twin paradox. So, when the two twins get together after the journey and compare clocks, less time has elapsed on the traveling twin's clock. Therefore, you would have slept less, not more. I hope you realize that the differences in time are trivially small unless your speed is comparable to the speed of light.

QUESTION: 
I want to give my students a relative understanding of the size of an atom. If a bowling ball were enlarged to the size of the entire earth, (and its atoms enlarged proportionately), how big would just one of its atoms be? the size of a house? a beach ball?

ANSWER: 
The radius of a bowling ball is about 0.1 m and the radius of the earth is about 6x106 m. So the magnification is about 6x107. The radius of an atom is about 1
Ĺ=10-10 m, so the magnified atom would have a radius of about 6x107x10-10 m=6x10-3 m=6 mm, about a quarter of an inch. Now, take it a step farther: magnify the whole atom to the size of the earth, magnification now an additional 6x106/6x10-3=109. So the net magnification is now 6x107x109=6x1016. The radius of the atomic nucleus is about 5x10-15 m, so if the atom is the size of the earth, the nucleus has a radius of 6x1016x5x10-15 m=300 m. Atoms are small, nuclei are really small!

QUESTION: 
These questions are in relation to faster than light communications and travel, are in the form of hypothetical examples, and come from my childhood... Let's consider the seconds hand of a clock that moves from 12 to 3 in 15sec. If the hand is absurdly long, isn't it possible to reach and exceed the speed of light at some point along the hand towards the tip since it is covering an increasing distance in the same 15sec window of time? Let's consider a stick where if I push on one end, a button that lights a bulb is pressed on the other end. If the stick is, again, absurdly long, then isn't it possible for my signal to have exceeded the speed of light?

ANSWER: 
This question is often asked in one form or another. You will see two similar questions not long before yours. You could find the answer on the FAQ page. The bottom line is that the "push" on a long rod travels with the speed of sound in the rod. And the long clock hand variation
—the mass of the tip would become so huge that you would be unable to exert enough torque to accelerate it further, never mind that the effect of your push would not reach the end for millenia.

QUESTION: 
if E=MC2 where as you approach the speed of light mass increases as we approach it thereby having to increase the energy then why is it that photons travel at the speed of light but do not have infinite mass?

ANSWER: 
Quite simply, because E=mc2 does not apply to photons. You could find this on the FAQ page (it is truly one of the most frequently asked questions).

QUESTION: 
A centrifuge simulates an intense gravity field as it rotates in a horizontal circle. That motion requires a centripetal acceleration directed towards the axis of rotation & hence, a force also directed toward to axis of rotation. I get the acceleration vector, but what is the source of the centripetal force? Is it the walls or bottom of the test tube? Is it the pressure gradient across a finite fluid element? The buoyant force? This is never mentioned in any literature I can find?

ANSWER: 
The side walls exert an inward force on pieces of the fluid in contact with it (called normal force in elementary books); in addition, these small elements feel an outward force due to the fluid just inside them. I think of the vector sum of these two forces as being the centripetal force for small volume elements at the surface of the wall. But, what about parts of the fluid inside? Imagine a small volume inside somewhere. The forces on it are a force in by the adjacent element just outside where it is and a force out by the adjacent element just inside. The vector sum of these is the centripetal force. If this volume element is more dense than the fluid in general, it will "sink" toward the vessel wall, very analogous to what happens when something sinks in a fluid. Hence, the ability of a centrifuge to separate out heavier stuff.

QUESTION: 
I know our atmosphere is made up of infinitely small particles that compose gases, liquids, and solids; however is our atmosphere completely saturated with these molecules or is there residual.... "space"?

ANSWER: 
These small particles (atoms or molecules) are not infinitely small, they are just very small (on the order of 10-10 m). In any gas only a small fraction of the volume is occupied by these particles, mostly it is empty space.

QUESTION: 
say in the future we are harvesting asteroids for their metal products and building things on earth. would adding so much new material to our planet have any effect on its rotation?

ANSWER: 
I cannot imagine that the mass of such things would ever get anywhere close to the mass of the earth, totally negligible.

QUESTION: 
What types of sub atomic particles are photons made of? If not then of what?

ANSWER: 
They are not "made of" anything, they are elementary. They are the quanta of the electromagnetic field.

QUESTION: 
Is empty space really empty? I can't imagine how it could be because in order for two things to be seperated there must be something in between to hold them apart.

ANSWER: 
Why does there need to be something to hold them? Anyhow, physicists believe, not for your reason, that space or "the vacuum" is actually populated by a sea of particle-antiparticle pairs popping into and out of existence. This may sound like it violates energy conservation (where does the energy come from to create the mass of the pair?), but it turns out that energy need not be conserved over sufficiently small time intervals; hence, the popping back out of existence. The physical principle which allows this is the Heisenberg uncertainty principle.

QUESTION: 
If you drive 100 km/hr, at the end you have travelled 100 km in space and 1 hour forward in time. If you go 100 light years, haven't you travelled 1000 trillion km and 1 jullian year forward in time?

ANSWER: 
100 light years is about 1018 m (so your 1000 trillion km is ok). At the rate of 100 km/hr it would take you about 60 million years. I do not know where you got that you could do it in one year.

QUESTION: 
My grade 9 science teacher believes for some reason that neutrinos are sub-subatomic particles because of beta decay. I`m certain that they are only subatomic, but she won`t listen. How can I scientifically prove to her that it is subatomic?

ANSWER: 
Do not make the mistake that semantics is physics. There is no operational definition of what a subatomic particle is, it is a generic term. And, if your teacher insists on putting another sub- there, that is just fine because it has no important meaning. Perhaps she is trying to convey, using this terminology, that neutrinos do not exist in atoms until they come into existence in beta decay.

QUESTION: 
Assuming matter could be ramrod straight for a lightyear: If I have a one light year pole one inch away from a glass of water, and I push my end of the pole at the same instant I pointed a laser at the glass, would the beam or the pole reach first? I assume the pole. Does this void casulty? Ignoring practicality issues, would this mean that a morse code system could allow Faster than Light Communication?

ANSWER: 
This question and ones like it are often asked. See the FAQ for an earlier answer.

QUESTION: 
My quandry is this: If e=mc2, how can a photon that has 0 mass, have any energy? Of course, I realize photons DO in fact have energy, since I can go outside on a sunny day and feel the warmth of the sunlight on my face. But when I plug "0" into the e=mc2 equation, I get a result of "0". Thanks for helping!

ANSWER: 
I have answered this question many times before and it is among the questions on the FAQ page.

QUESTION: 
I have a question about how sweating cools the body. I understand that sweat is produced by our bodies and makes our outer body wet, and when the heat from our body comes into contact with the sweat, it turns the sweat into water vapour, which evaporates and carries away some of the heat energy from our body. However, I have two questions about this. 1) Since heat rises, wouldn't the heat just come off our body and naturally rise away from us, leaving us cool? 2) Why do our bodies feel cool after the sweat start carrying away the heat if more heat is still being produced by the body? I guess I'm just having trouble understanding the "cooling" effect. Can you please explain this to me in a more scientific way (without the use of kilojoules and Newtons and the like)?

ANSWER: 
Evaporative cooling is very efficient because it takes quite a bit of energy to change the state of water from liquid to gas. Heat is not some fluid which will just rise up and away from you. It is not heat that rises, it is warm air that rises. Before that warm air can rise, it has to be warmed. Without evaporative cooling, the heat flow from your body can be via either conduction (your skin warming the air in contact with it) or radiation (actual electromagnetic radiation, mostly in the infrared). Both are very much slower than evaporative cooling. Even though your body continues to warm up, if you keep sweating you will attain an equilibrium situation where you the heat flows away as fast as you are warming up. You should also read my earlier answer regarding the meaning of heat.

QUESTION: 
Light-years. Is this a measurement of distance or is it also a measurement of time?

ANSWER: 
A light year is the distance that light will travel in one year. It is a distance.

QUESTION: 
If a buss is going 100mph and I’m seating on the back seat, I am traveling 100mph within a moving object in this case a (school buss). It happens a fly lands on my “hand” That fly’s speed is as well 100mph, suddenly the fly takes of my hand and flies towards the buss driver within moving object (school buss) that is doing 100mph. What is the speed of the fly combined the 100mph? I don’t know if I’m making myself clear enough! But what I’m trying to find out is the length of the territory covered by a fly within motion object.

ANSWER: 
Velocity is a relative thing. The velocity of the flying fly relative to the bus and you is however fast the fly is flying, let's say 1 mph. Then, relative to the bus, he will fly (1 mi/hr)x(1 hr/60 min)=1/60 miles in one minute. The velocity of this fly relative to the road is his velocity relative to the bus plus the velocity of the bus relative to the road, so it is 101 mph. So, relative to the road, he will fly 101/60 miles in one minute. Keep in mind that velocity is a vector, direction matters, so if the fly is flying from the driver back to you he is going 99 mph relative to the road.

QUESTION: 
I am trying to understand magnetic fields induced by electric fields. I can understand and calculate magnetic fields induced in small segments of wires, but I do not understand why the magnetic field at a point directly in the center of the center of a closed loop is not zero. Is there a difference between circles, squares, and other symmetrical polynomials?

ANSWER: 
I think you must mean magnetic fields created by electric currents. An electric field does not induce a magnetic field unless the electric field is changing. The reason the magnetic field in the center of a loop is not zero is that the current from each piece of the loop has a magnetic field through the area defined by the loop pointing in the same direction as all the others, so they cannot add to zero.

QUESTION: 
If you could stretch a wire (A---------B) in space a distance of say... 1 light minute. Then you tugged end A, would you immediately notice the tug on end B or would it take a minute?

ANSWER: 
It would take much longer than a minute; see my earlier answer.

QUESTION: 
Are theories testable. Or can a theory be based on speculalation?

ANSWER: 
According to the Wikepedia entry on theory, "(t)heories are analytical tools for understanding, explaining, and making predictions about a given subject matter." In philosophy, "(t)heories whose subject matter consists not in empirical data, but rather in ideas are in the realm of philosophical theories as contrasted with scientific theories." I personally would endorse these semantics with regard to what a physical theory is. For example, the theory of general relativity, which is the current best theory of gravity, makes predictions which may be tested; string theory, on the other hand, makes no predictions about nature and must, in my opinion, be classed as a philosophical theory based on ideas.
QUESTION: 
Since energy cannot be created or destroyed, only converted from one form to another, what does a computer processor convert the electrical energy into? (if a processor which receives 50 watts, only converts 40% of that into heat, where does the rest go?)

ANSWER: 
The way a processor works, essentially, is that many switches are being repeatedly turned on and off. Every time a switch is turned on or off it takes energy.

QUESTION: 
During a free fall ride at an amusement park, why do you not feel your seat when you're falling? You feel weightless, but you do not feel any force between your body and the seat of the car... why?

ANSWER: 
During free fall (neglecting air resistance) the only force on you is your own weight and when an object falls with only its weight acting, its acceleration is g=9.8 m/s2. If the seat exerted an upward force on you, your acceleration would not be g and, therefore, by definition, you would not be in free fall because the net force on you (which is what determines acceleration) would differ from your weight. [In the real world, since air resistance does exist, the acceleration will be slightly smaller than g and so the seat will exert a small upward force on you, but much less than the magnitude of your weight which is "normal" when not accelerating.]

QUESTION: 
In the absence of air resistance, a ball thrown vertically upward with a certain initial kinetic energy will return to its original level with the same kinetic energy. When air resistance is a factor affecting the ball, will it return to its original level with the same, less, or more kinetic energy?

ANSWER: 
Air resistance, a kind of friction, takes energy away both on the way up and on the way down.

QUESTION: 
I have been studying special relativity on and off for the past few years. I understand some of the concepts and find the ideas and theories fascinating. One problem i continue to have is with understanding gravity. When describing gravity, physicists often use a depiction of a well created in a fabric of space time. This does a great job of illustrating gravitation in a specific plane, but falls apart when one consideres space as a 3 dimentional void. Is this depiction the result of a dumbing down? Is there a better way of thinking of this concest of gravit? As it is, it seems incomplete.

ANSWER: 
I recently answered a similar question.

QUESTION: 
Special Relativity Irregularity! A 100m train passes through a station at half the speed of light. Lightning strikes the front and back of the train simultaneously from the station's reference point. After solving the problem, I discovered that an observer in the station would see scorch marks 100m apart on the tracks and scorch marks 87m apart on the length contracted train. how is this difference possible when the observer supposedly saw two normal looking lightning bolts? According to the numbers, one lightning bolt must have been 13m in diameter!

ANSWER: 
I believe you misunderstood the problem. If the lightening simultaneously struck two points 100 m apart in the station's frame, they would not strike the ends of an 87 m long train. There are different scenarios of this well-known thought experiment; I will give one which illustrates the point that simultaneaty is not absolute. A 100 m long train is moving with a high velocity on its track. It sees a bolt of lightning at each end simultaneously; it sees scorch at 100 m apart on the train and on the tracks. The observer at the station sees two flashes of lightning but not simultaneously; the first strikes the the rear of the train and, later, the second strikes the front just as it reaches a point 100 m from the first strike location. Hence, since the two events are not simultaneous in his frame, there is no paradox in having the marks 100 m apart on the track and 87 m apart on the train. I, personally, find a variation of this thought experiment to be easier to grasp. A flash of light occurs at the center of the train which strikes the ends simultaneously as seen on the train; the observer at the station sees the rear moving up to meet the flash and the front running away from the approaching flash, hence the rear of the train is struck by the flash first.

QUESTION: 
Let's say if a vaccuum cleaner standing on the floor turns through a small angle when the switched is on and then stops. Why does that happen?

ANSWER: 
When the motor is speeding up there must be a torque exerted on it, exerted by the rest of the machine. So the rest of the machine experiences an equal and opposite torque which causes it to also have an angular acceleration, so it turns. When the motor reaches operating speed, torque is no longer necessary. Similar things happen when revving up the engine of a car or airplane.

QUESTION: 
How would you go about finding the change in magnitude of velocity in a system just containing the earth and sun, relative to the center of mass of the system, over a 6 month period? I found this in my book and am not sure about how to approach it or what it means by change in magnitude of velocity.

ANSWER: 
First of all, the center of the sun is, for all intents and purposes, the center of mass; so we are talking about the earth. Since the earth's orbit is approximately circular, the magnitude of the velocity is approximately constant, so the change in magnitude of velocity is about zero. Maybe you meant the magnitude of the change in velocity; this would be twice the earth's speed.

QUESTION: 
what is the speed of a bullet?

ANSWER: 
It depends on the gun. A rifle bullet can be anywhere from about 180 m/s to 1500 m/s; that's about 400-3300 mph. A passenger jet airplane goes around 500 mph, the shuttle goes about 18,000 mph.

QUESTION: 
I want to build a circular runway 2,000 ft in diameter for an aircraft that lands at 50 mph. How many degrees of inclination will be required to make the vertical acceleration forces perpendicular to the horizontal axis of the aircraft? Same question with a 1,000 and 3,000 ft diameter runway? Pls provide the formula.
ADDED:
As a retired Navy pilot landing on a circular runway would be a walk in the park - particularly because I'd never have to land in a crosswind and could not possibly run out of runway and wind up in the wait-a-minute bushes just off the end. If I got the inclination right landing forces would feel exactly as they would landing on a conventional runway (vertical acceleration forces perpendicular to the horizontal axis of the aircraft).

ANSWER: 
This is the classic banked curve problem and the object is to find the angle such that a vehicle can negotiate the curve at a certain speed even on an icy (frictionless) road. So the idea is to make the the horizontal force the road exerts on the vehicle be the required centripetal force (mv2/R) and the vertical force the road exerts on the vehicle be equal to the weight (mg); here m is the mass of the vehicle, g the acceleration due to gravity, v the speed, and R the radius of the curve. The result is that tanθ=v2/(gR). For the numerical calculations it is good to use SI units, g=9.8 m/s2, v=50 mph=22.4 m/s, R=1000 ft=305 m; I find θ=9.50. You can test your prowess by doing the calculations for R=500 ft and R=1500 ft. A word of warning is that if you go too fast or too slow without enough friction you could slide off the runway (up or down, respectively) although it seems pretty unlikely, even at rest, that the plane would slide down a 9.50 incline. One thing that worries me a little is the moment of touchdown—will you try to be turning before you hit? Otherwise there will be that jolt of acceleration making the sudden transition from straight line to curved flight. But, hey, Navy pilots can land on a rolling carrier, right?

QUESTION: 
Our lecturer was showing us a demonstration whereby two cylindrical pieces of metal were allowed to free-fall down a hollow vertical tube made of copper.One of the pieces was a magnet and it took significantly longer to fall through the tube than the other. Can you explain why the fall times are different ?

ANSWER: 
The one which is not a magnet just falls. The falling magnet causes there to be a time varying magnetic field through a section of the tube. This changing field causes an electric current to flow around the circumference of the tube (Faraday's law). This induced current itself has a magnetic field and that magnetic field exerts an upward force on the falling magnet.

QUESTION: 
When traveling in a car and you turn you are now accelerating. As long as your turning you are still accelerating. Is it possible to cancel out aceleration while in a turn by slowing down without coming to a full stop?

ANSWER: 
There are two kinds of acceleration, one which changes direction (like when traveling on a curved path with constant speed) and one which changes speed (speeding up or slowing down). The first kind is always perpendicular to the direction of travel, the second always parallel. You cannot alter one component of a vector by altering another component perpendicular to the first. The answer to your question is no.

QUESTION: 
There are 3 people in a pitch-black room, spaced evenly apart, say maybe 5-6 meters each. The one in the middle has a laser pointer, turns it on and shines it straight ahead of himself perfectly parallel to the ground. Now, if the fact that we can see that light and the object(s) it illuminates is because the light is being reflected, how is it that all three people can see the laser pointer? The reason I don't understand this is that if light is composed of photons, and those photons reflect off of the object, I would think that only the person that is perpendicular to the laser pointer would be able to see the light because the photons would be reflected straight back at him. Sort of like if those same three people were bouncing a ball off the wall, the ball should only travel in one direction.

ANSWER: 
Because the object is not perfectly flat, light is reflected in many directions. If the object were perfectly flat and perfectly reflective, only the first guy would see it. (Of course, there is no such thing.)

QUESTION: 
Are photons subatomic particles? And what type are they?

ANSWER: 
Photons are particles, the quanta of the electromagnetic field. You may think of light as either a wave or a particle (photon). I don't know that there is a definition of subatomic particle, but let's say that it is smaller than an atom; in that case, some photons are and some aren't. Photons have no charge and no mass.

QUESTION: 
My curiosity combined with my lack of understanding of physics brings me to this strange question, which i can't seem to find the answer to on google. Basically I want to know: what is the 'weight' of music? In other words, when I copy music onto an iPod does it become heavier on any minute scale? Is there any number value, however small, that can be put to the 'weight' of the information that exists on the iPod/mp3 player?

ANSWER: 
It takes energy to put a song into the memory of your iPod. Therefore, the mass of the memory will increase. However, this energy comes from the battery and so the mass of the battery would decrease by the same amount, keeping the mass of your ipod constant. That said, let's try to estimate the kind of mass changes we are talking about. I have no idea how much energy it takes to write a bit of data to memory, but it can't be much. Suppose that it takes 1 nJ=10-9 J of energy to write one song. Again, this is a guess but can't be more than a few orders of magnitude from the actual amount. The increase in mass would be about (10-9 J)/c2≈10-26 kg (E=mc2, you know). This is roughly the mass of an atom, really unmeasurably small. I don't think you have to worry about the extra weight you will be carrying in your backpack!

QUESTION: 
Because everything is in constant motion relative to some internal and external position, can everything be manipulated? Or better put, even though much of the universe appears immovable to human beings, is all of it actually moveable/changeable? For example, even though a table presents itself as permanently solid, is it concievable that a human hand could be able to go through a table in the event that its atoms/subatomic particles/etc. randomly rearrange themselves simply because of the impermanence that the movement of such atoms, etc. affords?

ANSWER: 
I have no idea what you are asking in the first part of your question. Anything with mass can be "manipulated" by a force. The second part (table) seems to have nothing to do with the first part; however, your hand will never go through the table, I guarantee it.

QUESTION: 
Why can't humans exhibit wave-like behaviour when walking through the "single slit" of a doorway given that the de Broglie wavelength calculated is 6.1*10^-34m ?

ANSWER: 
To see diffraction from a single slit, the wavelength must be comparable to the size of the slit. Notice that you do not see any diffraction of visible light through a door either.

QUESTION: 
How much anti-matter hydrogen would be required to destroy the earths atmosphere when relesed? I'm writing a book that I want to be realistic.

ANSWER: 
The mass of the earth's atmosphere is about 5x1018 kg, so you would need that much antimatter to destroy it.

QUESTION: 
My question concerns the speed of light. My weak understanding is that nothing can travel faster than the speed of light. Matter becomes more dense and you would need an infinate amount of energy as you approach the speed of light. I've thought about this question a lot. I had a thought on the way to work this morning. What if I was big. Really big. Lets say I was big enough to hold a galaxy in my hand. Let's also say I had no problem with tempeture or pressure and I didn't have to breath air. I'm still human mind you. I'm just really really big. Big enough to hold a galaxy in one hand and another galaxy in the other. Now, I decide to move the galaxies closer together or a little further apart. As I move the galaxies together and apart, I don't see any issues with speed of light. From my point of view, it only takes seconds to do this. The galaxies would be moving at a tremendous speed for my wife who if still on earth calculating how fast the galaxies are moving back and forth. My question is: How does size figure in our perception of the speed of light?

ANSWER: 
Nothing "figures in" to the perception of light speed. It is a universal constant and any measurement of it reaches the same result. With your long arms you could move them fast, but never as fast as the speed of light. And, if each was moving toward the other with speed greater than half the speed of light, they would still be moving relative to each other with a speed less than the speed of light (see earlier answer). Another issue with your experiment is that you cannot instantaneously start accelerating the galaxies in your hands since, once you decided to move them, the push which you initiated with your shoulders would not appear at the galaxies more quickly than the speed of light (millions of years given that galaxies are millions of light years apart); in fact, it would take much longer since the push would more likely travel at the speed of sound since your arm is a mechanical thing. Again, see an earlier answer.

QUESTION: 
After must time trying to learn about special reletivity I hit a brick wall. The faster you travel the more time would slow down, right? If you were to travel towards a light source and calculate the speed of light by your frame that light should be traveling faster. Similarly if you see a light source in the opposite direction and measure that speed you should see that light at the speed of light because at your slow-down frame (time-wise) it is. If this is not true then the inverse has to be true, and the light behind is slower your frame is sped up to make the oncoming light the approperiate speed, because nothing can travel faster than 186000m/s.

ANSWER: 
You make the mistake to apply classical ideas about velocity addition to this problem, and these ideas are incorrect at high velocities or if light is involved. No matter what light you look at and no matter what your motion, the speed of light is always the same. Also, see my earlier answer on correct velocity addition relativistically.

QUESTION: 
If my severely limited understanding is correct, then the amount of matter in the universe is constant... never does anything pop in or out of existence; just changes form. If this is the case, then what does it mean when it is said that when matter and anti-matter come into contact they annihilate each other?

ANSWER: 
Sorry, not correct. The amount of matter is not constant but the amount of energy is; mass is one form of energy. So when mass disappears, as in your example, energy must appear in some other form, in this case photons appear.

QUESTION: 
Why does it seem like almost everything in the universe in rotating or revolving? The earth rotates and revolves around the sun. The moon rotates and revolves around earth. The sun rotates and revolves around the milky way. Satelite galaxies rotate and revolve around larger galaxies etc. etc. And I know the theory is the universe is expanding but how do we know the universe is not on some grand rotation or revolution around other universes? Oh yea I forgot cause you are a physicist things of the very small rotate and revolve also (electrons around nuclei).

ANSWER: 
Imagine a huge cloud of gas and dust, the one from which our sun and solar system formed. In the beginning there are many parts of the system very far away from the center of mass of the cloud moving in random motion. Think how unlikely it is that there is not some net flow around the center of mass. In physics terms, the cloud has angular momentum. Even if the motion of the particles is very slow, they are very far away from the center so they could have a very large angular momentum. Now, as this cloud collapses, it must obey the law of conservation of angular momentum since there is nothing nearby to exert torques on this cloud. So, what were originally very small rotational velocities become quite large. Think of a dancer or figure skater spinning with arms out and then pulling them in. Similar arguments can be made about larger or smaller systems. Atoms are not such a good example since the orbital model for electrons is really a quite poor description of what is happening; a hydrogen atom, for example, has no orbital angular momentum in its ground state. Elementary particles (like electrons) do have intrinsic angular momentum called spin.

QUESTION: 
I was watching a show on the Science network about super-massive black holes and was confused because they kept saying that they were proving the existence of super-massive black holes by the speed at which 'objects' rotate around the 'hole' before the event horizon (the bigger the black hole, the faster they spin - i think they were looking to obtain that equation, thereby linking a pattern between black holes based on mass and speed of rotation of the 'objects' rotating the black hole). This got me thinking about how they can measure speed in space. If v = d/t and time is relative (i.e. relativity theory), how can their measurements be accurate or at least not grossly biased based on our position in space/time? It was my understanding that they were looking at time lapsed photos and basically watching the 'objects' rotate, if that helps. I have been trying to research this and haven't found anything solid yet as an answer.

ANSWER: 
If the sun were a massive black hole, the force on the earth would be much larger and so it would be necessary for it to have a much larger velocity to move in the nearly circular orbit it has now. The distance to the center of the galaxy where a supermassive black hole is believed to be is pretty well known, so, by geometry, if an object at that distance moves we can pretty accurately deduce the distance it moves.

QUESTION: 
Why does AM radio reception go haywire at sunset and through the night?

ANSWER: 
Actually, AM transmission becomes better at night. The reason is that at night the radio waves can be reflected from the earth's ionosphere, something which is blocked during daylight hours because of the effects the sun has on the ionosphere. So, what happens is that distant stations with frequencies close to your favorite local station start to arrive at your antenna. Also, because of this, primarily local stations are required to turn down their power at night to keep from causing problems far away.

QUESTION: 
when were the atoms that comprise my body made?

ANSWER: 
Hydrogen atoms may have come from the origin of the universe, about 14 billion years ago. Heavier atoms, up to around iron, likely came from stars which have burnt out. Heavier yet atoms are believed to have originated in supernovas, exploding stars. Of course, there is no way I can put a precise time, other than older than the age of our sun and solar system, when those events happened.

QUESTION: 
regarding microwave ovens it is desired to calculate the amount (key word amount) of microwaves that are produced per second during the operation of a microwave appliance. how may this be done? regarding "key word amount", it is not know in what units this "amount" or "quantity" of waves may be measured in. basically it is desired to know the quantity of microwaves produced by a magnetron due to a given power input. perhaps the quantity can be expressed in terms of energy, as energy would be dependent on the quantity. what constitutes "one" electromagnetic wave. one particle? one photon? so perhaps the quantity could be measured in terms of number of photons?

ANSWER: 
The power rating tells the amount of energy per second consumed. So, assuming all the power goes into the microwaves, a 1000 watt oven will produce 1000 joules of energy per second. A common frequency used is f=2.5 GHz=2.5x109 s-1, and the energy of a photon is hf where h=6.6×10-34 m2 kg /s, so the typical energy per photon is 1.7x10-24 J. Therefore, there are on the order of 6x1026 photons of microwave energy per second if 1000 watts is the power rating.

QUESTION: 
I under stand the very nature of special relativity and am aware of the usual time dilation and length contraction experiments. I.e the moving spaceship and light beam theroretical experiment etc. A moving spaceship sees its light moving away at the speed of light because clocks in that frame of reference are slowed and length of the spaceship and all its rulers are contracted. However if a ship is moving in the opposite direction to the beam of light being fired, time must still be slowed as an observer (because it is moving) and the length contraction must remain, so in that case why doesnt the speed of light apear to be travelling faster then the speed of light (even when this is impossible)!?

ANSWER: 
You make the mistake of trying to understand the constancy of the speed of light mechanically. Time dilation and length contraction are consequences of the fact that the speed of light is a universal constant, so it is circular reasoning to understand the fundamental by using the consequences. Rather, appreciate that the constancy of the speed of light is a postulate of the theory which is fully verified experimentally. Actually, as I have written before, this postulate is unnecessary if you accept the principle of relativity which states that the laws of physics must be the same in all inertial frames of reference, so in that sense it is straightforward to understand the constancy of the speed of light.

QUESTION: 
Why rotation or spin gives stability to objects, like a spinning top or a spinning football?

ANSWER: 
A spinning object has angular momentum. It is a vector which points along the axis about which the object is spinning. It is one of the laws of physics that angular momentum stays the same unless there is a torque applied to it, and so if you keep the object isolated from external forces, the spin will always point in the same direction.

QUESTION: 
how is superconductivity used to produce strong magnetic fields?

ANSWER: 
What determines the strength of the magnetic fields is the magnitude of the electric current. It is possible to have much larger currents in superconductors.

QUESTION: 
Since a gravitational force slows time, what happens to time when it is in the mid-point between two equal very massive objects (when the net gravitational force is 0)?

ANSWER: 
If the clock is at rest and the gravitational field is zero, it runs at the same rate as if it were in empty space.

QUESTION: 
The atoms that constitute your body are mostly empty space, and structure such as the chair you're sitting are composed of atoms that are also mostly empty space. so why don't you fall through the chair?

ANSWER: 
See the FAQ page.

QUESTION: 
According to the theory and the tests that have been made, time appears to pass slower the greater you accelerate. Time is measured with a clock, and anything accelerating will experience g-forces. By that rationale a mechanical clock should tick more slowly due to the g-forces acting upon its gears. A digital clock should operate more slowly because of the g-forces acting on the electrons flowing through its systems. How do we know that the time dilation we observe is not a direct result of the g-forces involved affecting our measuring devices? (An experiment where a cesium clock was flown from the US to Britain on a Concord to test the theory comes to mind.)

ANSWER: 
Forget acceleration. A clock moving with a constant velocity, which experiences no "g-force", runs more slowly than the clock it is moving past. This is a fact of nature and has nothing to do with the actual design of the clock. You should read my earlier answers, and particularly the description of the light clock, to get a better feeling for this. If you are accelerating or in a gravitational field, time is also affected; these are effects in general relativity whereas the standard time dilation of moving clocks is special relativity. Experiments with atomic clocks take all effects into account. But the important thing is that time is not a universal constant, it depends on the motion of the clock, and this has nothing to do with clock design; if a clock does what you describe, it is an unacceptable clock because it is not measuring time.

FOLLOWUP QUESTION: 
Per your suggestion I took a look at the "Light Clock" applet, and either I am mentally incapable of grasping this concept, or it seemed to support my thoughts on Time Dilation being a result of instrumentation limitations. In that applet the hypothetical clock is ticking slower because light is taking longer to bounce between mirrors. At least from what the applet suggests it is taking longer because light is having to travel a longer distance to reflect on trips between mirrors due to their motion and the diagonal path the light must travel. (Perhaps I am missing something because the applet does not demonstrate the 'stationary' clock being time-dilated relative to the one in motion)

I am also aware of experiments in particle accelerators concerning the lifetime of particles, which also bothers me given that there is the old law that one cannot observe without having an effect on what they are observing, and slowing or accelerating these particles removes them from their natural state.

ANSWER: 
I will address your second question first: we do not do anything by observing a decaying particle which causes it to decay, we just sit and wait for it to decay. If there is a bomb with a 5 second fuse we might observe it to blow up after 8 seconds if it is speeding past; we did not do anything to cause the bomb to explode. And, you miss the whole point of relativity by saying that by moving with a large speed they are not in their "natural state". The principle of relativity states that the laws of physics are the same in all inertial frames of reference; there is nothing more natural about being at rest than moving with a speed of 99% the speed of light.

Regarding the light clock, the point is that the speed of light is the same in either moving or stationary frame. If this were instead a "sound clock", it would not be a good clock because, if there were a wind perpendicular to the distance to the reflector, the clock would run slow compared to other clocks you might have. (Think about it—it is like swimming across a flowing river which takes longer than across a still river.) But, with a light clock, there is nothing you could do in the rest frame of the clock which would cause it to not be synchronized with all other clocks in your frame because there is no medium through which light travels and so it cannot "flow". So, you have to admit, it seems to me, that if you are on the passing spaceship, all your clocks will run at the (normal) rate. But, the guy you are speeding past will see all clocks running at the same rate but slow because the light clock is demonstrably slow. Once you can accept the fact that the speed of light is the same for all observers, the rest of relativity just falls into place; read my earlier answer.

Again I need to emphasize that you should forget acceleration. You only need constant velocities to completely understand special relativity.


QUESTION: 
in a simple circuit (just a battery and bulb) electrons transfer their kinetic energy to the bulb by colliding with atoms in the filament, producing heat and light? [not sure how correct that is for a start]. If so, then wouldn't the electrons lose kinetic energy after the bulb causing the current after the bulb to be different than before the bulb? I know this is wrong as current is the same everywhere in a series circuit, but I am unsure where I am wrong,

ANSWER: 
First, you are correct that the wire gets its energy by electrons colliding with the atoms in the wire. But the electrons are constantly colliding and then get going again because of the voltage across the ends of the filament which means that the electrons are always seeing a force so they speed back up as soon as they stop. See an earlier answer for more detail.

QUESTION: 
An electron is at rest in my reference frame. I see a proton move by the electron at rest. There is an electrical component force (opposites attract), but will there be a magnet force since there is initially only one current?

ANSWER: 
There will be a magnetic field because you have a moving charge. However, there will be no magnetic force because the electron is at rest. If the electron is free to move in response to the electric field it experiences, it will then experience a magnetic force as well.

QUESTION: 
My 100 gram pepper drops at 9.8m per sec squared on Earth. According to my textbook we get this because The formula for the force of gravity is big G times big M times little m divided by distance squared I'm assuming that I can pick either the Earth of my little 100 gram pepper to be the big M or the little m. If I do that then the little g for the pepper is 1.64 x 10^-25. But isn't the Earth accelerating towards the pepper at the same rate that the pepper was accelerating towards the Earth when g = 9.8 m/sec squared?

ANSWER: 
The accelerations of the earth and pepper are, most certainly, not the same. What is the same is the magnitude of the force each exerts on the other (Newton's third law). Because the pepper has a small mass, it experiences a large acceleration; because the earth has a huge mass, it experiences a tiny acceleration (the "little g" you erroneously calculated for the pepper). GMm/R2 (Newton's universal law of gravitation) gives the force, but the acceleration is Force over mass (Newton's second law). So g for the pepper is GM/R2 and g for the earth is Gm/r2.

QUESTION: 
I'm reading "Relativity" by Albert Einstein and in it he says that simulteneity is relative and uses the example of two lightning strikes at points "A" and "B" and an observer at the mid point between the two "M". Einstein states that an observer on a train moving from "A" to "B" observing the lightning strikes just as the observer reaches "M" will see that strike "B" occurs before "A" (as the train it moving toward the light from "B" and away from the light coming from "A"). Does this mean that an event only becomes real when the light from it reaches the observer? Or could one legitimately says "These two events happened simultaneously, but I saw the flash from "B" before I saw the flash from "A"?

ANSWER: 
The relativity of simultaneaty is not "how it looks", it is how it is. In relativity you have to be careful how you define a time interval so that you correct for such things as the time it takes light to reach you. The converse is also true, a distant star is seen right now but as it was long ago. I always find the following example more convincing than the lightning at the ends of the train example. In the center of one of the cars there is a flashbulb which flashes. Light reaches the front and rear walls of the train at the same times. However, an observer on the side of the track watching the car go by sees the rear wall come forward to meet the oncoming light and the front wall running away from the light coming at it. Hence the event corresponding to the flash at the back wall occurs earlier for the observer at rest. (It is crucial to realize that both observers see all light moving with the same speed.)

QUESTION: 
i was trying to explain the quantum double slit experiment to my friend and he wasn't buying it so i wanted to tell him about quantum entanglement to help him better understand the "spooky action at a distance" (as Einstein called it). Seeing as i am 14 and have only the most basic of the basic understandings of physics i was flabbergasted when, while telling him about the experiment where 1 electron is taken to Chicago and another is taken to saaaay THE ANDROMEDA GALAXY but are both in a quantum state (spinning up and down at the same time i think) and are kept in a box (so we cant see them). If when you look at the one in Chicago, it spins down the one in the Andromeda galaxy spins the opposite of the one you look at WITHOUT EVEN LOOKING AT that one. After trying to explain that to him he asked "how do you know its spinning up and down at the same time when the second you look at it, its spinning up OR down and furthermore I cannot see an electron so how can we even detect whether it is spinning up or down in the first place". I tried and tried to figure it out in my head but to no avail so i ask, can you you tell me what piece of equipment, while doing the quantum double slit experiment, measures and therefore causes the spooky result and also solve my query about how the physicists know that an electron is spinning up and down at the same time when the second they measure (WITH WHAT) it, it chooses to change to only one.

ANSWER: 
What makes your question difficult to answer is that the quantum double slit experiment is not an example of entanglement, rather an example of wave-particle duality; so you are trying to convince your friend of something by using an inappropriate example. I will briefly describe both:
  • In the double slit experiment, the intensity of the light is turned down so low that only one photon at a time passes through the slits. Lo and behold, a double slit pattern appears on the screen and it appears one photon at a time. In other words, the photon somehow passed through both slits, not one as you would expect if the photon were a particle. But it is still a single photon when you detect it, so how could it have gone through both slits? Only if it were a wave when you needed to be. You get what you look for. The pičce de résistance is that if you place a detector at one of the slits to try to determine through which slit the photon passed, you destroy the pattern on the screen!
  • In an entanglement experiment you create two electrons whose total spin is zero. However, it is a mixed quantum system; so it is not simply one electron has spin up and the other spin down, rather each has spin half up and half down. Now, if you take one of them and measure its spin, the effect is that you "put" this electron into the state you measure, say up; but, since the pair is a single system, the total spin still has to be zero and so if you put one into the up state, you are simultaneously putting the other into the down state. It will be true even, as you specify, one is 2.5 million light years away. You could have some alien measure the spin there and send you his results but you wouldn't find out for 2.5 million years! This experiment has actually been done here on earth where you can actually find out the results before you die!

QUESTION: 
I am having trouble with calculating how fast a stationary hydrogen atom would move after absorbing a photon. Logic tells me that if a hydrogen atom absorbed a photon that had exactly the same amount of energy as it has, i.e. 9.3899x10^8eV, it ought to move at half light speed. As a photon does not have mass its momentum cannot be calculated from momentum, p, = mv. I tried substituting for m from the equation E = mc^2 but this gave me an answer much higher than light speed. I know that momentum is supposed to change with speed in accordance with the equation E^2 = m^2c^4 = p^2c^2 but I do not understand why or how to use this equation. Also, the units of energy change when m is replaced by E/c^2 and physicists get around this by choosing a unit of length so that the speed of light is 1. This seems to be a bit of of a fiddle to me. So could you please show me how you would calculate the speed of a hydrogen atom after absorbing a photon of equal energy.

ANSWER: 
The momentum of a photon is E/c. Take the rest mass of the atom to be M and the momentum of the recoiling excited atom P; the correct equation for P is P=Mv/√[1-(v2/c2)]. Then, energy conservation gives E+Mc2=√[(M*c2)2+(Pc)2] and momentum conservation gives E/c=P. Here, M* is the rest mass of the excited atom. You can expect that v<<c and so P≈Mv=E/c. That is the outline of what you need; if you want to be totally exact, just solve the two equations for the two unknowns (v and M*). I have assumed that the photon has a much smaller energy than the rest mass energy of the atom, whereas you have assumed otherwise, but a hydrogen atom is not capable of absorbing half its own rest energy because anything more than 13.6 eV will ionize the atom. You could go ahead, though, with the fiction that the atom did absorb a huge amount of energy and solve for the (physically unreasonable) velocity using the two conservation equations I give you above.

FOLLOWUP QUESTION: 
I have been led to believe that the absorption of photons that excite an electron are those photons with very specific energies given by the equation 1/wavelength = R(1/n^2 - 1/m^2). These are usually photons below x-ray energies and the highest energy photon absorbs first to excites the electron and then a less energetic photon excites it further and so on until the electron is expelled. What happens to photons of other energies. Are you saying that these are not absorbed or does the atom just absorb the right amount of energy from one that has more than it needs? If other photons are not absorbed then where do atoms obtain their kinetic energy from? And what happens to gamma rays when they pass into a substance. Although these can pass through some distance they must eventually be absorbed. If they just ionise the atoms is this the way that atoms accumulate kinetic energy by expelling electrons to make them move fast and then reabsorbing them to make the whole atom move faster.

ANSWER: 
For energies less than the binding energy, the photon will be absorbed only if it has just the right energy to excite the atom to one of its allowed states. Otherwise, the photon will scatter from the whole atom which will rebound in such a way to conserve energy and momentum but, for such low photon energies, it will look, for all intents and purposes, that the photon stays about the same except changing direction (like it had bounced off a mirror). (Incidentally, the energy of the incident photon must be very slightly larger than the energy of the excited state for absorption to occur because some of the final energy will be in the kinetic energy of the recoiling atom.) If the energy is larger than the binding energy, the photon will most likely scatter from the electron which will be ejected from the atom and you end up with three things, the photon, electron, and proton; the proton, being as massive as it is, can be ignored to an excellent approximation and the energy and momentum are shared between the photon and electron. This is called Compton scattering. It is also possible that the photon will scatter from the proton and the result is again that, approximately, the photon just changes direction (again like a mirror). The electron and proton will not get back together—the electron is moving so fast that it leaves the scene.

QUESTION: 
I have lots of questions regarding the nature of space-time...and especially gravity. I have yet to hear a satisfactory explanation of why mass causes a curvature (or is a curvature) in space-time and why that curvature results in an attractive force to all other objects. (the bowling ball example is flawed since the animation always shows the rolling ball pulled towered the depression that represents the curvature. So it seems to assume a gravitational force pulling the object toward the depression.

ANSWER: 
General relativity is a very mathematical theory, so it is almost inevitable that explanations are going to be qualitative and not really rigorous. The bowling ball on the trampoline, for example, is just a qualitative example of how bending the space around something will make it look like other objects are attracted to it; also, it is meant to give an example of how mass can alter the space around it. I think that a more persuasive argument to convince yourself that space is warped by gravity is the equivalence principle which states that there is no experiment that you can do which can tell you whether you are in a gravitational field with an associated acceleration g or whether you are in empty space accelerating with acceleration g. This is the cornerstone of general relativity. So the experiment you think of is being in an accelerating spaceship and a beam of light, perpendicular to your direction of travel, enters from the side; you will then see the beam of light bend as it crosses the spaceship. Hence that beam of light will bend if passing by a massive object like a star. You should read my earlier answers on general relativity too.

QUESTION: 
It is understood that objects give off infrared radiation and can be imaged in this part of the electromagnetic spectrum. Further, photons are ejected by atoms as electrons move between energy states. So, I wonder if you had a single atom...in a 'dark, cold' space, would you be able to sense that the atom is there by viewing the infrared heat emissions? That's the basic question. But, beyond that... I can't find a good answer to, "Would the atom continue to emit photons forever? Or, would the atom eventually experience a sort of 'heat death'? (Where does it get the photon to begin with?)

ANSWER: 
The mechanism by which objects emit infrared radiation is not the same mechanism by which atoms eject photons. An isolated atom will not radiate if in its ground state and, if it is excited, it will emit photons until it reaches its ground state and stop. It obviously cannot radiate forever because each photon carries energy and the atom does not have any source of energy once in its ground state. A macroscopic object radiates because of the thermal motion of all the atoms. This radiation differs from the deexcitation of atoms in that the spectrum is continuous and contains all wavelengths; individual atoms emit photons only of certain specific energies. The continuous radiation spectrum peaks at some wavelength and, for objects not too hot, is mainly in the infrared region. The object will continue to radiate until it comes to equilibrium with its environment at which point the rate of radiation will equal the rate of absorption of radiation from its environment.

QUESTION: 
I am constructing a machine/quasi-invention which involves a fairly large tank of compressed gas (air with high O2 concentration. I am trying to get a handle on how the internal surface area of the tank and the final PSI of the gas inside the tank will affect the stress on the various areas of the tank. Assume for now that it is a 7 ft. tall, 3 ft. diameter cylinder with compressed air at 2.0 atmospheres. Would I the tank's number of square inches of the tanks surface area (about 12,000), and then multiply that number by the extra 14.7 pounds per square inch I add to get up to the second atmosphere and conclude that every inch of the tank is under 180,000 lbs of inner pressure?

ANSWER: 
You are confusing pressure and force. The pressure is the force per unit area (like PSI, pounds per square inch) and that is what is important for a tank. The total force on the inside of the tank is not important, how it is spread around is. In your situation, the total pressure over the surface of the tank is 1 atm (2 atm pushing out, 1 atm pushing in); this is normally called the guage pressure. The pressure inside is 2 atm.

QUESTION: 
I need to spin a low-friction disk without making a direct physical connection. I am using 2 neodymium magnetic cylinders, magnetized across their diameters. One is mounted in a wood dowel, chucked in a drill press. The other is pressed into a teflon bearing at the center of the disk. If the driving magnet is turned very slowly (maybe 10 or 20 rpm) the rotation is coupled as expected, and the disk turns. If the drill press rotates at 250 rpm, the disk simply vibrates, and does not rotate at all. I expected the rotating magnetic field to cause the disk to accelerate, even if slowly. It does not. Why?

ANSWER: 
I suspect that the problem is the fact that the disk has inertia and will not start spinning instantly when it experiences a torque from the rotating magnet. So the magnet rotates 1800 and now the other pole tries to rotate it the other direction, and so forth. What you would need to do is to start it very slow and gradually increase up the speed you need.

QUESTION: 
I am running a Traveller gaming campaign (space sci-fi). I need a formula to quickly calclate transit time from point a to b within a star system. A space craft has to go from one point in space to another. Velocity to begin is zero they then accelerate at a constant 1g to arrive in orbit, assume ending velocity is also zero for simplicity. Assume at the half way point they begin decelerating also at a constant 1g. Assume 500,000 km is to be covered or at least a specific distance to be choosen. What is the time needed to complete this manuever?

ANSWER: 
It is not technically possible to have a constant acceleration forever since eventually one approaches the speed of light which is as fast as you can go. If you are interested in these effects of relativity, see earlier answers. Let us assume that, for your conditions, relativity is not important. That is, I will use the classical kinematic equations for constant acceleration, x=˝at2 and v=at (assuming the craft starts at rest at the origin). If my calculations result in a speed v which is not very small compared to the speed of light, then I will need to redo everything using relativity. So, the time t it takes to go halfway is found using 2.5x108 m=˝(9.8 m/s2)t2; solving, I find t=7.1x103 s. This is about 2 hours. The speed would be v=9.8x7.1x103=7x104 m/s (about 160,000 mph). This is much less than the speed of light (3x108 m/s), so the answer to your question is that it takes about 14,200 s, about 4 hours, to make the 500,000 km (about 1.3 light seconds) trip.

QUESTION: 
Circular motion requires constant acceleration, I understand. Change in velocity in a straight line requires acceleration. So what is going on when there is a change in circular motion velocity?

ANSWER: 
Your first statement is technically wrong because the acceleration of uniform circular motion is not constant although its magitude is; the acceleration, just like the velocity, is constantly changing direction which always points toward the center. If an object moves in a straight line with changing speed, it does have constant acceleration, and it has a direction along (if speeding up) or opposite (if slowing down) the velocity. If an object moves on a curved path (and therefore changing its direction) and is also changing its speed, its acceleration has two components, one parallel to the velocity and determined by the rate of change of speed (called the tangential component), and the other perpendicular to the velocity and determined by the rate of change of direction of velocity (called the radial component). If the curved path is a circle (your question), the radial component is v2/R where v is the speed at a particular time.

QUESTION: 
please tell me about the physical interpretation of gradient divergence and curl

QUESTION: 
how does gradient, divergence and curl matter in physics?

ANSWER: 
I guess you guys had the same assignment? These quantities play the role of the derivative in three dimensions. To answer your questions would require volumes. The derivative, you should know, is very important in elementary physics (it was invented by Newton to be able to sensibly talk about velocity and acceleration), so the importance in three dimensional physics should be pretty self evident. The gradient is a derivative-type operation which generates a vector function from a scalar function. The divergence converts a vector function to a scalar function. The curl converts a vector function to another vector function.

QUESTION: 
Okay, so nothing can travel faster then the speed of light, in theory, so what if you have a rope in space with two guys holding it, and the tension is at the max and the length, lets say, 5 light years. if one guy at the other end of the rope tugs it (remember its at maximum tension) would the guy 5 light years away from the rope feel it? would he feel it right away? if not when will he feel it, if he can feel it at all?

ANSWER: 
This question has been answered before (that question was a pushed rod instead of a pulled rope, but the answer is the same). It is also so frequently asked that you could have found it on the FAQ page.

QUESTION: 
If light in a vacuum travels 300 million meters per second and it enters a pane of glass and slows to 200 million meters per second, then exits the glass into the vacuum again and speeds up to 300 million meters per second, what gives it the impetus to increase its speed? Where does the photon get the energy to accelerate?

ANSWER: 
You are thinking of it classically where we accelerate something and it takes a force because the object has mass. However, even classically, if something has no mass it can change speed without there being any force on it; think about it—if you exert a force on a massless object, it would experience infinite acceleration. In the case of light, the light changes wavelength as it enters the medium but the frequency stays the same. Since the energy of a photon is proportional to the frequency, the energy inside is identical to the energy outside.

QUESTION: 
I have a quick question in regards to aerodynamics. I understand that Bernoulli's law demonstrates that velocity and pressure have an inverse relationship, and the air flowing on top of an airplane's wing is moving faster than the air flowing on the bottom, which means there is more pressure on the bottom, therefore creating lift. The only thing that I don't understand is why the air on top is arriving at the back of the wing at the same time as the air on the bottom. If the air on top has a longer distance to travel, why doesn't it just travel at normal speed and reach the back of the wing later than the air on the bottom?

ANSWER: 
Maybe it would be easier to see thinking about the problem from the frame of reference of the air which is at rest. The wing moves through the air and pushes the air up above the wing and then it moves back down when the wing has passed. Viewed from the airplane, the air passing over the top takes the same time as the air passing under the bottom, hence moves faster.

QUESTION: 
Whilst checking out Kepler's laws on paper (as I often need to prove these things to myself) and attempting as an aside to apply them to the Bohr mode of the atom, I noticed a somewhat glaring problem with the latter. In the gravitational force balance the centripetal force (or whatever you want to call it) on the orbiting body is easily worked out as mv^2/(2r), equal to the gravitational force attracting the two bodies and Kepler & Newton are vindicated. However, applying the same equation to the electron/proton using the coulomb force, the centripetal term on the electron comes out about half what it should be. In other words, the centripetal force is taken to be mv^2/r, which is completely wrong as far as the maths is concerned. Also the Lorentzian energy of a relatavistic particle as (gamma)mv^2 is missing a factor of half. I cannot reasonably entertain the belief that physics has missed this completely for a hundred years, nor can I see how it is mathematically possible to just leave the half out for the sake of making it all add up. Where did it go and why?!

ANSWER: 
I have no idea what you are talking about. Whether in the Kepler problem or the Bohr atom, centripetal force is mv2/r. This is set equal to GMm/r2 for the Kepler problem or ke2/r2 for the Bohr atom. And, relativistic energy is γmc2, not γmv2 as you have it, and there is no factor of ˝ missing. It is pretty easy to show that, for v<<c, γmc2mc2mv2. 

QUESTION: 
I understand that mass and weight are different, and that weight can change in your relation to Earth's gravity. Since we mass items on Earth, like in my science classroom, isn't the difference between weight and mass really more philosophical? As I place on item on the pan of a triple-beam balance, does gravity have an affect on the measurement, or does the nature of the instrument (a balance vs. a scale) render gravity inconsequential? Is there a demonstration I can set up, perhaps using a TBB and a spring or even bathroom scale, to show the difference? The terms weight and mass, as well as their units, get used so interchangably that it's been a very confusing concept for my students.

ANSWER: 
Mass and weight are completely different things and the confusion has its roots in an amazing accident of nature. Contrary to your statement, you do not "mass" something when you use a balance, you weigh it because what you are doing is measuring the force which the earth exerts on it, called the weight. Only because the weight is proportional to the gravitational mass are you able to infer the mass, that is W=mGg. But, what is the first thing you teach your students about mass? If it doesn't involve the concept of inertia, then I suggest you rethink how you teach them about mass. What we usually mean when we talk about mass is that it is a quantity that measures how resistant something is to being accelerated when we push on it. This is what we call inertial mass and it is the mass of Newton's second law, F=mIa. One of these masses measures how strongly gravity affects something and the other measures how much resistance it has to having its speed changed. What on earth do these two concepts have to do with each other? That is the amazing accident of nature—they are exactly the same, mI=mG. Of course, it is not really an accident and its existence is what provided the opening for Einstein's theory of general relativity, the most modern and complete theory of gravity.

I have given a really long-winded answer because it is a subtle and important point. I have not really answered your questions. Neither your balance nor your scale would work without gravity, they both measure (indirectly, because you infer mass from weight) mass mG. You cannot measure mG without gravity because that is what it is all about. What might be fun is to devise an experiment where you measure the acceleration of an object (horizontally so that you get gravity out of the picture) by a known force; then you would be directly measuring mI and could compare it to mG you get from a balance.


QUESTION: 
Suppose I had an object at rest upon a frictionless table and gave it a push to make it move at vm/s. Let us say that according to the equation KE = 1/2mv^2 the object has acquired 10joules of kinetic energy. Now I give it a second push exactly the same as before and it accelerates to 2vm/s. According to the equation it now has 100joules of KE. Why is it that two identical forces can give 10 joules in the first occurrence and 90 joules in the second?

ANSWER: 
First, there is an error in your premise. If the object with speed v has kinetic energy of 10 J, then if it has a speed 2v its energy will be 40 J. Your question, though, is still in tact—why does the same force give more energy. The simple reason is that you cannot tell from what the force is what energy it will impart. And, I think you could have easily figured that out if you had thought a little more about it. If you push something for an inch with a 50 N force will you impart the same energy as if you pushed it a mile? As I have said many times before, there is a tremendous misconception that if you know the speed of something you can determine what force caused that speed. You either must think of a force and the time it acts or a force and the distance over which it acts.

FOLLOWUP QUESTION: 
With regard to my earlier question concerning pushing an object upon a frictionless table, you said that, I, like many others, have a serious misconception about force and speed. You must have misunderstood my question because I stated that if you gave an object a push with a certain force and for a certain time and found it moved that object at vm.per s. and then gave it another push with exactly the same force and for exactly the same time so that it now moved a 2m.per s., why does the first push impart 1 unit of kinetic energy to the object and the second push 4 units of kinetic energy? If you say that the second push is not the same as the first then what about an object subjected to a constant force such as gravity? After 1 s the speed has reached 10m.per s., 2s it reaches 20m.per s. etc. but the kinetic energy increases as the square of the speed. Where does the kinetic energy come from and if you say from potential energy, where does this come from, how and where is it stored in an object , how does gravity convert potential energy into kinetic energy and how does it do so at an ever increasing rate? After 100 s the kinetic energy being added is an astonishing 10,000 units per second.

ANSWER: 
You did not say that the second push was for the same time, in fact you did not say anything about time. So, let's start all over again. You are correct, the speed will be v after the first push and 2v after the second push. Let us askwhy? The force times the time it acts is called impulse and impulse changes linear momentum, m(v2-v1)=Ft (true only for constant force in a straight line). But, just because the momentum changes by a certain amount does not mean the energy changes by the same amount because impulse is not what changes energy. Work changes energy and work is force times distance (again, for a constant force in a straight line). Now, think about your situation: the first push does a certain amount of work depending on how far you push during the (constant) time. The second push, however, finds the object moving, on average, much faster than the first push so it acts over a larger distance and hence does more work in the same time. That is why the second push imparts more kinetic energy than the first. An object freely falling is having work done on it by its own weight, that is, the earth is doing work on it. The amount of work the earth does is mgs where s is the distance it falls. It falls a short distance the first second, a larger distance the second second, and so forth such that as each second ticks by the rate of energy gain gets larger and larger. You do not need to introduce potential energy at all, it is just a clever bookkeeping device to keep track of the work the earth does. (One minor correction to your question: the second push imparts 3 units of energy if the first imparts 1 unit since energy went from 1 unit to 4 units.)

QUESTION: 
The static friction and the kinetic friction do not depend on the area of contact as long as the normal reaction R is the same.why?

ANSWER: 
First, it is important to realize that friction f being proportional to normal force N only is not a law of nature, it is an approximate statement of experimental measurements. I think it is not too hard to realize why it might be true. Suppose that you have a 10 lb box on the floor and it experiences a kinetic friction when sliding of 20 lb; now, if you have a second identical box, it will also experience a 20 lb frictional force. Attach the two together and you get a total force of  40 lb and so we still have f proportional to N even though the area has doubled. However, there are definite exceptions, notably wide tires which reputedly give more traction. To read a discussion of that situation, see my earlier answer.

QUESTION: 
Why is it difficult to carry a load with the hands stretched out in front than carrying the same load with the hands lowered at the sides?

ANSWER: 
Because of the design of your body. Look at the simple model of an arm to the right. The weight hangs from the outstreched arm and the arm is held horizontal by the biceps modeled here as a string from shoulder to elbow. The tension in the muscle must be very large to hold up the weight, much bigger than the weight itself. If the arm were straight down, the tension in the muscles holding it up would be equal to the weight.

QUESTION: 
If a circuit has 12V and a bulb is attached it will glow twice as bright as the same bulb with 6V. From ohms law it says that for the 12V circuit twice as much current will flow. My question. Does the voltage make the electrons have more kinetic energy? and is this kinetic energy then transferred to the light bulb? If so does this mean in both circuits there are the same number of electrons but in the 12V circuit they are just faster?

ANSWER: 
I will assume that the resistance of the bulb stays constant. (It actually does not because as it gets hotter its resistance changes; we'll ignore that.) Your very first statement is false. If you double the voltage you will double the current. Since the power (to which brightness is proportional) is the product of voltage times current, the brightness will increase 4 fold. The number of electrons participating in current flow is mainly independent of the amount of current flowing, so, yes, more current means faster average electron velocity.

FOLLOWUP QUESTION: 
You gave me an answer the other day about current and voltage which I really liked, and you showed me that the brightness of a bulb would increase 4 times (not 2) if voltage was doubled, thanks very much. I am working towards my theory papers for a 1st yr electrician and I just don't get how the energy from voltage is supplied to a load. Like, is it due to the kinetic energy of the electrons? Do the electrons always have the same charge (intrinsic electrical energy?) but are just moving quicker then release their energy in a collision with the load/resistance atoms, make heat/other energy? or do the electrons 'carry' extra charge? my teacher always talks about electrons as being like dump trucks. They have a little motor (the fundamental charge) then load up with extra energy and drop it off?

ANSWER: 
A conducting material has approximately one electron per atom which is essentially free to move around. Even if there is no voltage, they are zipping around inside the material randomly; the first approximation of a model of a conductor is to just treat the electrons like a gas and it works pretty well. Now, when a voltage is applied, there is an electric field set up inside the material and the result is that each electron experiences a constant force from the field. Now, if the electrons were truly free, they would accelerate from the negative to the positive terminals. But they are, in fact, in a material and they just get going and they hit an atom. This collision essentially stops the electron and causes the atom to bounce back a little, in other words the electron's acquired kinetic energy is transferred to the atom. But, giving atoms in the material more energy means increasing the temperature of the material. That is how the load gets the energy from the current. Since you are studying to be an electrician, I should perhaps mention here what I hinted at last time—the light bulb changes its resistance as it heats up, that is it is not truly an ohmic device. If you double the voltage, the current will not double because the resistance does not stay the same as the bulb gets hotter but increases.


QUESTION: 
What is the effect of distance on the brightness of a light bulb?

ANSWER: 
The intensity falls off like the square of the distance. For example, go 10 times farther away and the intensity is 1/100 as strong. (This assumes that the bulb looks like a point source which it will if you are far away compared to the size of the filament.)

QUESTION: 
Recently newspaper reported that at the Brookhaven National Laboratory, New York, the atom smasher was used to smash gold ions and the resulting quark-gluon plasma reached a temperature of four trillion degrees Celsius. How is such extra-ordinary high temperature measured?

ANSWER: 
I do not know in detail but am sure it is something like estimating the average kinetic energy per particle which is essentially what temperature measures.

QUESTION: 
when a car is driving at a steady speed, is it "accelerating"?

ANSWER: 
Acceleration is a vector concept (rate of change of velocity), speed is a scalar concept (magnitude of velocity). A car traveling in a circle with constant speed is accelerating because the (direction of) the velocity is changing. A car traveling in a straight line at constant speed is not accelerating.

ADDED NOTE: 
More detail can be found here: http://physics.aps.org/articles/v3/28


QUESTION: 
My friend proposed the idea that there is no such thing as pull. He explained that when you pull something, you actually push it, for example, when you pull open a door by its knob, you actually reach around the knob and push it from behind with your fingers. I said "what about tug-of-war or what about when you drag a piece of paper across a table by pressing your finger on it and pull it toward you?" He answered "that's probably particles pushing particles". Anyway, I wanted your take on this. What is pull?

ANSWER: 
"Push" and "pull" are qualitative ways of expressing force. I would say it is nonsense to say something like there is no such thing as a pull. In the simplest sense I would call any attractive force a pull and repulsive force a push. There are certainly plenty of attractive forces in nature.

QUESTION: 
Why do my TV and computer gather dust so fast?

ANSWER: 
If they are cathode-ray tubes the acquire a net charge, probably negative. A nearby dust particle will become polarized with positive closer to the screen and be attracted to the negative screen.

QUESTION: 
How does gravity effect weight and density, and why is mass density more important than weight density?

ANSWER: 
Weight is affected by gravity since weight is, by definition, the force something feels because of the gravitational field it is in. Mass is a property of the matter and therefore not dependent on gravity. It follows that weight density is affected by gravity and mass density is not. How important something is really subjective; I guess you could consider mass density more fundamental than weight density since it is independent of gravity.

QUESTION: 
As we know from our spectroscope lab, hydrogen and helium (Elements from which the sun is mostly composed of) emit specific frequencies of light, which one can measure and view via a spectroscope. We know that the reason is because of energy levels, and the traveling to and from a ground state to an excited state. If this is so, than the sun should not be a continuous spectrum of light, as it is. It should be, in fact, a very specific mix of both hydrogen and helium lines on our spectroscope. Why than is it a continuous spectrum?

ANSWER: 
When something gets very hot it emits a continuum spectrum, not discrete lines. The reason is that the atoms in the material are violently vibrating around and so they look like a bunch of oscillators, and they have charge distributions which are oscillating. When electric charges are accelerated they emit electromagnetic radiation and this is different than the line spectra of atoms. The sun is mostly not hydrogen and helium atoms but hydrogen and helium ions with most of the electrons freed from the atoms—called a plasma. A hot plasma has this sea of positive and negative charges zipping around, colliding with other, and consequently radiating a continuum of radiation. If you look closely, it is not really a continuum but has dark lines in the spectrum. These are wavelengths which are just right to be absorbed by atoms mostly in the sun's outer region, called the absorbtion spectrum. So there are still some unionized atoms still around.

QUESTION: 
Why does visible light only travel through clear things? How come a proton can go thourgh the atoms of glass but not copper or silver? Or, why does a light wave move through my car window but not my roof, sound seems to make it in through both?

ANSWER: 
Read an earlier answer. Sound is much easier to deal with because being simple pressure waves, it can easily travel through most continous media.

QUESTION: 
In an earlier question, a teacher asked about electromagnetic forces during an eggdrop in her science class. You told him/her that as the egg falls, the atoms in the egg are colliding with the atoms in the air, and the energy that's involved between those colliding atoms is electromagnetism. How exactly is electromagnetism involved here? Aren't the atoms just colliding because gravity is pulling the egg down and the air molecules are in the way of the egg?

ANSWER: 
But, you must ask what colliding means. What it means is that two objects interact with each other via some force. An atom or molecule at very close range looks like a cloud of electrons which are negatively charged. So when one atom (in the air) and another (in the egg) come close to each other, they repel each other and the mechanism is the repulsion between like (negative) charges—electromagnetism.

QUESTION: 
Are there within the electromagnetic spectrum of visible light (say 380 - 750 nm) an infinite amount of different wavelengths? For example, are there wavelengths such as 380.1 nm, 380.11 nm, 380.111 nm, 380.1111 nm, etc?

ANSWER: 
There are, indeed, an infinite number of possible wavelengths. It is, however, a moot point because the uncertainty principle forbids a light wave from having a wavelength precisely equal to 380.11 nm, for example, unless it is infinitely long which no wave, of course, is. All light waves are composed of a continuous distribution of different wavelengths; the distribution might be very narrowly peaked around a particular length, but never exactly one wavelength.

QUESTION: 
Sound waves are just vibrating fluid molecules, but isn’t energy responsible for those vibrations? Why aren’t sound waves a part of the Electromagnetic Spectrum, or am I misguided and the entire Electromagnetic Spectrum is actually responsible for sound?

ANSWER: 
Sound and electromagnetic waves are totally different phenomena. The only thing they have in common is that both are waves. Sound waves get their energy from their source, a vibrating vocal cord, a running motor, an explosion, a violin,

QUESTION: 
Minus any obstructions like smoke or dust to reflect the light into our eyes, why can't we perceive a light beam from a 90 degree angle, (standing perpendicular to the direction that the beam is traveling)? When light bounces off an object into our eyes, does this redirection at all result in a slight deceleration of the light?

ANSWER: 
Because the way you see light is to get some of its energy and convert it to electrochemical energy. The energy in light is transported parallel to the direction the light moves. Light never leaving a medium certainly does not undergo a deceleration since the speed of light is constant; there may be a delay or phase shift depending on the nature of the reflection, but the speed of light through air depends only on the properties of the air. [This wasn't really a single question (see groundrules) was it?]

QUESTION: 
My girlfriend and I both have iPhones. My in protected by a rubber case and hers by a plastic case. I told here that when dropped, the plastic case is more likely to transfer the force of the impact to her phone, whereas the rubber surrounding my phone is more likely to absorb the force of impact thus reducing the rusk of damage to my phone. Who is right?

ANSWER: 
Here is the main thing: the force needed to stop something depends on how long it takes to stop it. If you fall from 20 feet onto a thick slab of foam rubber, you are unhurt because, since the time it takes to stop you is relatively long, the force you experience is relatively small. On the other hand, if you fall onto a slab of concrete you get hurt because you stop very quickly and experience a very big force. The rubber of your case is likely to result in a longer time to stop than the plastic of your girlfriend's and that is why your phone is better protected. It is not really accurate to use the phrase "absorb the force of impact" because there is no such thing as a unique "force of impact", rather the force can be anything depending on the time as I explained above.

QUESTION: 
If gravity can bend light, does this imply that light therefore has mass?

ANSWER: 
Certainly not. Light does not have mass. You should read an earlier answer. I am also often asked "if light has energy and E=mc2, why does it not have mass?" So just to complete your futile quest for mass of light, read another earlier answer!

QUESTION: 
if the hight of the tv tower is increased by 21% how much will its range increase?

ANSWER: 
I presume you mean line-of-sight distance to the horizon. Well, it depends on how high the tower is. The distance L from the top of a tower of height h to the horizon is fairly easy shown to be L=√[h(h+2R)] where R is the radius of the earth.

QUESTION: 
I was wondering if you could explain how length contraction works. I've already done some background research and I understand the mathematical reasons my text book gives me, I was just wondering if you could give some kind of analogy that would enable me to picture the effects of length contraction, and better yet allow me to explain it to my friends in a way they can understand.

ANSWER: 
I don't know a simple one-step way to intuitively understand length contraction. But, I know a good two-step way starting with intuitively understanding time dilation. Time dilation is pretty easy to understand in one simple example, the light clock. See my earlier answer about the light clock and be sure you understand that and have a good intuitive feeling for why moving clocks run slow. From that point, length contraction can be understood as a natural consequence of time dilation. Here is how it goes. Imagine a bomb which has a fuse of 1 s. That is a clock. If that bomb is moving by us with a speed of 99% the speed of light, the elapsed time before detonation will be 1/√[1-(0.99)2]=7.09 s. So, the distance it will travel is 0.99x3x108x7.09=2.1x109 m. But, the bomb will measure in his own frame that he should last 1 s and go 0.99x3x108x1=2.97x108 m, only about 1/7 the distance we measure. If you think about the distance we measure as a long stick of length 2.1x109 m, then the bomb sees this stick moving by him with speed 99% the speed of light, so to reconcile his results with ours he must measure that length to be 2.1x109x√[1-(0.99)2]=2.97x108 m.

QUESTION: 
Is it possible for a longitudinal (eg. sound) wave to interfere with a transverse (eg. water) wave? Can 2 such waves generate "negative interference" and mutually annihilate each other or cancel each other out ?

ANSWER: 
There is no reason why both cannot both be present. Light and sound both go through air. And the superposition principle still applies, but the same thing has to be "waving" for them to add up. For example, light is waving electric and magnetic fields whereas sound is waving molecules, so those would not interfere. Some media can support both transverse and longitudinal waves of the same type. For example, water waves are a mixture of transverse and longitudinal. I do not know what "negative interference" is but the two waves would certainly interfere by virtue of the superposition principle. I can certainly not imagine a situation where they would "annihilate" each other.

QUESTION: 
place a card over the open top of a glass filled to the brim with water, and invert it. Why does the card stay in place? Try it sideways.

ANSWER: 
The force of the atmospheric pressure pushing up on the card is greater than the force of the water pushing down (which is numerically equal to the weight of the water). It's hard to get it to work on its side because the force the water exerts on the card depends on the height and the card wants to slide under the influence of its own weight. Also, even the tiniest bubble of air will cause the seal to break.

QUESTION: 
During Nuclear Fission, how does the atom actually split? I understand that it becomes unstable with the absorption of a neutron but what I do not understand is how the atom can separate into two atoms. Eg. U-235 + n > U-236 > various products My main concern is how an atom can split.

ANSWER: 
The atom does split, but the heart of the matter is the nucleus of the atom splitting. So, we usually do not call them atomic bombs anymore, we call them nuclear bombs (or, if you are George W. Bush, nuculer). Think of it like this: a nucleus is a very dense, very small object, often a sphere but sometimes more like a football, egg shaped. Uranium nuclei are more egg shaped. It also helps to think of the nucleus as a fluid. Now, if you add a neutron to 235U the resulting 236U nucleus is very excited. By this I mean that it is not just sitting there but vibrating wildly. So, think of this egg-shaped fluid vibrating so that it gets more elongated, in fact starts to develop a thinner "neck" in the middle, sort of like a peanut. What happens is that the two halves of this peanut-shaped thing break apart and, voilŕ, fission!

QUESTION: 
What is the maximum number of poles a magnet can have?

ANSWER: 
There is no upper limit. However, the lower limit is 2 and the total number must be even. The reason is that there are no magnetic monopoles (which would be the magnetic analog of a point electric charge) in nature. Every electron is a tiny magnet, so any chunck of stuff may be thought of as having like ~1025 poles!

QUESTION: 
Hello, my questions is simple and you please explain the speed of light and the speed of electrons moving in a wire carrying a current?

ANSWER: 
I do not really understand what you are asking. Maybe it has to do with the speed that information propogates through a wire and how fast the electrons move. When a potential difference (battery, for example) is connected across a wire, an electric field is established. This field is established essentially at the speed of light, so, for all intents and purposes, all the electrons in the wire start moving as soon as the potential difference is applied. The individual electrons, though, all move, on the average, with very small speeds because they keep bumping into atoms in the wire; so it is a start-stop-start-stop affair. Usually electrons move with average speeds of much less than one millimeter per second.

QUESTION: 
We always hear that it would be impossible to power a spaceship to the speed of light because as the ship neared the speed of light the mass would increase to near infinity and therefore require an infinite amount of fuel. Wouldn't the fuel, as it also increased in mass, produce more energy? I've always found this to be puzzling?

ANSWER: 
The simple fact is that the amount of energy it takes to accelerate an object to the speed of light is infinite; there is not an infinite amount of energy in the universe. See an earlier answer for more detail.

QUESTION: 
Sports announcers and governing bodies of sports say that extra weight is a benefit in downhill speed events. For example, it is illegal to add extra weight to bobsleds and soapbox derby cars. Announcers often say that a heavier bicyclist has an and vantage going down hill. I can understand that perhaps on snow or ice the extra weight could help melt the snow or ice, but on a bike or other wheels this would make more surface area and friction. Obviously there is the belief that mass plus gravity = greater velocity. How does this reconcile with Galileo’s experiment with the two cannonballs?

ANSWER: 
I have addressed this question numerous times before. You should read these discussions by linking here and following subsequent links. You will see that I never really came up with a satisfactory answer. This is a problem to which simple physics seems not very successful in describing—there are just too many variables and friction is a very difficult thing to deal with except in the simplest cases. However, do not let it get in the way of accepting that objects freely falling all have the same acceleration; this is a simpler experiment because it is easier to either eliminate air friction or calculate it relatively easily. If there were no friction, all cars would have the same acceleration on a ramp, I can guarantee that.

QUESTION: 
When we stretch a metal wire then energy is stored in it. My question is that when we lift the stretching forces and the wire unstretches then where does this energy go ?

ANSWER: 
It goes into "helping" you lift the weight stretching the wire.

QUESTION: 
What would a 50 gallon plastic trash can full of water weigh on impact after falling 12 feet to the concrete? I assume the weight of the thrash can is approximately 400 pounds. Could you show me the formula you would use to calculate the problem.

ANSWER: 
The weight is the weight is the weight! See an earlier answer.

QUESTION: 
Would a pin-ball machine with a higher spring constant result in its ball moving faster or slower than a regular pin-ball machine ball?

ANSWER: 
Faster if the spring were compressed by the same amount to launch the ball.

QUESTION: 
how would you find the maximum number of interference maxima observed in a Young's experiment with two identical slits if you know the wavelength of the light and the spacing between the slits?

ANSWER: 
This isn't homework is it? Starting with nλ=dsinθn, set θn=900 and solve for n.

QUESTION: 
is it possible to have a planet so dense that instead of that solar system being heliocentric, it becomes geocentric (basically would the sun then revolve around the planet instead of vice versa)

ANSWER: 
Not in our universe! There is a certain mass a star must have to "ignite" and that is much larger than chunks of matter you might identify as a planet are likely to be. There are lots of binary star systems in the universe. Of course, you should realize that the planets do not move around the sun, the whole system moves around its center of mass. It is just that the sun is so much more massive than everything else that it looks like it is the center because the center of mass is always very close to the center of the sun.

QUESTION: 
Why is sonic boom audible and visible?

ANSWER: 
A sonic boom is a shock wave which is a very intense pressure pulse. By definition, sound is variations of pressure transmitted through a medium, so it is to be expected to be audible. We do not expect to see variations of pressure (we do not see sound normally) but if the conditions are right (very rapid high-low pressure variation and ample humidity in the air), water vapor can condense to form droplets like a cloud to make it visible.

QUESTION: 
Dose weight affect the speed at which an object travels through a nonnewtonian liquid?

ANSWER: 
If you mean the terminal velocity as it is falling through the fluid, then yes, the weight is important. For example, ketchup is a nonnewtonian fluid and a marble will certainly fall faster through ketchup than a grain of sand.

QUESTION: 
Does the moon spin on an axis? If so, why can't we see it spin? Since we are also spinning...

ANSWER: 
The moon revolves around the earth (once every 28 days) and also rotates on its own axis (also once every 28 days). The moon appears to not be spinning on its own axis because it always presents the same side to us, but if it were not spinning we would see the other side of the moon 14 days from now because of its orbital motion.

QUESTION: 
I am preparing a test plan for a consumer electronics device. One of the test plans is to measure the microphonic distortion created when the device is impacted on any of its planes. The typical impact test can be as simple as dropping a deadweight (typical a sphere) from a short distance on to one of the surfaces of the device. I want to intelligently convey this in proper physics terms. It is my understanding that if I drop a mass of 350 gms from an elevation of slightly more than 20 cm as sea level, there will be an energy transfer of ~0.7 joules at impact (.35 kg * 9.81 m/sec^2 * .21 m). This leads me to the conclusion that I should state my subjective, witness microphonics test as follows: The device must not produce perceptible audio or video distortions when subjected to an impact energy transfer of 0.7 joules. For the record my co-worker believes that the test should be stated in terms of Newtons. I told him I can produce loads of Newtons merely by sitting on the device, but this would not produce any microphonic distortion.

ANSWER: 
I don't know what "microphonic distortion" means, but in terms of measuring "impact" effects, you and your coworker are both wrong, it seems to me. Rather, you should talk about the rate at which energy is delivered to the device, power. For example, if you were to cover your device with foam rubber the falling ball would deliver just as much energy but over a much longer time and likely cause much less "microphonic distortion" (whatever that means!) Hence, you should talk about joules/second, watts (W). To take this a bit further, I would also guess that the area over which the impact was delivered would matter; e.g., energy delivered by a "pointy" object would likely cause more "microphonic distortion" than an object with a big flat surface. If so, you should quantify the impact with W/m2. W/m2 are the units used to measure the intensity of a wave, that is intensity is areal density of energy flow rate.

QUESTION: 
I have a question regarding electron density, wave function and radial distribution of electrons in electron shells. Sorry if its more related to chemistry, but I thought I'd try this site first. What are electron density, wave function and radial distribution defined as? I understand that they are all in one form or another describing the probability of finding an electron in a position in space, but i do not understand the difference between them, and how they are derived. More specifically, what really confused me is in a graph of electron density in a s orbital, it seems to suggest that there is infinite electron density at the nucleus, while the radial distribution graph contradicts that. If you could clear this up it would be greatly appreciated.

ANSWER: 
The wave function is the solution to Schrödinger's equation. The wave function, often a complex function, has no physical meaning on its own, but everything knowable can be extracted from it in one way or another. If you multiply the wave function by its complex conjugate (insuring a real number) you get what is called the probability density Ψ*Ψ=|Ψ|2. So, let's talk about density. If you have a mass density, it does not tell you what the mass is at a point, it tells you what the mass per unit volume is at a location in space. The mass at a point is zero since a point has zero volume. Therefore you have to be careful not to look at probability density as a picture of probability. Probability can only be really visualized for a particular volume; the probability, for example, of finding the electron inside some small volume dV is |Ψ|2dV. Atomic physics is usually done in spherical polar coordinates where, instead of the Cartesian (x,y,z), we use (r,θ,φ) where r is the distance from the origin (taken as the nucleus), θ is the polar angle (latitude) and φ is the azimuth angle (longitude). For your purposes, let's just stick to the 1s orbital of hydrogen. s orbitals are spherically symmetric, that is they depend only on r and not on the angles. The wave function for the 1s state is Ψ=Cexp[-r/a0] where C and a0 are constants. Therefore, |Ψ|2=C2exp[-2r/a0]. But this does not tell you what the probability of finding an electron looks like. |Ψ|2 is not infinite, as you seem to think, at the center of the atom but it is biggest there; this certainly is not where you most expect to find the electron. So, we need to multiply |Ψ|2 by dV. Here is the tricky part: dV=r2sin2θdθdφdr in spherical polar coordinates. If you don't know much or any calculus, this is probably all Greek to you. But, let's get rid of the angular part because the probability density for the 1s state does not depend on angle. In order to understand what the probability of finding an electron is, you can see that a factor of r2 must be multiplied in because it appears in the volume element dV. So the electron probability function, let's call it, is Kr2exp[-2r/a0] where K is a constant chosen so that the probability of finding the electron somewhere in all of space is 1 (called the normalization constant). Note that this is zero at the origin and zero very far away and has a maximum somewhere in between which you might call the radius of the "electron orbit". It turns out that a0, called the Bohr radius, is where it maximizes. (If you know a little calculus, you can prove this by differentiating the electron probability function and setting equal to zero and solving for r.) I am not familiar with the terminology "radial density". Likely it is the probability density with angular dependence taken out (which would render it the same as probability density for all s states). Higher orbitals (p, d, f, etc.) have wave functions which depend on (θ,φ). If you want to see the wave functions in more detail, go here.

QUESTION: 
I am in the 8th grade. I am doing a project for my science class on the physics of arm motion in swimming. Can you give me some information? I get extra credit if I get information from a professional.

ANSWER: 
I do not quite know what you want. Here is something: what is it that pushes you forward? Is it your arm? No, it is the water that pushes you forward. Here is how it works. When you move your arm backwards under the water toward your feet, you are pushing on the water, pushing it backwards. Newton's third law says that if one object (hand) exerts a force on a second object (water), the second object (water) exerts an equal and opposite force on the first object (hand). Therefore, the water pushes you forward.

QUESTION: 
How much energy is required to remove the air of a container with a volume of one cubic meter to obtain a perfect vacuum?. Assume that the container is in a room at one atmosphere pressure and that the machine used to remove the air has an efficiency of 100%. I just made it up this problem from a talk with a friend in which we talked about if I can use the force produced for letting the air in to a vacuum container. I suppose it is the reverse process to the problem that I set and you will obtain (ideally) the same power that you used to extract the air.

ANSWER: 
It would just be PV=(1x105 N/m2)x1.0 m3=105 J. This is the work necessary to start with a volume of zero and expand it to V. Imagine a piston in a cylinder of cross section A being pulled out from being completely inside the cylinder: the work would be Fdx=∫PAdx=∫PdV=PV.

QUESTION: 
I was told that in free fall your mass is doubled every time you fall your length is this true if so can you explain?

ANSWER: 
That is absolutely not true. Your mass is a constant (unless your speed becomes comparable to the speed of light). A recent question was similar to yours.

QUESTION: 
when sound travels a longer distance,a person standing at a very far place from origin of the sound, hears it very softly,but the person standing close to it hears it loud due to doppler effect.but, now in case that is with the ultrasonic sound (which is at the audible and ultrasonic limit)can this be also heard at a distance very far from its origin ?

ANSWER: 
The loss of intensity with the distance from the source has nothing to do with the Doppler effect and almost nothing to do with the wavelength of the wave. It is because the sound waves spread out and so you get less and less of the wave front in your ear as you go farther away. The intensity falls off approximately quadratically for most sources, that is, if you are twice as far away you hear four times less loudness. The way you make the intensity fall off less is to focus the sound, much as a car headlight focuses light. That is how a megaphone works, or simply cupping your hands around your mouth.

QUESTION: 
i was just wondering...do satelites move randomly in space? or do they move in a defined path?

ANSWER: 
Any object orbiting another much heavier object (in particular, satellites) move in a path which is an ellipse (an oval shape). A circle is a special case of an ellipse.

QUESTION: 
not sure how to explain this... what is the formula for calculating weight in motion? for example... If someone is hanging from a trapeze and weighs 60 kilos. how much weight does the rigging point need to be able to take if the trapeze is swinging 1 foot in each direction or 3 foot in each direction.

ANSWER: 
I know what you are getting at, but allow me to correct a semantic point: weight is the force that the earth exerts on something and it has nothing to do with the motion of that object. However, something which is swinging from a rope may cause there to be a force on the rope rigging point which is greater than the weight. The force on the rigging point is applied by the rope which has a tension in it, so we might just as well ask what must the strength of the rope be? Suppose we have a 5 lb test fishing line (which means it will break if its tension exceeds 5 lb). Can we lift a 5 lb bass with this line? We can hang the fish from this line but we cannot lift it because, in order to lift it, we must exert an upward force greater than the weight to accelerate it upwards. In fact we could not even lift a 4 lb fish if we tried to do it too quickly, a fact which has left many fishermen feeling that their line was not as advertised. The same thing happens when an object is moving on a circular path, like your trapeze. The weight on the trapeze is accelerating because its velocity is changing direction as it moves along its path; the tension in the rope will be greatest when the rope is vertical. Unfortunately, you have framed the specifics of your question in a way that I cannot answer it because the 1 or 3 foot measure isn't enough information—you need to know the length of the rope too. If instead you specify the angle of the swing, then you do not need the length of the rope. The tension in the rope will be W(3-2cosθ) where W is the weight and θ is the angle. For example, if θ=200 then cosθ=0.94 and so the rigging point must support (3-2x0.94)=1.12 times the weight. Here is a table for angles a trapeze is likely to have.
 

θ (degrees)

cosθ

3-2cosθ

0 1.00 1.00
10 0.98 1.03
20 0.94 1.12
30 0.87 1.27
40 0.77 1.47
50 0.64 1.71
60 0.50 2.00

QUESTION: 
What feels the strongest pull of attraction toward the earth and why? A box containing 10 baseballs or a box containing 15 baseballs?

ANSWER: 
The "pull of attraction toward the earth" is called the weight. The weight of something is proportional to its mass and so 15 baseballs has 50% more mass than 10 baseballs. Even though the "pull" is greater on the greater mass, both accelerate the same because the acceleration is inversely proportional to the mass. (I suspect from your question that maybe you are trying to make a case for the heavier box falling faster.)

QUESTION: 
Identical springs made of steel and aluminium are equally stretched.on which more works will have to be done?

ANSWER: 
The work done to stretch a spring of spring constant k by an amount s is ˝ks2. If the springs are geometrically identical, then the one which has the larger Young's modulus has the larger spring constant. The Young's moduli for steel and aluminum are 200 and 69 GPa. The answer is steel.

QUESTION: 
If two objects, one of greater mass than the other, are dropped on the moon from the same height, wouldn't the object of greater mass hit the moon first (albeit to a negligible extent) due to the greater gravitational pull it exerts on the moon compared to the smaller object? Or, to put it another way, does my mass pull the earth towards me more strongly than, say, the mass of an ant would. Or to put it one last way, in the depths of space, would two very massive objects, independent of the gravitational pull of any other objects, accelerate towards each other more quickly than two less massive objects? no one's ever quite answered this one to my satisfaction.

ANSWER: 
Yes, you are correct. You have to be careful that the center of each object begin at a certain height above the ground and that you take "hit the moon" to mean "fall a given distance". To see this for a specific example, consider two pairs of objects, each in empty space. The first two objects have equal masses, m. The second two objects have masses m and 2m. Both pairs are separated by the same distance. The force on each of the second pair is twice the force on each of the first pair. Therefore, since acceleration is force divided by mass, both objects of the first pair and the 2m mass of the second pair have the same accelerations; the mass m of the second pair, however, has twice the acceleration of all the others. Therefore, the second pair falls together faster.

QUESTION: 
Is it possible that no two independent events can begin at precisely the same moment? Or end at precisely the same moment?

ANSWER: 
There is no reason why two events cannot be exactly simultaneous. What is important is that simultaneity, like time itself, is not an absolute thing and different observers may disagree regarding whether two events are simultaneous. Do not read this as appearing to be simultaneous; two events simultaneous for one observer may not be simultaneous to another.

QUESTION: 
How would one find where in an orbital an electron is? Is it even possible?

ANSWER: 
It is not possible. The picture of a well-defined electron orbiting a nucleus is a very simplistic picture of an atom. Rather, you should think of a cloud surrounding the nucleus, the density at any particular place being proportional to the probability of finding the electron there if you made a measurement. The probability of finding an electron at a specific point in space is zero because the volume of a point is zero.

QUESTION: 
If you push for a half hour or a whole hour against a stationary wall how much work is done?

ANSWER: 
Since the wall does not move and work is force time distance, no work is done on the wall. However, your question now verges on biology rather than physics if you are asking yourself "how can you say I am not doing work when I push on the wall when I know energy is required to do so?" That is, you will get tired pushing on the wall. 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. There is no physics way to quantify the amount of work; it would depend on, for example, how hard and long you pushed.

QUESTION: 
if a curling stone weighs 20 kilograms and is traveling at a speed of 0.5 meters/seconds3, with how much force did the curler throw it in N?

ANSWER: 
Why the speed in m/s3? It should be m/s. Anyhow, it seems like I have answered questions like this a thousand times! You cannot determine the force needed to give a particular mass a particular speed. Just to make that plausible, suppose you push on the 20 kg stone with a force of 2 N for 1 s; surely it will have a different result than if you push on the 20 kg stone with a force of 2 N for 2 s. There are two (in the end, equivalent) ways you can think about this problem:
  1. The impulse delivered by a force F in a time t is Ft. Linear momentum of an object with mass m and speed v is mv. The change in momentum is equal to the impulse and so, if the object starts at rest, Ft=mv. For example, in your case Ft=10 kg-m/s so you could push with a force of 10 N for 1 s.
  2. The work done by a force F pushing over a distance s is Fs. Kinetic energy of an object with mass m and speed v is ˝mv2. The change in kinetic energy is equal to the work and so, if the object starts at rest, Fs=˝mv2. For example, in your case Fs=2.5 kg-m/s2 so you could push with a force of 10 N for a distance of 0.25 m.

In both cases, be sure to note that what the force is depends on how long or far it is applied.


QUESTION: 
The question I have is about relativity I would like to know why the speed of light is squared in E=MC squared, what is the significance of this number.

ANSWER: 
For starters, it would not be dimensionally correct. Energy must have the dimensions of mass times (length/time) squared. For example, a kg-m2/s2 is called a joule. Your question is sort of like asking why is velocity not measured in kilograms. The derivation of the famous equation is a bit technical but may be seen in an earlier answer.

QUESTION: 
I recently did an egg drop lab with my students and when I asked "What forces were acting on the egg", most students identified gravity and air resistance. A few others also noted electromagnetic and nuclear forces. My question is: Were electromagnetic and nuclear forces acting on the egg?

ANSWER: 
While it is falling, gravity and air resistance are indeed acting on the egg. You could ask: "what is the origin of the air resistance?" The atoms of the air are colliding with the atoms of the egg and the force between atoms (at these energies) is all electromagnetic. You could therefore replace air resistance by electromagnetic. There is another (which is very small for an egg) forcethe buoyant force associated with the egg's being in a fluid (air); this is an upward force and, if the egg's weight were smaller than the weight of an equal volume of air, the egg would "fall" up! When the egg hits whatever it is falling onto, there is an upward force due to the contact force (often referred to as a normal force in physics texts); this force, like air resistance, has its microscopic origins in forces between atoms, electromagnetic forces. At no time do nuclear forces come into play because the energies involved are not great enough for nuclei to come into contact with each other.

QUESTION: 
A submarine rises to the ocean's surface to take on passengers and the sinks back underwater. How can I expain how this works to my 10 year old daughter.

ANSWER: 
A submarine has balast tanks. They start out filled with air. Now, sea water is permitted to flow into the tanks and it is just like a boat springing a leak, so it sinks. However, the air which was in these tanks is not allowed to escape, rather it is pumped into small tanks under high pressure. When the submarine wants to come back up, the air is pumped back into the tanks pushing the water back out into the ocean.

QUESTION: 
I was wondering if you could help me understand something. Here is the scenario; I hit a golf ball with a golf club. the ball travels 6" and impacts a wall of equal hardness as the golf club that struck the ball. Does the ball take more force (newtons?) from the initial impact or the impact with the wall? If at all possible please describe why.

ANSWER: 
It is not meaningful to ask how much force the collisions took because what matters is the impulse which is essentially the product of the average force times the time it acted. This should seem reasonable since you could stop a freight train with a modest force if you were willing to push for a long enough time. So, for the sake of argument, let us assume that the time that the golf club was in contact with the ball was the same as the time the ball was in contact with the wall. Then we can compare forces. The impulse delivered to the golf ball is proportional to its change in velocity. The golf club changed the ball's velocity by only half as much as the wall did (from 0 to v for the club, a change of v and from v to -v for the wall, a change of 2v). Therefore the wall exerts twice the force in an equal time as the club does. (I have assumed an elastic collision with the wall so the rebound speed is unchanged and that the club did not lose much of its speed when it struck the ball.)

QUESTION: 
how to know that if you double a car speed, the time it takes to stop is doubled, but the distance it takes to stop quadrupled. I understand that it is the distance, not the time, that determines what you will or will not hit or what will or will not get you into trouble. Please kindly guide me with this.

ANSWER: 
The equations of kinematics for constant decceleration are v=v0-at and x=v0t-˝at2 where t is the time, a is the magnitude of the acceleration, v is the speed at time t, v0 is the speed when t=0, and x is the position at time t; I have assumed that x=0 when t=0. The time t=0 is when you put on your brakes. So, if t is when you stop then v=0. Then, t=v0/a (t is proportional to v0) and, putting this t into the x equation, x=˝v02/a (x is proportional to v02). All this assumes that the wheels are locked which results in nearly constant negative acceleration.

QUESTION: 
My son is doing a science fair project for 5th grade. His question is "does the distance from a starting point to a ramp, affect the distance of a jump?" He is using a remote controlled car...We've done the research having performed jumps at 5, 10, 15, 20, and 50 feet. We've determined that the greater the distance (ie, 50 ft). the greater the jump will be. I'm having trouble determining what scientific rule support this. [Clarification: Yes we have a starting line.Trying to figure out how the distance from the starting point to the ramp that goes up affects the distance that the remote controlled car (electric) will jump. We did it at various distances, 5, 10, 15, 20, and 50 feet away from the starting point. I mean the distance from the starting point until you reach the ramp. The ramp is about 10 inches tall and 3 feet long.]

ANSWER: 
The only thing which will affect how far the car goes is how fast it is going when it becomes airborne. For your experiment the car starts at rest and accelerates as it goes. Of course, it won't keep accelerating forever, so it is surprising to me that there is any difference between 20' and 50' since I would have thought that the car would have acquired all the speed it was going to acquire after 20'. I would guess that your data would eventually level off and after some distance all greater distances would give the same result. One thing your son could do to is measure the distance and from that deduce the "launch" velocity. I estimate roughly for your geometry that if v (in m/s) is the speed and L (in m) the distance it goes before hitting the floor, then v1.1L√[20/(1+1.2L)]. (You might have to teach your 5th grader what a square root is!) For example, if L=1 m then v1.1√[20/(1+1.2)]=1.1√9.1=3.3 m/s. To convert m/s to mph, multiply by 2.2, so v would be about 7.2 mph. One additional tip would be to be sure the results are reproducable, that is each datum should be taken several times and the average value of L calculated; the spread of the data around the average value give you some idea of the accuracy of the measurement.

QUESTION: 
does the length of an object moving with high speed really shrink!

ANSWER: 
Two of the most surprising results of the theory of special relativity are that moving sticks are shorter (along the direction of motion) and moving clocks run slower. These have been thoroughly verified experimentally. And it must be appreciated that moving sticks really are shorter, they don't just look that way; in fact they may very well appear longer. It is important to define what you mean by the length of something: length is the distance between the ends where the ends have been observed at the same time.

QUESTION: 
Does a rubberband lose it's potential energy the longer it is kept in the wound position? We are running a Cub Scout space derby and would like to wind the rockets ahead of time to help speed up the process but we do not want any issues with parents if the one rocket wound first would have a disadvantage because it lost some of it's potential energy.

ANSWER: 
Yes, it certainly does. Rubber is a particularly unusual elastic material which exhibits strong hysteresis which means that when it is stretched it tends not to return all the way to its original shape. Under conditions of high temperatures or long times this hysteresis becomes more pronounced because the long molecules tend to relax when stretched if held there for a long time.

QUESTION: 
You're walking on the surface of a hollow planet. Imagine something like a pumpkin without the seeds and guts. The surface is thick enough to walk on and hold it's shape. The planet is also large enough to have an Earth-like gravity that attracts you to the surface. So you're walking along when you fall in a hole. The whole goes all the way to the hollow center. How far do you fall?

ANSWER: 
During the time you are entering the hole, you accelerate because the part of you not yet inside feels a gravitational force toward the center. Once you are completely inside, you feel no graviational force and so you move with constant velocity across the cavity.

QUESTION: 
In a collision experiment, how would the conservation of momentum equation change if one metal sphere is replaced with a) rubber sphere; b) paper sphere; c) rock sphere of the same mass?

ANSWER: 
Since momentum is always conserved in a collision (for an isolated system), the equations always look the same regardless of the material.

QUESTION: 
Why is there a percieved difference in time only a few miles above the earth? Ie, gps satelite daily time reset.

ANSWER: 
Because general relativity has a result that the rate at which a clock runs depends on the gravitational field strength and the field gets weaker as you get farther from the center of the earth.

QUESTION: 
How planets orbit the sun without stopping? Wouldn't this be considered perpetual motion, which is, according to the conservation of energy, impossible?

ANSWER: 
Perpetual motion is not forbidden by energy conservation. In fact, perpetual motion is perfect energy conservation. The reason satellites orbit (planets around the sun, moons around planets, etc.) is that they are in perpetual free fall. For example, see Newton's cannon.

QUESTION: 
Why do microwaves cause water molecules to heat up where as waves smaller and larger fail to do so? Why is it a person can be exposed to visible light which has a shorter wave length with no ill effects, and be exposed to longer radio waves with no damage as well? I can understand how waves smaller then visible light cause damage as they have more energy, but why is it a certain ban of IR and Microwaves cause heat? Is it sort of like a harmonic effect?

ANSWER: 
It is basically a resonance thing. Electromagnetic waves with frequencies corresponding to microwaves are strongly absorbed by water, fat, and other organic molecules whereas other frequencies are not. See the Wikepedia article.

QUESTION: 
I would like to know how the size of a planet assuming the density is the same as the earth effects the graviy i.e if you double the mass do the gravity double and if the daimeter is doubled does the gravity go up 8 fold.

ANSWER: 
I assume you want to know the strength of the gravitational force on the surface of the planet compared to on the surface of the earth. If the density remains constant, then doubling the mass means increasing the radius by a factor of 21/3=1.26. The force is proportional to the mass and inversely proportional to the square of the radius, and so F/FE=[2ME/(21/3RE)2]/[ME/RE2]=21/3. If you double the radius, you increase the mass by a factor of 8. The force is proportional to the mass and inversely proportional to the square of the radius, and so F/FE=[8ME/(2RE)2]/[ME/RE2]=2. ME and ME are the mass and radius, respectively, of the earth.

QUESTION: 
The potential (relative to a point at infinity) midway between two charges of equal magnitude and opposite sign is zero. Is it possible to bring a test charge from infinity to this mid point in such a way that no work is done in any part of the displacement?

ANSWER: 
Imagine the plane perpendicular to the line and bisecting it. Moving a charge anywhere around this plane requires zero work.

QUESTION: 
As earth is moving around the sun and also rotates around itself is it possible that we can only lift the airplane without moving it towards ahead and we can reach to another place of the earth? If not then why?

ANSWER: 
No, because an airplane flies relative to the air, not relative to the earth, and the air moves with the earth (more or less).

QUESTION: 
How does heat cause wind?

ANSWER: 
The details are very complicated but the general ideas are simple. Adding heat to a gas causes changes in pressure, temperature, and volume. In a large volume of gas like the atmosphere, these changes cause movement of the air.

QUESTION: 
My chemistry teacher said that when electrons becomes exited in form of energy absorbed it releases a photon when it falls back to its original place. So, does this make the photon entangled with the electron?

ANSWER: 
No. Both the photon and the electron are in definite states, not mixed with each other.

QUESTION: 
why is it necessaryto use two coeffeocient kinetic and static to describe the fritional force b/w two force how do u decide whoich cofficent to use when solving a problem

ANSWER: 
Because they are two totally different things. For kinetic friction, the object must be sliding on a surface and the coefficient of kinetic friction tells you what the friction force is (coefficient times normal force). For static friction, you cannot use a formula to get the frictional force; it can be anything from zero up to the coefficient times normal force. The static is always larger than the kinetic coefficient since the object "pops away" when it starts to slide. Teflon is an example of a material with static and kinetic coefficients very nearly equal.

QUESTION: 
given Mars' gravitational field, would it be possible to catapult a vessel into space from the surface (of Mars)? And if it is possible, how much force would be required? Sorry, I know this is a rather vague question, I guess all I'm really asking is if it would be possible to launch a vessel into space via catapult at all... Don't worry, I'm not trying to build a space catapult or anything, I'm just wondering if it's even possible.

ANSWER: 
The escape velocity from Mars is 5 km/2, about 11,000 mph. Whether you could do that with a catapult is an engineering question more than physics. I would not think this would be a practical way to launch a spacecraft since it would require a huge acceleration (because the size of the catapult means the speed would be acquired in a relatively short distance). A huge acceleration, of course, means a huge force so having people in the craft would certainly be out of the question.

QUESTION: 
a co-worker of mine was describing a time where he threw a box weighing about one hundred pounds out of a window on the eighth floor of a building. In an effort to dramatize the story a little, he wanted to express to us how forceful the impact was of the box on the ground below. he mentioned what I considered to be a rather dubious rule-of-thumb: an object's weight doubles for every foot it falls. At home in the evening I relayed the story to my roommates, and we discussed how unlikely it was that this was true--just for being such a simple, neat multiplier. Our subsequent research on the internet (involving many formulas, calculators and conversion charts) seemed to (roughly) confirm his statement. Being poor physicists, we are open to believing the calculations, but, perhaps as good scientists, we continue to doubt. Can you help? As a general rule-of-thumb, does his statement hold any water: does an object's "weight" double for every foot it falls?

ANSWER: 
I must have written this a hundred times—the weight of something is the force that the earth pulls on it. The weight of the box is the same no matter where it is dropped from (unless the height is not small compared to the size of the earth). So, what might he be trying to say? Suppose we ask how much force would be needed to stop the box in a distance of, say, one foot. The work to stop an object with an energy of 500 ft-lb is equal to 500 ft-lb; so, to stop a 1 lb box which has an energy of 500 ft-lb, you would have to exert a 500 lb force to stop it in a distance of 1 ft (work is force times distance). To acquire this energy, it would have to be dropped from 500 ft up; the energy something has at the ground is proportional to how high you drop it from, so it has twice as much energy than if you had dropped it from 250 ft. Clearly, the energy does not double with every foot, so I don't know what he means about doubling every foot of fall. With every foot of fall a 1 lb box increases its energy by 1 ft-lb.

QUESTION: 
I keep pondering this scenario about centrefugal force, but there seems to be some principle I'm missing. If I rotate two balls connected by a rope here on earth, the centrefugal force will cause the rope to tighten as the balls move away from each other. Now I imagine I am watching the two balls in deep space, rotating, in relation to me perhaps, not in relation to anything else. If motion is relative, in what way are the two balls moving and is the rope taut? What am I missing? If I wasn't there to observe them, could the balls be rotating and the rope be taut?

ANSWER: 
Imagine yourself in empty space with these two balls and rope. First find a frame of reference where the balls can be at rest with the rope totally slack. This is called an inertial frame of reference, defined as a frame in which Newton's first law is true. Now set the balls spinning. They now have an acceleration (relative to the inertial frame) and a force is required to maintain this acceleration—the tension in the rope. It makes no difference whether you are there or not, the rope must be taught to provide the acceleration. The idea that all "motion is relative" is often overstated: there is a big difference between inertial frames and all others. What you are suggesting is like saying that there would be no difference between how things are in a car moving with constant speed and a car having a very large acceleration (like hitting a brick wall).

QUESTION: 
I understand that gravity is thought to propagate at the speed of light. Consider two large bodies moving initially at the same high speed and parallel to one another, close enough to exert a gravitational attraction and far enough away from other bodies that they can be considered isolated. Now there will be a gravitational attraction between the two bodies drawing them together. But surely the direction of the vectors of attraction would lag slightly behind the actual position of the bodies. This would over a long period of time result in an overall slowing down of the bodies.

ANSWER: 
This is really too technical a question. You are asking about what is often referred to as "retarded potentials" and, you may be sure, there is no force which will slow these particles down, only the force pulling them together. In the rest frame of the two masses they clearly have no acceleration other than toward each other.

QUESTION: 
I read that if electrical power is transmitted over long distances the resistance of the wires becomes significant. Why is this the case and which mode of transmission would result in less energy loss-- high current and low voltage or low current and high voltage?

ANSWER: 
Power dissipated by a current I passing through a resistor R is I2R. A conducting wire has low resistance so this is not much of an issue in your house, say. However, R is proportional to the length of the wire and over many miles R, and therefore power loss, can become significant. Since R cannot be controlled, I is minimized. The way to do this is to use a very high voltage and low current.

QUESTION: 
What would happen if an object traveling at one half the speed of light passes through the earth's atmosphere?

ANSWER: 
It depends entirely on the object. Cosmic rays go faster than half the speed of light and they hit the earth all the time; they tear up atoms as they go but mainly leave no significant trace. Something large would burn up and probably cause catastrophic damage.

QUESTION: 
Why does fusion reactions produce more energy than fission reactions, given that by Einstein's equation, energy is proportional to mass.

ANSWER: 
Fusion reactions convert a higher percentage of mass to energy than fission. First you should look at an earlier answer about fusion and fission. You will see that the slope of the binding energy curve is much greater for light nuclei fusing than heavy nuclei fissioning. One uranium fission will give more energy than two hydrogens fusing but the ratio of mass converted to energy to mass of the fuel is much bigger for fusion.

QUESTION: 
does time move faster on the surface of the moon as opposed to on earth?

ANSWER: 
Yes. Time moves more slowly as gravitational force gets larger. This is a consequence of the theory of general relativity.

QUESTION: 
Tie an object such as a pencil eraser to a string. Use the string to swing the object around your head at a constant speed. Can the object be swung so that the string is parallel to the horizontal, flat, and even floor on which you stand?

ANSWER: 
No. The reason is that the object must in equilibrium in the vertical direction since it has no vertical component of its velocity (or acceleration), so the forces in the vertical direction must add to zero. One force in the vertical direction is the weight, straight down. There must be another force which points up and is equal in magnitude to the weight; this can only be the vertical component of the tension in the string and a horizontal string has no vertical component of its tension.

QUESTION: 
the time it takes for one heavenly body to make one complete revolution around another heavenly body is called what?

ANSWER: 
The period.

QUESTION: 
why do physicists relate the four dimension to time more then just another physical dimention , much like the third dimension to flat-landers?

ANSWER: 
Suppose that you have a two-dimensional coordinate system and the coordinates of some point are (x,y); now, if you rotate this coordinate system the new coordinates of the same point are (x',y') where x' depends on both x and y, and y' depends on both x and y. That is, the coordinates become mixed up with each other. Now suppose that one observer moves in the x direction of some coordinate system; this observer carries a clock which measures a time t' and measuring sticks which measure a distance x'. If (x',t') are compared with (x,t), analogous quantities measured by a second observer not moving in the coordinate system, it turns out that x' depends on both x and t, and t' depends on both x and t. This analogy tells you that it is fruitful to treat, mathematically at least, time on an equal footing with space, that is, special relativity can be developed using four-dimensional vectors. It is, however, a mistake to think of time as a fourth space dimension like a flatlander would think of the third dimension. Indeed, you should think of there being something which encompasses both space and time which we call space-time, not that space is now four dimensional.

QUESTION: 
Suppose that the mass holders in an experiment all have the same mass.May their masses be neglected in calculation of the acting force?

ANSWER: 
Maybe for some particular experiment, but certainly not in general.

QUESTION: 
In my high school physics class, we have just finnished the section on electricity. The two equations my question relates to are: the relation between capacitance, voltage, and charge C=Q/V the force between two point charges (Q and q) as a function of distance F=(kQq)/d^2 My question, which none of my teachers can answer, is: Why don't these two eqations work when used together? I'm assuming they dont work because a capacitor of 100uF at 10v holds 1000uC and two charges of 500uC separated by 10cm should, according to those equations above, exert a force of roughly 225,000N. I've done the math many times and can't find any issues. Is it a calculation error or improper use of the equations?

ANSWER: 
The reason is that the two equations are essentially unrelated. The force equation applies to two point charges separated by a distance d. The plates of a capacitor are not point charges. You also have some misceptions about capacitors. The capacitor carries a charge Q. This means that one plate of the capacitor carries a charge Q and the other a charge -Q such that the total charge is zero. The plates of a capacitor therefore exert an attractive force on each other. It is pretty easy to compute the force one of the plates of a parallel plate capacitor feels because the electric field between the plates is nearly uniform. The field is given E=4πkQ/A where A is the area of the plates, and the capacitance is C=A/(4πkd). The force felt by each plate is F=EQ/2. If I put in your numbers for Q=10-3 C, V=10 V, and d=0.1 m I find that F=0.5 N, a far more reasonable number!

QUESTION: 
Is a nuclear submarine called nuclear because it has nuclear weapons or because it is run with nuclear power?

ANSWER: 
Because it is nuclear-powered.

QUESTION: 
I got in an argument with my friend about the weightless feeling in space. I say that if you're in a spaceship orbiting the earth, you'll feel weightless because you are technically in constant freefall. He says you'll feel weightless because theres no gravity. Who's correct?

ANSWER: 
You are correct. If there were no gravity, what force would hold your spaceship in orbit? It is also insightful on your part to recognize that, when in orbit, you are actually in freefall.

QUESTION: 
Will a rock cool faster in deep space in sitting on a dinner plate at stp? (room temperature at my house) lets say it starts with a 300 degree F temp and cooling to 100 F

ANSWER: 
I do not think you can say for sure. It would depend on the shape of the rock, the composition of the rock (in particular, how good a radiator the material is), the amount of contact of the rock with the plate, etc. The rock in deep space can cool only by radiation and would continue cooling below 1000F. The rock in your house would also cool by radiation but it would also absorb radiation being constantly emitted by the environment; if radiation were the only consideration, the rock in space would clearly win the race. But the rock in your house can also cool by conduction (via contact with the dinner plate) and convection (air moving over it carrying away heat). But the the rock in you house is tending toward room temperature, maybe 700F, so I would think that it would approach 1000F much more slowly than the space rock. So, my best guess is that the space rock will win the race, but careful tailoring of the conditions might reverse the situation.

QUESTION: 
I would like to ask a quick question regarding laser and color. I tried googling it but found no result. I'm trying to find some ways to detect the color of a material for a small project. I thought of two ways so far
1. through an optical lens that will capture the color and get it analyzed.
2. shoot a beam of laser at the material and analyze the wavelength that gets bounced back.
Obviously, the first example does not concern physics too much, but I was wondering if the second example is feasible in a relatively cheap manner.

ANSWER: 
The laser will not work because laser light is of one color and that is the only color you could see reflected back. I do not see what you mean that "the first example does not concern physics". Optics is physics. But to "capture the color" is not the essence of what you want to do, it is determining the color that is the real task. For this you need a means of measuring the wavelength(s) of the light, e.g. using a prism or a grating as a spectrometer.

QUESTION: 
what makes people think that fundamental physical constants can be anything other than what they are? Is this a mistake akin to imagining a temperature of, say, −274.15°C (where −273.15°C is the number we use to say that atomic movement -- i.e. heat -- ceases). Just because we can change the numbers in our minds does that give us a basis for imagining what our universe would be like if the cosmological constant was anything other than it is? A recent article I read in SciAm seemed to treat it as a given that these constants are arbitrary and proceeded to hypothesise about multiple universes. Am I missing something, or is this baseless physics?

ANSWER: 
For starters, absolute zero is not a fundamental constant. No matter what fundamental constants are, how units are operationally defined, absolute zero will always be that (unattainable) state of matter with zero kinetic energy. Examples of fundamental constants are the universal constant of gravitation, G, which quantifies the strength of gravity, and the permittivity of free space, ε0, which quantifies the strength of electrostatic interactions. And, you do not look at the numbers since those depend on the system of units; rather you look at the implications of the relative values for physics. The values of G and ε0 tell you that gravity is incredibly weak compared to electromagnetic forces. And we have no idea why they have the values they do. Amazingly, just relatively minor changes in the fundamental constants would make our universe as we know it impossible—stars would not form, atoms would not form, nuclei would not form, life could not exist, etc. One of the ideas advanced is that what the constants turn out to be is random but that there are many universes with many values of constants and few are anything like ours; whether this is "baseless" physics is open to debate, but there is certainly no physical evidence that it might be true.

QUESTION: 
if a water-filled container's weight with an object in it is equal to the container plus the buoyant force on the object if it has not sunk, Why then does the buoyant force point opposite to the direction of the weight of the container? Would you then not subtract the buoyant force from the weight of the container to get the total weight?

ANSWER: 
Aha, you make the classic mistake made by thousands of physics students going before you—you are not focusing on one body at a time. Let me run through the various possible scenarios here and see if you understand.
  • Look at the object. What are the forces on it? Its weight w down and a force b which the water exerts on it, up. (We call b the buoyant force.) Newton's first law (N1) requires b-w=0, so b=w.
  • Look at the container and water. What are the forces on them? Their weight W down, a force N from the scale they are sitting on, up, and a force f which the object exerts on the water. Newton's third law tells us that f=b and points down. N1 now tells us N-W-f=0=N-W-w, and so N=W+w. There is the answer to your question—the scale reads the total weight. But let's look at another possibility.
  • Look at the container and water and object all together. The forces are the weight, W+w down, and the force of the scale N up. The buoyant force does not come into it at all because the forces the water and object exert on each other are internal forces and cancel out (or, b-f=0). So N1 tells you that N=W+w. Again, there is your answer—the scale reads the total weight.

QUESTION: 
In case one I accelerate an apple to one mph. It requires x amount of work to accelerate the apple by one mph. Next I accelerate the same apple by another one mph. This requires the same amount of energy as in the first example, ie the energy necessary to accelerate one kg by one mph. Thus I have expended twice as much energy to accelerate from one mph to two mph. But the apple now has 4 times as much energy. I would so much appreciate an explanation in laymen's terms. I must be really stupid. I just cannot see the logic.

ANSWER: 
No, you're not stupid; it is a little subtle. Energy is not what we call an invariant quantity, that is the kinetic energy something has depends on the frame from which you observe it. What your thought experiment does is shift frames for the second acceleration; you have put yourself in a frame where the object starts at rest. When you do this, you move into a frame where the apple is at rest, that is has zero energy. Therefore, your second experiment is nothing more than a repeat of your first experiment. If you calculate the work you have to do in the original rest frame to accelerate the object from speed v to speed 2v you will find W=˝m(3v2) which then leads to the correct total energy and work of ˝m(4v2).

FOLLOWUP QUESTION: 
This is what I have trouble understanding. Assuming no friction or wind force, etc. it requires x amount of fuel to accelerate a car by 1 mph, and x amount of fuel for each additional 1 mph. Thus change in velocity is directly proportional to energy input. But change in kinetic energy is proportional to the square of change in velocity. Energy input should equal the change in kinetic energy of the car, but apparently does not. I know there is a basic flaw in that reasoning, but where is it?

ANSWER: 
It does not take the same amount of fuel to accelerate the second 1 mph. It is a little tricky to talk about a car where it is actually the force of friction of the tires which push the car forward to accelerate it. That is were the work is done on the car, a forward force F which is constant if you are accelerating at a constant rate. Now, if the car starts at rest and increases its speed at a constant rate to speed v, how far, d, has it gone? Well, simple kinematics tell us that d=˝(F/m)t2 and v=(F/m)t, so d=˝(F/m)[mv/F]2=[˝mv2]/F. Therefore, the work F does to bring it to v is W=Fdmv2. Now, let's calculate the work F must do to increase the speed from v to 2v. The new distance traveled, d', is d'=vt+˝(F/m)t2 with 2v=v+(F/m)t => v=(F/m)t; note that the time the force acts is the same but the distance over which it pushes is larger, so more work is being done. If you do the algebra here (put t=mv/F into the d' equation), you will find d'=3d. It takes three times the energy to increase the speed by another 1 mph in your example. I suspect you are still bothered! And I think I can say why. If you burn fuel at a constant rate you will not accelerate at a constant rate; to accelerate at a constant rate you have to increase the rate at which you burn fuel as you go along. If you burn fuel at a constant rate, the time for the second 1 mph gain will be longer than for the first 1 mph; and guess how much—exactly 3 times longer! (I always go that extra mile for folks nice enough to make a donation!)

QUESTION: 
Gas particles travel at speeds really big like 400 or 500 meters per second. Why does it take so long for gas molecules to travel the length of a room? This came up in class and i couldn't figure it out.

ANSWER: 
Because they collide with other molecules in the air and therefore have a zig-zag path. The mean free path, sort of an average distance between collisions, is around 10-7 m in air at STP.

QUESTION: 
I need to prove innocence in a car accident and need to find out the minimum speed a car would be travelling to push a stopped car forward about 2 metres. Scenario. 3 car crash where 2 front cars pulled up hard but were still a sufficient distance from each other eg 1.5-2.5 metres when the 3rd car rammed hard enough into the middle car to push it forward and bump the front car. Sealed road, dry conditons. the middle car weighs 1322KG and the 3rd car weighs 1703KG

ANSWER: 
First of all, you are not going to "prove" anything. There are too many variables. The coefficient of sliding friction for rubber on dry asphalt is in the range 0.5-0.8. I will use 0.7. I will assume that the brakes of the middle car are engaged so so the tires slide, do not roll. It is crucial what car #3 does after the collision. One simple collision is called perfectly inelastic; in this case the two cars move together after the collision, essentially as one. At the other extreme, the collision can be perfectly elastic; in this case no energy is lost in the collision, sort of like two billiard balls colliding (almost elastic). I am not going to include all the details, just the results. First, I calculate the speed the middle car needs to have to slide 2 m and stop: 5.24 m/s=12 mph. If the collision is perfectly inelastic, the speed of car #3 at the instant of impact is about 9.31 m/s=21 mi/hr. If the collision is perfectly elastic, the speed of car #3 at the instant of impact is about 4.65 m/s=10 mph. I would guess that the collision would be more like the inelastic collision, that is, the speed would be closer to 20 mph. Of course, the speed of the middle car could have been greater than my minimum of 12 mph because it was probably moving when it hit the front car; so, to make a better estimate one needs to estimate the speed it had on the second impact. My final estimate based on limited data would be the speed of car #3 on impact was greater than 20 mph.

ADDED THOUGHT: 
If you are the driver of car #3, it is my impression that you are at fault regardless of anything. It is the responsibility of all drivers to be able to stop to avoid hitting another car at rest.


QUESTION: 
If you are driving a car going 60 mph and you shine a beam of light from your car why isnt the light moving at a speed of the speed of light plus 60? The driver would measure the speed of the beam at the speed of light, but I'm pretty sure that to an observer the speed of that beam of light is moving 60mph faster than the speed of light. So technically, since the observer is at rest, isnt the beam traveling faster than the speed of light?

ANSWER: 
See earlier answers.

QUESTION: 
A jet plane is traveling 1000 mph to the West, a gun mounted on the airplane facing East fires a bullet with a muzzle velocity of 1000 mph, does the bullet travel in either direction, East of West, or fall straight to the ground?

ANSWER: 
To an observer on the airplane the bullet has a speed of 1000 mph east. To an observer on the ground, the bullet has zero velocity and, as you say, falls straight down.

QUESTION: 
the pressure at the bottom of earth's atmosphere is about 100,000 N/m squared. this means there is a force of 100,000N acting on every square meter of area! your body has about 1.5 square meters of surface.why arent you crushed by the atmosphere?

ANSWER: 
Because we evolved in this environment and therefore there is a balancing pressure from inside. Every cell has an internal pressure of approximately one atmosphere. Think of a bottle which you have put a cork in—why doesn't it get crushed?

QUESTION: 
I noticed an answer of yours and you made a point that the fastest possible speed is the speed of light. Has this been proven? Or is it still a theory? Also isn't possible that there is some type of particle that we are unable to detect (as of now) that may actually move at a speed greater than the speed of light?

ANSWER: 
This is the result of the special theory of relativity and the special theory of relativity has been shown to be correct in more experiments than you can imagine. The "speed limit" is clearly evident in our biggest accelerators where we push and push on the accelerated particles and they keep gaining energy but do not gain any significant speed once they get close to the speed of light; we can get them to 0.9999999 the speed of light, but never to it. There have been speculations about there being particles going faster than the speed of light (dubbed tachyons). But, "you can't get there from here", that is they have to have always been there because it takes an infinite amount of energy to accelerate something to the speed of light, let alone beyond.

QUESTION: 
I am not sure if this is a "highly technical question" or not. My question involves the relationship between a photon's energy, frequency, and wavelength. I am having trouble understanding what a photon's "wavelength" is exactly! I know that the energy of a photon is E = hc/lambda, where the frequency = c/lambda. When an electron falls from the conduction band back down to the valence band, a photon is emitted. If frequency = c/lambda, then what determines wavelength? Is it the distance that the electron falls between energy gaps? Is it wrong to view "frequency" as vibration or rotation of the photon in this case?

ANSWER: 
This is the kind of conundrum you can get into when dealing with wave/particle duality. A photon of a given energy and momentum has a one to one correspondence with electromagnetic waves of a given wavelength and frequency. Energy and momentum of a photon are related as E=pc; frequency and wavelength of a wave are related as f=c/λ; the two are connected by E=hf. I think it is a mistake to try to visualize the frequency or wavelength of a photon.

QUESTION: 
I am an 8th-grade science teacher. We are teaching chemistry now and taught physics earlier this year. I did a small demonstration with dropping blue dye (food coloring) into water and watching the diffusion. We connect this back to physics by asking students to think about the dye molecules and what needs to happen for them to accelerate horizontally (there must be a force, Newton's 1st law) and then talk about where this force comes from -- the random motion of the water molecules. So we are using the physics the students know to introduce the basics of kinetic molecular theory. The blue dye also moves vertically and in cold water this motion is much faster than the horizontal diffusion. I don't think the density is substantially different and indeed the dye does not settle to the bottom, it diffuses throughout. So we talked about gravity as a force acting on the dye molecules. But one of my most astute students quite reasonably said, if gravity is an unbalanced force acting on the dye molecules and causing them to accelerate toward the bottom of the beaker, then why wouldn't there also be unbalanced force on the water molecules, which doesn't seem to be the case? So here is the question: Is it in fact gravity that immediately disperses dye vertically when dropped into water, or is it just the momentum of the falling drop? In general what are the forces on the molecules in a liquid that is "still" (not moving)? How do the forces from random molecular collisions relate to gravitational downward pull -- is one much stronger than the other, say for water at room temperature? Since the molecules are not accelerating downward, at least on average, then what force opposes gravity? Would you call it buoyant force when thinking about this at a molecular level? I would think not, the molecules aren't really floating. But it is not normal force either. Can you clarify?

ANSWER: 
So, I tried this experiment myself. I believe that what happens is that in cold water the drop of dye keeps its integrity long enough to sink just like anything more dense than water would. As it falls, the influence of the moving water molecules causes this drop to "bloom" and, as it spreads out, its fall becomes less dramatic. In the hot water, this "blooming" just happens quicker before the drop can fall very far. Certainly, gravity acts on all the molecules, water included. However, water is essentially incompressible and so the molecules can't all fall to the bottom of the jar; the effect of gravity is that the pressure in the water gets larger as you go deeper. When I did my experiment, carefully letting the glass of cold water sit to still all currents, the dye did initially have a large fraction settle to the bottom. In the end, though, the molecular bombardment did redistribute it throughout the whole volume.

QUESTION: 
I need to know what is the mass of one milliliter of hydrogen protons! I need to know the answer in mass and show my work! I'm lost

ANSWER: 
You can estimate the volume of a proton by assuming it is a sphere of radius 10-15 m. Then, looking up the mass of a proton you can get an estimate of its density, mass divided by volume. Now that you have the density, you can get the mass of any volume (like a milliliter).

QUESTION: 
I think I correctly understand the idea of the following but not the actual physics of it... If there's MORE light going out a window than there is going in, I will see a reflection of what's on my side and if there's LESS light going out the window than there is going in, I will see what's on the other side. Why is this?

ANSWER: 
I do not know where you got this from, but it is wrong. Light striking the window from either side will be partly reflected and partly transmitted. If there is much more light coming in from outside than there is reflected inside, you will not notice the reflection but it is still there.

QUESTION: 
How can we find the initial velocity of a marble when we have the angle it's launched at, the final distance traveled, and the mass of the marble (special equations might be helpful) Please help us! We are trying to calculate the velocity of a marble with a different mass.

ANSWER: 
The short answer is that, if air friction is negligible, it makes no difference what the mass is. Ignoring air friction, we can write the equations for the x and y positions of the marble as a function of time t (assuming that x=y=0 at t=0). Here, x is the horizontal position and y is the vertical position. These equations also include the angle θ the initital velocity v0 makes with the horizontal and the acceleration due to gravity g (9.8 m/s2 or 32 ft/s2).
x=v0tcosθ
y=v0tsinθ-˝gt2
Now, if you put in y=0 (assuming the marble came back to the altitude it started) and x=d (distance traveled), you can eliminate t and solve for v0:
v0=√[gR/(2
cosθsinθ)].
Note that this is all independent of mass. The reason that mass does not matter: as Galileo himself discovered, all objects have the same gravitational acceleration (neglecting air friction).

QUESTION: 
I am trying to help my 10 yr old son with a project. He is an avid hockey player, he wanted me to help him figure out the force (easiest explained in lbs or kg) of a hockey puck hitting a goalie at various speeds. I suggested we look at Newton's second Law, F=ma. I am having a problem with units, though for a 6 oz puck, my results suggest 10 mph = 3.75 lbs, 40 mph = 15 lbs, 80 mph= 30 lbs etc. Am I on the right track?

ANSWER: 
I don't know how you got your answers, but it is impossible to get any answer by simply knowing the mass and the speed because the speed does not determine the acceleration (which is rate of change of speed). So you need to know or approximate the either the time it takes the puck to stop or the distance the puck moves before stopping. Then you can calculate the average force over that time or distance. This should be evident because if the goalie wears a thick foam chest protector (which causes the time or distance to be bigger than if he uses a hard shield), he will feel much less force. And, working in ounces and pounds is really hard—the English system just does not lend itself very well to doing this kind of calculation. So, let's do a simple calculation. The mass of the puck is 0.17 kg. Suppose that the puck stops after the chest protector has compressed by about an amount d=2 cm=0.02 m (a little less than an inch). A 20 mph puck has a speed of about 8.94 m/s. To calculate the acceleration a, use a=(v2/(2d))=8.942/0.04=2000 m/s2. Now that you have the acceleration and the mass you can calculate the force: F=ma=0.17x2000=340 N=76 lb. The time it took to stop was about t=v/a=8.94/2000=0.0045 s=4.5 microseconds. Keep in mind that I have just guessed at the distance to stop. Note that the acceleration is proportional to v2 so, assuming d stays the same (which it might not) the force at 80 mph would be 4 times bigger than that at 40 mph. (Incidentally, if you want to be able to easily convert back and forth between different units, I recommend a nice little free converter called, appropriately, Convert.)

QUESTION: 
What actually puts force on electrons to cause them to accelerate in a complete circuit?

ANSWER: 
By causing there to be a potential difference across the ends of the circuit (by a battery, for example), an electric field is established in the conductor. This electric field causes electrons to experience a force in the opposite direction of the field (because they are negatively charged). This force wants to accelerate the electrons, but they accelerate only a short distance and then collide with one of the atoms in the conductor. Then this happens all over again, and again, and again, etc. The net result is, on average, a slow drift of electrons with constant speed opposite the direction of the field.

QUESTION: 
What happens if one section of a complete circuit is replaced with a good insulator? I know that in insulators, electrons are more tightly connected to individual atoms, so would the circuit slow down?

ANSWER: 
What do you mean by "circuit slow down?" The total current in the circuit would get smaller because you have increased the resistance of the circuit by replacing a piece of conductor (low resistance) by a piece of insulator (high resistance).

QUESTION: 
I have a seemly simple question about conservation of energy (but I am confused). This question pertains to raising an object up and then letting it fall. I understand that it costs energy to raise an object to some height “h” which gives the object gravitational potential energy. Now let it be released and accelerated back toward the ground. The universe does work to accelerate the object back to z = 0. There is a force times a distance so work is done. So if a human does work to get the object up, and the universe does equal work to bring it back down, that should equal zero, where does all the energy come from to “smash” the object into the ground (assuming its broken into pieces which happens all the time). I’ve asked and they say, “no” is the kinetic energy which does the smashing. I’m not too worried what we call the energy, it took E to get it up, - E to get it down, then Es to do the smashing, which may get converted into heat, etc. It seems people ignore the energy the universe spent to bring the object back down considering that to be free. In the reverse direction it makes more sense. Drop and “bouncy ball” from some height “h,” and it can (in theory) bounce right back to height “h” having zero velocity. But the ball doesn’t just explode at the top which would violate conservation of energy.

ANSWER: 
You are right, you do work E to get it up and, when you drop it, you get all that energy back just before it hits the ground. Energy is not conserved going up (you are adding it) but is conserved going down. What happens when it hits the ground? Let us take a simpler example first, where all it does is stop; maybe it is just a ball of putty. It has energy (kinetic) one moment and not the next. In the meantime there was another force doing work, the force the floor exerts on the ball. This force takes all the energy away from the ball (it must do negative work, right?). And, if the ball is putty, you cannot get the removed energy back. If, however, the ball is "bouncy", the compressed ball will uncompress and, as it does that, the lost kinetic energy is restored; the floor does positive work on the ball during this time. So, the energy to "smash" say a glass ball comes from the work which the floor does on it; this is evident because if you take away the floor, the ball will not smash (or squish or compress).

QUESTION: 
If an object is held stationary above the ground at a height of 10 cm, would work be done on it or not? I told my dad that work would be done because the object possesses gravitational potential energy but my father said that work would not be done since the object is stationary and work is force multiplied by displacement.

ANSWER: 
Both and neither of you are correct. If the object is at rest, no work is being done on it. If it was previously at a different height, say the ground, work was done on it to raise it; that is where the potential energy came from.

QUESTION: 
Does the statement "a ball dropped from the top of a building increases in speed until it hits the ground" violate the law of conservation of energy?

ANSWER: 
The law of conservation of energy is not a universal law, it is only true under certain situations. If you define a conservative system as one in which no external forces do work, then the total energy of that system remains constant. In the case of the ball, it is not a conservative system (the force the earth exerts on it, its weight, does work) and so you do not expect its energy to remain constant; hence its kinetic energy increases as it falls. If you are clever, though, in many cases like this you can introduce a potential energy function which is a trick to take a previously external force and internalize it. So, as the ball falls its kinetic energy increases but its potential energy decreases and the sum of the two remains constant. So, in this case the energy of the ball is either conserved or not, depending on how you define energy. But the case where it is not conserved is not an instance of "violation" or the law because the law ought not apply to that case.

QUESTION: 
When a body is being rotated in a circle by applying centripetal force, why doesn't it come towards the center of the circle since centripetal force acts towards the center of the circle?

ANSWER: 
When a force acts on an object in a direction perpendicular to the object's direction of velocity, it causes the direction of the velocity to change but not its magnitude. In the case of a centripetal force, the result is that the object moves in a circle with constant speed, constantly changing its direction so that it is always moving tangent to the circle.

QUESTION: 
Just a quick question, Im a biologist and no virtually nothing of physics. There is currently a lot of debate here in the UK on the efficacy of homeopathic "medicines". According to homeopathists a 1M solution serially diluted 10^-50 (i.e. more than Avagadros constant) in water will somehow inprint a memory of the diluted molecule in the water. The pro homeopath lobby are trying to explain this with Quantum mechanics, is there any scientific basis for this? an example:
 "From what I've read I think that Werner Heisenberg's theory of energy-time indeterminacy and Erwin Schrödinger's thoughts on there being many indeterminate states possible until a conscious observation is made are the most fitting regarding homeopathic efficacy. These try to address the curious "tunneling" of electrons into unexpected areas of space, and the "wave function" of particles which are said to "collapse" into a specific state due to the act of being observed."

ANSWER: 
In my opinion, this kind of statement is total nonsense. There is no basis in physics or chemistry to support the claims of homeopathy that somehow water molecules have a "memory" of previously dissolved chemicals. And, if it were so, what about all the other substances which must have been previously dissolved in the history of the water?

QUESTION: 
We were discussing in a biochemistry class about atoms, how is it if you have atoms in your hand, and a table has atoms, Why doesn't your hand go through the table?

ANSWER: 
This is one of the FAQs.

QUESTION: 
Two balloons that have the same weight and volume are filled with equal amounts of helium. One is rigid and the other is free to expand as the pressure outside decreases. When released, which will rise higher? Why?

ANSWER: 
On each balloon there are two forces—its weight (which makes it want to fall) and the buoyant force on it (which makes it want to rise). The weights are the same and never change. The buoyant force is proportional to the volume, so whichever has the greater volume has the greater buoyant force. The expanding balloon wins.

QUESTION: 
I recently found out that a bottle full of helium gas will weigh less than a bottle full of air. But would the helium bottle now weigh more or less than a bottle with the air pumped out of it?

ANSWER: 
There is also the proviso that the pressures and temperatures of the two gasses must be the same; for example, you could have the helium bottle weigh more if you put enough helium in it. If you now evacuate the air bottle, it will weigh less.

QUESTION: 
Does the statement "a block sliding freely on level ice increases in speed until it hits a wall" violate the law of conservation of energy? Why or why not?

ANSWER: 
It certainly does. If the ice is level and there are no external forces on the ball which do work, where does the increased kinetic energy come from?

QUESTION: 
I'v always thought of a light wave as coming towards me going up/down or left/right is it true that it is actually spinning in a circle as it goes along

ANSWER: 
Any of those are possible. What you are talking about is called the polarization of the wave and, depending on the source or the preparation, several polarizations are possible. in all cases, the electric and magnetic fields are perpendicular to the direction of travel.

QUESTION: 
we were discussing time travel in class and learned that it is theoretically possible. We were thinking about this and wondered if it is possible to actually perform an experiment to prove this. Have there been any experiments that demonstrate this phenomena. Will traveling at great speeds actually cause a temporal shift? Also, does going back in time and "changing the past" affect present events or is it impossible to affect the present. We are aware of the "paradox of time travel" (going back and killing Mozart...would his music cease to exist?....or....going back and preventing your parents from uniting, would you cease to exist?)

ANSWER: 
Here is what physics has to say about time travel: it is entirely possible to travel forward in time but impossible to travel back in time. In the theory of relativity, moving clocks run slow which is what allows forward time travel; see my earlier discussion of the twin paradox. Moving clocks running slow (called time dilation) has been verified experimentally in many ways. Since, as far as we know from physics, backward time travel is not possible, we do not need to worry about the kinds of paradoxes you ask about.

QUESTION: 
we were discussing rational and irrational numbers in math class one day. First of all, we were told that the numbers in an irrational number will never show a repeating pattern. We believe that eventually it will have to repeat. Our teacher told us that experiments were done with "pi" that showed no repeating pattern for a huge amount of decimal places. We are convinced though, that the digits will have to repeat at some point. What do you think?

ANSWER: 
First, I think I am a physicist, not a mathematician! But let me opine that your "belief" is entirely unscientific. I suspect that there is a quite rigorous proof that there is no repeating pattern. You need to have a basis for making such statements other than "we believe".

QUESTION: 
We were wondering about the "Planck Distance." First of all, is it true that this is the shortest distance possible? Secondly, if that is true, wouldn't the reciprocal of the Planck Distance be the greatest distance possible?

ANSWER: 
See an earlier answer. The flaw with your hypothesis about the reciprocal being the largest possible distance is that it would be dimensionally incorrect, that is its units would not be meters but rather meters-1.

QUESTION: 
Ok you are on the north end of a south bound plane going 300 miles an hour and you throw a baseball forward at 95 miles an hour. So at what speed is the baseball traveling????

ANSWER: 
You must specify who is observing the baseball. The speed relative to you is 95 mph, relative to the ground is 395 mph.

QUESTION: 
Can you please tell me what if any visible changes to the structure and composition of wood will result when electro magnetic energy passes through it or near it?

ANSWER: 
There is no single answer to this. For example, a powerful laser is em energy and it would burn the wood. But radio waves usually have no affect on the chemistry. Over long periods, visible light can bleach or fade the color of wood.

QUESTION: 
I dropped a bowling ball and ping pong ball from high above the floor. They hit the ground at the same time. A student asked about a balloon I also had. In that match, the ping pong ball easily bested the balloon (~8inch round). Why? Both are round, lightweight and filled with a gas. Is it aerodynamics/ air resistance? Smoothness of surface? I don't know what to tell them. Please Help!

ANSWER: 
In the absence of air, all will fall together. But, air friction is clearly not negligible for a balloon. Air friction depends on the geometry (both are round, so that is not an important factor), cross sectional area (balloon has a bigger area), and weight (ping pong ball is heavier, probably). The area here is probably the main factor; if you were jumping out of an airplane, would you want a parachute of diameter 5 m or one of diameter 5 cm? To get a little more quantitative, the terminal velocity, the maximum speed v achieved by something falling through a fluid of density ρ, is v=[2mg/(ρACp)]˝ where mg is the weight, A the cross sectional area, and Cp a constant which is determined by the shape. You link to a bunch of other answers on air friction on the FAQ page.

QUESTION: 
A 100,000 N car is raised a distance of 5 m by an effort force of 500 N. (consider this an ideal frictionless situation) I have to solve for work output and work input, don't I need to know another force to do this? Out of school too long.

ANSWER: 
The only way that you can lift a 100,000 N car with a 500 N force is on an incline. Determine the angle where the component of the weight along the incline is 500 N and then how far you would have to pull up the incline to lift it 5 m. I have no idea what is meant by "input" and "output" here. One would simply calculate the work done by the 500 N force, 500x(distance along incline). That is the concept. There is an easier way. Once the car is 5 m up it has an increased potential energy of mgy=500,000 J which must be the amount of work done on it. So, you see, not only do you not need another force, you do not need the 500 N force either!

QUESTION: 
why isotopes are same in chemical properties and different in physical properties ?

ANSWER: 
Because the chemical properties are determined by the electron structure, not the nuclear structure. So, changing the number of neutrons in a nucleus has almost no effect on the electron structure which is determined by the charge of the nucleus, not its mass.

QUESTION: 
if the repulsion force between two protons in nucleus of iron atom is big value,the nucleus of iron is not destroyed?why?

ANSWER: 
Because the Coulomb force is not the only force acting. The nuclear force, also called the strong interaction, is an attractive force which is much stronger than the repulsive Coulomb force at close distances.

QUESTION: 
Suppose an object tied to a string is being rotated. The string applies a centripetal force on the object. The object exerts a centrifugal force on the string as a reaction in accordance with Newton's third law.My question is that why we consider centrifugal force to be a fictitious force ?

ANSWER: 
The object does indeed exert a force on the string. But this is not what is called a centrifugal force, it is what is called the force of the object on the string and exists because of Newton's third law. The thing referred to as a centrifugal force is a force which seems to be on the object but is not really there. See my earlier discussion of centrifugal forces.

QUESTION: 
Here's a relativity question I've been losing sleep over. If you're on an object that's traveling slightly slower than the speed of light and you fire a rifle, why does the bullet not exceed the speed of light?

ANSWER: 
This has been discussed in several earlier answers; see this one for the most detail. In essence, it is that things just do not behave as they do in classical physics when traveling at very large speeds. Although you would see the bullet traveling with its usual muzzle velocity relative to you, an observer at rest would not see the speed of the bullet as the speed of you plus the muzzle velocity of the bullet. Try to get some sleep.

QUESTION: 
Hello, I was at a baseball game and my friend had a chance to catch a ball that came into the crowd. Being into Physics myself, I've always wondered to catch it safely, should you move your hands toward the ball, hold them still, or move them in the same direction as the moving ball?

ANSWER: 
Move them in the same direction as the ball. This allows you to maximize the time during which you are stopping the ball thereby giving it as little acceleration (the rate of slowing down) as possible. Acceleration of the ball matters because of Newton's second law which says the force you must apply to stop the ball is proportional to the acceleration it has. Hence if you minimize the acceleration you minimize the force you exert on it; because of Newton's third law, if you exert a force on the ball, it exerts an equal and opposite force on you. So you will also minimize the force (which is what hurts) on you.

QUESTION: 
I know that to measure the half life of a radioactive element you just take a certain amount of it and count the rate at which the decay products change over time. But how do you measure the half life of neutrons where you do not have a pool of neutrons upon which to base your measurements as you cannot isolate a group of them?

ANSWER: 
The difficulty of measuring you note is one of the reasons that the half life of the neutron has a larger uncertainty than some radioactive nucleus. There are, however, copious amounts of neutrons in reactors and that is where such measurements are made. Hence one must deal with beams of neutrons, not simply a box of them.

QUESTION: 
In a newton's cradle(which has usually 8 bobs or balls)if i let 5 balls bang on the other 3 balls then after collision 5 balls would move on the other side.Why 5 balls are moving and not three balls?

ANSWER: 
Both energy (˝Σmv2) and linear momentum (Σmv) must be conserved where Σm is the mass. Suppose that 5 come in and 3 go out. Then momentum conservation says 3m0u=5m0v (where v is the speed of the balls coming in, u is the speed of the 3 balls going out, and m0 is the mass of one ball). So, u=(5/3)v. So now, the energy of the incoming balls is ˝(5m)v2=2.5v2 and the energy of the outgoing balls is ˝(3m)u2=˝(3m)((5/3)v)2=4.17v2; so energy would not be conserved. The only way to have both energy and momentum conserved is to have the same number of balls going out as went in.

QUESTION: 
Today in my physics class my teacher explained how a car being pulled by two ropes each having a force of 700 newtons could equal a magnitude of 1000 newtons, he said it had to do with mass and acceleration, i didn't quite understand this though, could you please explain this to me?

ANSWER: 
Actually, it has nothing to do with mass and acceleration, it is just addition of vectors. Two guys pulling on two ropes attached to a car, one pulling south with 700 N and the other pulling north with 700 N: no net force, zero. Two guys pulling on two ropes attached to a car, both pulling south with 700 N: a net force of 1400 N south. Two guys pulling on two ropes attached to a car, one pulling southeast with 700 N and the other pulling southwest with 700 N: a net force of 990 N south. In fact, their net force can be anything between 0 and 1400 N.

QUESTION: 
An object is pulled at a constant F. KE0+PE0=0, so W=PE+KE. If there were no friction, the slope of a graph (KE+PE=Y-axis, W=X-axis) would be 1 and the y-intercept would be 0. What would the addition of friction do to the slope and y intercept (would the y intercept be more, less, or equal to 0 and the slope more, less, or equal to 1)

ANSWER: 
The energy would still be equal to the work done assuming that the energy of the system is zero at the beginning of the experiment. So, even though the total work being done is both by F and the friction, the graph you describe would still have a slope 1 and an intercept 0.

QUESTION: 
If a charged particle passes close by me, I will experience a magnetic field because of the electric current that the moving charged particle represents. I will also experience a magnetic field because as the charged particle approaches me and then receeds away from me, the electric field stength will change in an inverse square of the distance between myself and the particle. So do I experience two magnetic fileds combined ? or is the magnetic field from the current the exact same thing as the magnetic field from the change in electric field, just by way of a different explanation ? What if the charge is stationary and it's me that's moving ?

ANSWER: 
You experience an electromagnetic field. The sources of the magnetic field you experience are both the current density and the time varying electric field. Similarly, the electric field you experience is both from the charge and the time varying magnetic field.

QUESTION: 
According to the twins paradox [Relativity], one of the twins who goes on a space travel at a speed close to that of light, is much younger that his brother who stays back in Earth. Does that mean travelling at a speed close to that of light slows down biological processes also, as aging is a biological process? No virus found in this incoming message.

ANSWER: 
All clocks, including biological clocks, slow down.

QUESTION: 
I have read about a theorized elementary particle called the graviton. As I understand, or think i understand, gravity is not a force at all but a result of the warping of spacetime. Am I wrong?

ANSWER: 
This has been answered before. See FAQ question.

QUESTION: 
Can the size of a photon be measured and if so, is it bigger or smaller than the electron and positron it is said to be changed into under the proper conditions? Or is the electron emitted larger than the photon from whence it came?

ANSWER: 
See an earlier answer.

QUESTION: 
a vertical conducting sheet is permitted to fall under the action of gravity between the poles of a powerful permanent magnet. is the motion of the sheet affected by the presence of the magnet? explain.

ANSWER: 
Yes. Eddy currents are induced in the sheet which experience a force (as all currents do) from the magnet. This is used as a means of braking sometimes.

QUESTION: 
What will happen scientifically on 21 dec, 2012?

ANSWER: 
See earlier answer.

QUESTION: 
I’m having a problem with the issue of gravity and force and with the issue of what laws of the universe in which I should use. When it comes to the issues of gravity and force, should Newtonian physics be used or should Einstein’s general theory of relativity be used? (Main Question I Want Answered: Are the matter of which the inner planets, such as Earth, Venus, and Mercury made of pushed together by Einstein’s space-time curvature or are the matter of which the inner planets, such as Earth, Venus, and Mercury made of pulled together by gravity, which is described as weight or mass times gravity?)

ANSWER: 
Newton's universal law of gravitation is what is called an empirical law: it is merely a statement of experimental facts, a mathematical expression of how nature works. What it says is that the force between two objects is proportional to the product of their masses and inversely proportional to the square of their separation. It says nothing about why this force exists or how it is caused. General relativity, the warping of space time by gravitational mass, addresses the why and how questions. For most practical applications like celestial mechanics (calculating how planets move, for example), classical mechanics is just fine. In fact, however, Newtonian gravity is not exactly correct; for example, it fails to predict that light is bent by gravity or that clocks run at different rates in different locations in a gravitational field. One can make corrections for general relativity, but they are usually very small.


QUESTION: 
According to my understanding of mass and gravity, two objects (in a frictionless environment) fall at the same speed regardless of their relative mass. For example, a basketball and a bowling ball dropped from the same height would hit the ground at the same time. My question to you is this: what if you held an object that had the same mass as the earth over the earth? Would that object hit the earth at the same duration as a basketball dropped from the same distance from the hearth? Or would the gravitational pull of the earth-size object pull the earth toward it and half the time they hit each other relative to the basketball?

ANSWER: 
Let's talk about what happens. The earth exerts a force on the object which causes it to accelerate toward the earth; because of Newton's third law, the object exerts an equal and opposite force on the earth which causes it to accelerate toward the object. As long as we are talking about objects (like your bowling ball and basketball) which have a mass much smaller than the mass of the earth, the earth, for all intents and purposes, does not actually measurably accelerate toward the ball (because its mass m is big and the force F is small so the acceleratation, F/m, is exceedingly tiny). If you had an object the mass of the earth but the size of a basketball, the earth would accelerate up to meet it and they would meet halfway between. If, however, it was also the size of the earth, it would be a much more complicated problem because the acceleration due to gravity decreases as you go farther away; at a distance one earth radius above the earth's surface, the acceleration due to gravity would be only g/4.


QUESTION: 
Suppose two unlike charges are executing simple harmonic motion with no phase difference b/w them. Will there be a phase difference b/w the EM waves produced by the two charges?

ANSWER: 
The electromagnetic waves created by each would be 1800 out of phase because the difference in the charges. Hence, if you were far away (far compared to the distance between them) from these two oscillating charges, you would see no EM waves at all.


QUESTION: 
My teacher says we are able to stand on the ground because of newton's third law i.e. the reaction from the ground balances our weight but we are unable to stand on water or a pool because there is less reaction to balance F=mg.But I don't agree with this.What newton law says is that there must be an equal and opposite force and hence even in the case of standing in water there must be an equal reaction and hence we must stand on water or a pool.But this is not the case.So I am confused with this.Please help me and tell me the reason why we are able to stand on the ground and not on the pool.

ANSWER: 
I am sorry but your teacher is not right. The force that the ground exerts on you is not a "reaction force" to your weight. It is a reaction force to the force which your feet exert down on the floor. The reaction force to your weight is the force which you exert on the whole earth because your weight is the force which the whole earth exerts on you. Let us get clear what Newton's third law says: if body A exerts a force on body B, then body B exerts an equal and opposite force on body A. Newton's third law never refers to only forces on a single body, only to forces on two bodies. So, on you, there are only two forces, that which the earth exerts down on you and that which the floor exerts up on you. The reason that you are in equilibrium is because of Newton's first law: if you are at rest, the sum of the forces on you must be zero. Therefore, since you are at rest, the force from the floor must be equal and opposite to your weight. Now suppose that the floor were made of tissue paper. The floor would be incapable of exerting an upward force equal to your weight and so you would not remain in equilibrium. Now suppose that the floor is the surface of a lake. The surface of the lake is apparently incapable of exerting the requisite upward force.


QUESTION: 
We've learned in class that there are several ways in which one can experience weightlessness: during free fall, like jumping off a cliff, in orbit about a planet, going over a hill on a roller coaster or someplace deep in space where the force of gravity is very weak. There is one other place you can be and experience weightlessness. The hint was you don’t have to leave the Earth to find it, but it exists on other planets as well.

ANSWER: 
As I always do, I should first remind you that your weight is the force exerted on you by the earth; therefore you are not really weightless during free fall, roller coaster, or in orbit, it just seems that way. Only the situation where you are free of gravitational forces are you actually weightless. I am not sure what your instructor is looking for, but if you could place yourself at the center of the earth you would be truly weightless because there would be no gravitational force on you.


QUESTION: 
why gravitational effects are ignored when considering motion of electrons in electric fields

ANSWER: 
Because gravitational forces are insignificantly tiny compared to electric forces. Example: an electron a distance of any distance r from a proton. Gravitational force is Fg=MpMeG/r2 and electrostatic force is Fe=ke2/r2. Hence Fg/Fe=MpMeG/ke2=1.7x10-27x9x10-31x6.67x10-11/(9x109x(1.6x10-16)2)=4.4x10-46.


QUESTION: 
How fast is gravity communicated to the object it is pulling on? For example, I know that the light from the sun takes around 8 minutes to get to the earth. If the sun were to magically "blip" out of existence, would the earth start to fly out of orbit instantaneously, or would it take 8 minutes before it stops being affected by the sun's gravity? Or is there another answer?

ANSWER: 
I have previously answered this question.


QUESTION: 
Suppose I had an airtight cylinder filled with water that extended to space. If I placed a bouyant object in the cylinder, would bouyant force propel the object to the top of the cylinder (into space)?

ANSWER: 
I don't see why not.


QUESTION: 
Some years ago I have seen an experimental device like a bulb with a shaft in it. This shaft could rotate. When I sent a light beam onto the shaft, it started rotating. Can you explain this? What is the name and where I can find such a device?

ANSWER: 
Maybe what you are talking about is a Crookes radiometer. There are lots of places you can buy one; just Google radiometer or physics toys.


QUESTION: 
Why does acceleration due to gravity always remain constant regardless of mass of object falling on earth ????

ANSWER: 
See an earlier answer.


QUESTION: 
i know that nothing can travel at or faster than the speed of light. but, just simply why? what equations or whatever says no...

ANSWER: 
Because the mass of an object, that is its inertia, increases as the velocity increases. Therefore it gets harder and harder to accelerate it as it goes faster and faster. The expression for the mass of an object m as a function of its velocity v is m=m0/√(1-(v2/c2)) where c is the speed of light and m0 is the mass when it is at rest. Note that as v approaches c, m approaches ∞ so it is impossible to push beyond c. Another way to look at it is from the perspective of energy. The energy of a particle is E=mc2=m0c2/√(1-(v2/c2)), so the energy required to accelerate the mass to the speed of light is infinite and there is not an inifinite amount of energy in the universe.


QUESTION: 
I know that energy is stored in Electric field b/w plates of a capacitor, Is there any energy stored in Electric field of an isolated charge? If yes then where does this energy comes from?

ANSWER: 
There are sophisticated mathematical proofs which establish energy density, momentum density, etc. of electric and magnetic fields. But let us be a little less technical. Imagine two point charges on top of each other, one positive and one negative. There is a net charge of zero so there is zero electric field. Now, move them apart and an electric field appears. If you move the negative charge very far away you will eventually be left with the field of the positive point charge and, as you note, this field has energy. But you had to do work on the system in order to move the negative charge away and that work may be thought to now reside in the field. I just made that up, but the point is that it takes energy to create any electric field.


QUESTION: 
I am curious if the earths electro magnetic field has any impact on the moons rotation around earth. if so, what effect? I am under the impression that the earths magnetic field shifts frequently between polarities. if this is so, then would there be any effect on the orbit of the moon either immediately or long term? to summarize further, does the Em force effect G or vise versa? what about strong nuclear and weak nuclear forces? where do these forces come in contact with each other in such a scenario if at all?

ANSWER: 
As far as we know, electromagnetism and gravity are independent and do not affect each other. There is no known effect of the earth's magnetic field on the moon.


QUESTION: 
With the theory that the universe started from a single point and thus expanded is there any thought as to where that single point came from it seems the so called 'answer to everything theory' starts just after the creation of the universe, for me if it is agreed the universe started from a singularity then where did it come from.

ANSWER: 
This is answered on my FAQ page.


QUESTION: 
How are electrostatic fields set up in space? I believe I understand how to interpret such fields. What I am puzzled by is how they originate or establish themselves, and how they are maintained. Does the boundary of the field propagate at the speed of light? Assuming it does, what is propagated? A wave? Or is it a pulse, as in a 'nothing then something' pulse? Are there particles like photons associated with this propagation? For example, consider the field for a large charged flat sheet. It has the same strength at any distance (less than say the smallest dimension of the sheet), but it must set up some how? I'm puzzled by how. Then once established, what is established? Does a electrostatic field change a region of space for charges that find themselves in that region of space? Also, the presence of a single charge will impact any number of charges that enter the field (superposition), yet its effect on any one charge is not diluted by the presence of others.

ANSWER: 
Think of a crack starting at one edge of a frozen lake and propogating across the lake. That is essentially what electric and magnetic fields do but they propogate at the speed of light. So, if you suddenly create an electric charge, the field takes time to establish itself; so, if you were 300 m from where the charge was created, you would not see a field from that charge for 3x102 m/3x108 m/s= 10-6 s=1 μs. To answer the "…once established, what is established" question, a field at some point in space will simply result in an electric charge Q being placed there experiencing a force QE in the direction of the vector E. Now, are they simple mathematical constructs to help us visualize forces or, as you ask, do they actually exist in the space where we visualize them? I believe that the view that they are just a construct is wrong because an electric field has an energy density, so energy resides where electric or magnetic fields reside.


QUESTION: 
Why does the mass of a particle increase as its speed approaches the speed of light? And how does this relate to the equation E=mc2?

ANSWER: 
The reason that mass increases is discussed in an earlier answer. In a nutshell, it is because we need to redefine linear momentum (mass times velocity in classical physics) so that momentum is conserved for an isolated system. This results in mass increasing as speed increases but faster than the rate the speed is increasing. The mass may be written as m=m0/√(1-v2/c2). Since this quantity becomes infinite as v approaches c, the energy required to accelerate something to the speed of light is infinite, obviously impossible. The total energy of a particle with speed v is mc2 where m is as given above. See another earlier answer for more detail on E=mc2.


QUESTION: 
why does earth has magnetic poles? is it just a point or certain area? what caused it to occur?

ANSWER: 
The origin of the earth's magnetic field is rather complicated. Essentially there is a portion of the earth's core which is molten iron which is ionized. The magnetic field is generated by convective motion of the electric charges in this ionized liquid and energy is supplied by various sources including radioactive decay of heavy elements. I recommend that you read the Wikepedia article on dynamo theory.

FOLLOWUP QUESTION: 
well i know about dynamo convection theory... but dont know much about magnetic poles... how and why does the magnetic poles exist?

ANSWER: 
Well, a magnetic pole is actually more or less a qualitative construct. If you have a region of space where magnetic field lines are coming out it is a north pole; if the field lines are going in it is called a south pole. Actually, the earth's north geographic pole is a south magnetic pole and vice versa. So, these two pictures show the poles of the earth and a bar magnet. The precise location of the earth's magnetic pole is determined by where the field lines come in (or go out) vertically. One important concept about magnetic poles is that they never exist alone, that is you will never find an isolated magnetic pole.


QUESTION: 
If heat causes most materials to expand, why do certain clothes shrink when put in the dryer?

ANSWER: 
Obviously, shrinkage has nothing to do with thermal expansion since the clothes do not return to their original size when they cool down. So, we are really out of the realm of physics. What happens is that cotton contains quite a lot of cellulose and celluose is very good at soaking up water. But if the water is removed very quickly, the cellulose fibers become considerably shorter than they were, causing the whole garment to become smaller. It is usually possible, by rewetting the garment, stretching it, and drying it slowly, to reverse shrinkage.


QUESTION: 
I want to know what actually electricity is? Is it flow of electrons or just the electrons vibrate. If its of wave form then whats the need of free electrons , even electrons in shell may vibrate and pass the energy. And also what exactly is resistance, i thought resistance is obstacle which reduces amount of current as it passes throught it, but i find current remains constant, and instead it affects the circuit as a whole, How ?? And when current remains same then in what form energy is consumed by resistance like when it generates heat or any way ???

ANSWER: 
In a conductor, some electrons are pretty easy to move (called conduction electrons). When a potential difference is applied across the ends of the conductor it causes an electric field in the conductor which, if the electrons were perfectly free, would cause them to accelerate from one end to the other; in that case, all the energy available from the power source would be converted into the kinetic energy of the electrons. However, each electron, when it starts to accelerate, quickly collides with one of the atoms in the material and loses the energy it just gained and the atom gets that energy. So the effect of the electric field on the electrons is start-stop-start-stop-start-stop. There is a net flow of electrons (that's the current) but on average there is no acceleration, just a slow drift of electrons opposite the direction of the field. Each collision gives an atom some added energy so the material heats up due to the motion of the electrons (that's ohmic heating, resistance).


QUESTION: 
Why is "back and forth" considered a different dimension than "side to side"? If you are on a sphere (i.e. Earth), aren't they really the same thing, depending on your perspective? If not, then why isn't "diagonal" considered a different dimension?

ANSWER: 
There is no difference if you are on a sphere which is not rotating. However, if it rotates, you are in an accelerating frame of reference and Newton's laws do not work in accelerating frames. A rotating system is a particularly complicated system to analyze, but we usually do that by introducing "fictitious forces" which, when added, allow us to use Newton's laws; fictitious forces you may have heard of are centrifugal force and coriollis force. These "forces" depend on the direction of the angular velocity of the sphere and also on the direction of the velocity of the particle you are analyzing. I wouldn't say they are "differrent dimensions", rather that the dynamics depends on the direction something moves.


QUESTION: 
I have recently been pondering the potential benefits of quantum entanglement. Basically, after knowing just a little about the phenomenon, I thought that it might be possible to transfer classical information at faster-than-light speed (actually, maybe even at "immediate speed"). After doing a simple thought experiment I confused myself and now I am unclear as to whether or not it is actually possible. The way I had setup my thought experiment is this: 1. Entangle two pairs of particles. 2. Send 1 particle from each pair to Pluto (obviously, this part will take a long time). 3. If you want to communicate a "0" value for a classical bit, measure your two particles on Earth along some axis A. Else, you want to communicate a "1" value for a classical bit, so measure your two particles on Earth along some perpendicular axis B. 4. Now this is the part I am confused on -- is there some way to measure the two particles on Pluto in such a way that results in knowing whether or not a "0" or "1" was intended? I thought maybe you could measure one Pluto particle along axis A and the other Pluto particle along axis B.... In any case, I really just wanted to know if any kind of real faster-than-light communication can be achieved using quantum phenomena.

ANSWER: 
Here is the problem with your scheme: Your measuring device must define a preferred axis. The usual discussion of entanglement assumes the first device makes a measurement and thus "puts" the first particle in either an up or a down state thereby putting the second particle into a down or up respectively state. The second observer, orienting his apparatus similarly to yours (which I will call "vertical" since I have called states up/down) can verify that this is true. But, suppose that the second observer orients his device horizontally instead of vertically. Then his measurement will either put the second particle in either the right or left state, each with 50% probability. If you think carefully about the logic of what I have said, you will see that it is not possible to determine the direction of the spin of the second particle even though you can verify its direction if you know the orientation of the first instrument. Thus, your scheme fails since you cannot tell which direction the second spin points, horizontally or vertically. Thanks to M. M. Duncan who helped me understand this situation.


QUESTION: 
While dealing with induced emf why do we always take the example of a coil ? Is no emf induced in a straight wire? why? Is self induction only the property of a coil and not a straight wire why?

ANSWER: 
Any area through which a changing magnetic flux passes has an induced EMF around its edge. A long straight wire does not define an area and therefore cannot have any flux through it. If a straight wire moves through a magnetic field (with a component of its velocity perpendicular to the field) there will be an induced EMF between its ends because it "sweeps out flux". However, a changing current in a long straight wire can induce an EMF around an area through which the flux passes, for example an area in a plane which the wire is in. A long straight wire cannot have self inductance for the reason stated above: there is no defined area around through which a flux could pass.


QUESTION: 
I have been reading up on the photoelectric effect and I have a couple of questions: (1) I understand that the cutoff frequency of incident light (below which no photoelectrons are emitted) occurs because not enough energy can be imparted to electrons by low-frequency photons for the electron to overcome the work function of the metal. However, could a given electron absorb successive photons and in that way build up enough energy, or will the photon typically be re-emitted before the electron can absorb another? (2) Electrons bound in atoms can only absorb photons with appropriate energies (frequencies). Can free electrons (e.g. in a metal or plasma) absorb photons of any energy?

ANSWER: 
(1) See an earlier answer about multiphoton ionization. (2) Bound electrons can also absorb photons with any energy above the ionization potential (the amount of energy necessary to remove the electron from the atom. Indeed, free electrons can absorb any energy. In a metal where the conduction electrons are essentially free, most if not all the photoelectrons are from conduction electrons.


QUESTION: 
what would happen if earth stopped rotating and if its axis were perpendicular to its orbit

ANSWER: 
The length of a day would become one year, a half year of light and a half year of dark. This would play havoc with weather. If the earth were not rotating, then it would not have an axis.


QUESTION: 
why there is no effect of gravity in the motion of gas particles?

ANSWER: 
There is. If not for gravity, there would be no atmosphere since all gas molecules would just zip off into space. See a recent answer to a related question.


QUESTION: 
I have a few questions relating to matter and antimatter. 1. I know that if matter comes into contact with antimatter, they are both annihilated. What if an antimatter element with a low mass such as hydrogen comes into contact with a matter element with a larger mass such as gold. Would the difference between the electrons and positrons, protons and antiprotons, and neutrons and antineutrons still remain or would the entire atom be annihilated? 2. If the above scenario can happen, what happens to the remaining particles? Will they annihilate with their opposites if they come into contact with an atom? 3. If combining matter and antimatter creates nothing, then could you create matter and antimatter from nothing?

ANSWER: 
First of all, as far as I know hydrogen is the only antimatter atom which has been made; because of the predominance of matter, it is extraordinarily hard to manipulate antiprotons, antineutrons, and positrons. If you did have, say, an oxygen atom and an oxygen "anti-atom", positrons and electrons, neutrons and antineutrons, and protons and antiprotons would interact pairwise. (Other interactions besides "annihilation" occur leaving some residual particles and/or antiparticles; the possibilities are very many.) Regarding "annihilation" (which I take to mean the disappearance of the pair with no mass present after the interaction), you do not have "nothing" afterwards, but a pair of photons; energy must always be conserved and so the energy after the annihilation of each photon is the rest mass energy of an electron (for electron-positron annihilation).


QUESTION: 
My question involves the position of the stars in the night sky. Is there a way of working out the ACTUAL current position of very distant stars and galaxies? I'll explain what I mean: We are told that when we look at the most distant objects in the universe; right at "the edge of the observable universe" then we are seeing those objects as they were 13bn or so years ago. We are presumably seeing where they WERE 13bn years ago too, so where would they be today? In my thought experiments on this subject I imagine objects much further away than they appear and I would also expect them to be shifted in all 3 dimensions. IE if an object appears to be directly in front of me, then it's real position could well be many degrees up and to the right, or possibly even in the complete opposite direction, if it's moving fast enough. Ok, I understand that many of the stars we can see are no longer with us but it would still dramatically change a map of the skies if the 'current' positions of celestial bodies were mapped.

ANSWER: 
There is no way to know where distant stars are "right now" because special relativity forbids information being transmitted at faster than the speed of light.

QUESTION: 
Ok, I understand "knowing exactly" but surely if you know where they were, roughly when it was, what direction they were travelling and how fast, it could be estimated quite accurately I would have thought...

ANSWER: 
If you assume that you understand all the forces which act on those stars (including "dark matter" and "dark energy"), and if you assume you know all forces from its neighbors which you do understand, including those neighbors that you cannot see because they are too far away, you could do a calculation to predict where they would be "today". Don't forget that for the more distant objects you will have to know how to calculate all these forces over a time of billions of years.


QUESTION: 
Are all isotopes of iron magnetic or has nobody ever performed experiments upon the isolated isotopes to find out?

ANSWER: 
Ferromagnetism is an atomic, not nuclear, effect and all isotopes of iron are ferromagnetic.


QUESTION: 
Does gravity effect a magnetic field?

ANSWER: 
Not that we know of, but the relationship (if any) between gravity and electromagnetism is not well understood.


QUESTION: 
Can you settle an argument raging between physics teachers? When a capacitor is charged by a battery in a series R-C circuit, how much energy is 'lost' during the charging process? Some say 'always 50%', some say 'less' and some say 'none'.

ANSWER: 
At the end of the charging process (technically infinite time, but for practical purposes much greater than RC), the voltage across the capacitor will equal V, the voltage of the battery, since no current is flowing. The energy stored in the capacitor is therefore ½CV2. During the charging process, the current through the resistor is given by i=(V/R)e-(t/RC) so the instantaneous power loss in the resistor is i2R=(V2/R)e-(2t/RC). If you integrate the power from t=0 to ∞ you will find the energy lost to ohmic heating in the resistor is ½CV2. So, exactly the same energy stored in the capacitor is dissipated in the resistor. Hence, since the battery is the only energy source, half the energy supplied is lost.


QUESTION: ;
earth is a magnet in rotation if so by faradays laws it must induce current on all metallic objects on earth is this possible and deductable???

ANSWER: 
But objects on earth rotate with it and so the field they see is constant.


QUESTION: 
Law of conservation of Linear Momentum states that the total Momentum of the system remains constant provided no external force acts on the system. Which forces can be regarded as external, is Friction an external force ?

ANSWER: 
Friction between pieces of the system does not affect momentum conservation. But friction from an external agent would be external and cause momentum to be not conserved. One simple example: a 1 kg mass sliding on a floor with a speed of 1 m/s. Then the momentum right now is 1 kg m/s. But, if there is friction between the floor and the mass, it will be slowing down, that is losing momentum. Another example: a block of mass 1 kg sitting on a floor with friction is struck by a bullet of mass 1 gram going with a speed of 1000 m/s which lodges in the block. This is a question you often see in books and you are asked to find out how fast the block/bullet are going immediately after the collision using momentum conservation. However, momentum is not actually conserved because the bullet takes some short time to come to rest in the block and during that time the block is sliding (with friction). The reason it works (approximately) is that the time is very short and therefore the impulse (approximately the sliding friction force times the time of the collision) is very small.


QUESTION: 
I have internet WiFi set up in my house, because I have two different computers in different rooms. The wireless router and the wirelessly-connected computer both use dipole antennas, operating on a frequency range of 2.4210 Ghz to 2.4835 Ghz. This system is put together using common components from the local big box store. I’ve discovered a few web sites that show how to increase the range and signal strength by making and installing a parabolic trough reflector at one or more of the dipole antennas, constructed using cardboard and aluminum foil. In the case of parabolic dish reflectors (like the satellite TV dishes), there is apparently an optimum ratio of f/D, which is the focal distance divided by the diameter. This ratio, of course, defines the how “fat” or “thin” the parabola looks. In my application, is there an optimum shape of the trough parabola?

ANSWER: 
The parabola has a focus but D is not a useful concept. The advantage of a parabola is that if a source is located at the focus the rays all come out parallel to each other. In essence, instead of spreading in all directions the waves come out in one direction. However, it will not be perfect and the main effect is probably to take waves which would have gone out away from the house and put them into the house. The equation of a parabola, y=ax2, tells you the "fat/thin" you want to know; the larger a, the "thinner" the parabola. You can't use a simple D because where you cut off the parabola is arbitrary. If the thing worked ideally (which it won't), the thinner the parabola, the narrower the output beam so you would have to "aim" very carefully at the other computer. Here is a great opportunity to experiment with different shapes to see which suits your purpose best. I would guess that the improvement, if any, would not be very dependent on the detailed shape and that a cylindrical reflector would work just as well in this application.


QUESTION: 
I recently read an article about new imaging of atoms and molecules using extremely brief pulses of electrons. This made me wonder where do these researchers get electrons, like what process? And then how is it possible to capture something so small as an electron and then control it?

ANSWER: 
Electrons are very easy to get. If you have a wire and heat it up red hot in a vacuum, electrons will stream from it. This is how old-fashioned TV sets worked (cathode ray tubes) with a filament as a source of electrons. Once you get electrons, they are easy to manipulate with electric and magnetic fields.


QUESTION: 
Suppose a ball is spinning at a rate at which the surface is moving at the speed of light. Now what would happen if giant rod was attached to the surface of the ball. Would the end of the rod be moving faster than the speed of light since it would be covering more distance at the same time as the surface of the ball as the ball spins?

ANSWER: 
First of all, groundrules forbid questions assuming something goes the speed of light. Trying to rotate a rod such that the end moves faster than the speed of light is futile. The rod could not be strong enough to compensate for the fact that the mass increases with velocity. Also, you could not get the information to the other end of the rod that you were trying to rotate it more quickly than at the speed of light (see earlier answer).


QUESTION: 
why two objects of different masses reach the ground at the same time? and what are the factors that affect their motion?

ANSWER: 
The motion of a mass is determined by Newton's second law, F=ma where F is the net force on the mass, m is the mass, and a is the acceleration of the mass. A mass in free fall (no air friction) has only one force on it, its own weight which is the force with which the earth pulls on it. It turns out that the weight is proportional to the mass, that is W=mg where g is a proportionality constant called the acceleration due to gravity. So, if you have two masses, m and M you can calculate their accelerations, a and A respectively.Therefore A=W/M=g and a=W/m=g; since a=A the two fall identically. (You can see why g is called the acceleration due to gravity.)


QUESTION: 
I know that Kinetic energy and Momentum are related to each other. I have read that in every collision momentum is conserved but K.E may or may not be conserved.How is it possible ?

ANSWER: 
While it is true that kinetic energy and linear momentum each depend on each mass and velocity, they are different quantities and one can change while the other does not. To determine whether momentum and/or energy are conserved, you must know the physical conditions for conservation: if there are no external forces on the system, linear momentum remains constant; if there are no external forces doing work on the system, energy remains constant. Also, techincally, if you know the kinetic energy you cannot calculate the linear momentum because energy is a scalar and momentum is a vector (although you can calculate the magnitude of the momentum).


QUESTION: 
When a radio signal is sent between two points, by what means is the actual energy conveyed? Are areas of electron density occurring between the points (matter waves?)? Radio is part of the electromagnetic spectrum, so might they be photons instead? I ask because I understand how signals are sent, but I just thought about it a day or two ago and realized that I actually don’t know the communicating force. Yipe!

ANSWER: 
Read an earlier answer about electromagnetic waves. Since changing electric fields cause magnetic fields and changing magnetic fields cause electric fields (again, see an earlier answer), the wave is self sustaining, no "force" or medium is required for its transmission. And, yes, electromagnetic waves may always be thought of as photons.


QUESTION: 
How much does a gas compress under its own weight? For example, does a container 100m tall of xenon (a heavy gas) have a greater pressure/density at the bottom than a 100m tall container of helium ( a much lighter gas)? Thanks for your help.

ANSWER: 
I do not want to get quantitative here by answering the "how much" part; that does not have much general interest. Certainly a gas is compressed because of the weight of the gas above it. That is why the atmospheric pressure gets smaller as we go up in altitude. And, a vessel of any height will have a higher pressure at the bottom than the top for this reason. The weight of the gas will certainly make a difference and if your xenon and helium have equal numbers of atoms in identical containers, the pressure at the bottom of the xenon container will be greater.


QUESTION: 
how do physicists come up with equations? i know that may sound vague, but i was watching a program on Einstein coming up with his equations, and the program said things like "he would sit and come up with equations". i know it sounds like a stupid question, but I am really interested in physics. I am currently a sophomore in college majoring in Biomedical/ Mechanical engineering, and am thinking about minoring in physics. I've been really interested in physics for a while, and I've noticed that I can think in terms of physics to do better in my mechanical lab/ design classes. So I guess what I'm asking is this: What books are there out there on how to come up with equations to model phenomena with math/ physics equations?

ANSWER: 
A question of this kind essentially reveals that the questioner does not understand what science is. Knowing equations is not what matters. What matters are basic fundamental ideas or principles from which equations emerge naturally. Here is a general example. Einstein most certainly did not one day, by some insight, say to himself "aha, E=mc2!" What he did was get the idea one day that the laws of physics must be the same for any observer in our universe and that the speed of light was the same for all observers. Starting with no more than these basic principles and a knowledge of classical physics, he was able to write all the equations of the theory of special relativity, including the biggie above, by just doing the basic mathematics and physics. Equations are the end, not the beginning, of a theory; you "come up with" principles, not equations.


QUESTION: ;
Hydrogen is the most abundant element in the universe. A star, such as the sun in our solar system can produce all of the naturally occurring elements on the periodic table. Are all of the naturally occurring elements on the periodic table created by adding hydrogen atoms together under extreme temperatures and extreme pressures? (Ex: 1 hydrogen atom equals hydrogen, 2 hydrogen atoms added together equals helium, 3 hydrogen atoms added together equals lithium, and so on.)

ANSWER: 
Starting with hydrogen, stars fuse nuclei together to get heavier elements, but not all heavier elements. The net effect is to make helium from two hydrogen, but the process is a little circuitous. Heavier elements are made by fusing lighter elements, but not three hydrogen for a lithium, rather something like a helium and a hydrogen; for example, see this link to see how carbon, oxygen, and nitrogen are made.. After iron, energy is lost when fusion to heavier nuclei occurs so it will not happen. The elements heavier than iron are made when the star explodes (supernova) and the energy from the explosion can be used to fuse heavier elements.


QUESTION: 
Is there a mechanism that create Hydrogen? It seems to me that over billions of yrs all of the H would be burned up in stars. Where does it get converted back to H after its been used up???

ANSWER: 
This is the nature of our universe. When all the hydrogen is burnt up (that is, converted to heavier elements), that will be the end. After more than 14 billion years, there is still quite a bit left.


QUESTION: 
Can radioactive material such as Plutonium be used to generate electricity in the same fashion as electromagnetics? It just seems archaic to use nuclear material to heat water and so forth as is currently done in reactors.

ANSWER: 
But, heat is mainly where the energy from fission ends up (kinetic energy of fission products), so it is perfectly reasonable to use it to heat water.


QUESTION: 
A geostationary orbit of any diameter above earth would have variable speeds but the time completed of any geostationary orbit would be relative to a 24 hour period on earth. Even if the orbit were 2 light years, the geostationary object would complete 2 light years in 24 hours time, the exact same as a 50 thousand mile geostationary orbit.. This seems to violate what I know about Time dilation. What am I missing? It seems impossible but all I would need is a long enough hypothetical pole and I could violate the laws of physics (as I know them). I'm sorry if this question is too elementary and I'm missing something very obvious but I have searched and searched for this answer and I just cannot understand enough physics vernacular to find a simple answer to this.

ANSWER: 
There is only one geostationary orbit with a particular radius (about six and a half earth radii). See satellites here.


QUESTION: 
An astronaut steps away from the shuttle in orbit and activates a retro fuel source to begin re-entry. Before entering the atmosphere, a parachute is deployed. Question is: as air molecules begin to inflate the shute, is it possible to realize a gentle re-entry into the atmosphere and to the Earth's surface?

ANSWER: 
I don't really know. You have to realize that he is going at a tremendously fast speed (like 18,000 miles/hr) and the parachute would probably not slow him enough before the air density got big enough to burn him up. Perhaps with a well-designed parachute it could be accomplished.


QUESTION: 
one section in my ice cube tray formed a 'mountain' of ice. it stands 9mm above the surface of all the other cubes. in that there is no water source to this economy fridge and water [to my knowledge] does not flow uphill, how is this possible?

ANSWER: 
I have previously answered this question.


QUESTION: 
I am studying circular motion, and I understand when something is swinging in a circle when attached to a string or rope that is above it, say attached to a ceiling. The gravitational force equals the y component of the tension, and you can set the equation for the centripetal force equal to the x component of the tension. Find both tension in the x and y directions, and you can solve for the vector tension. What is not so clear to me is when you are swinging something over head and the object is above the string or rope. I came across a problem in my text that has a ball being swung above your head such that the string made a 14 degree angle with the vertical. So instead of the string being hung from somewhere and the object attached to it being below it, the object is now above the string. I hope my description makes sense. Anyway, in the solution, it says that the tension in the y direction is equal to the force of gravity, mg. But wouldn't the tension in the y direction plus mg be equal to ma in the y direction?

ANSWER: 
After pondering your question for a while, it just occurs to me that maybe you are talking about the conical pendulum where the string traces out a cone and the object moves in a horizontal plane. For your second part, you cannot have a conical pendulum with a string, it has to be a stick. Then the stick would have to exert a force on the object which has vertical component up and equal to the weight to keep it moving in a horizontal plane; a string, which can only pull along its own length, could not do this. For your first (below support) problem, there is a unique solution. For the second (above support) problem, you cannot find the speed and the "tension" because the force the stick exerts will not be along its own length. So, presumably, the second problem asks you to find the force the stick exerts on the object for a certain speed.


QUESTION: 
I understand that gravity slows down time. So this had me thinking. What would the speed of time be like in the absolute middle of dead space (no matter whatsoever) compared to time on Earth? Would it be so fast that it would almost be like it was infinite? (I don’t know if that makes sense…) Would that be an absolute reference point for time? I can’t seem to find any answers, so thanks for your website and in advance for the consideration of this question.

ANSWER: 
The effect of gravity is very small, at least gravity in the vicinity of something of the size of the earth or the sun. So going to empty space (there is no such thing, of course) would have little effect on rate of time.


QUESTION: 
The force of gravity between two objects gets weaker as the distance between them increases. Why is it more correct to say the square of the distance?

ANSWER: 
I wouldn't call it "more correct", I would call it more quantitative. The first statement is a qualitative statement which tells you it gets weaker. The second statement is a quantitative statement which tells you how much weaker it gets; e.g., if you double the distance you quarter the force.


QUESTION: 
I seem to remember a law of physics from high school about the collision of two objects with the same mass and density which says something along the lines that the one traveling at the higher velocity will break the other one. Do I have this right or even close? What is the name of the law?

ANSWER: 
There cannot be any such law for more than one reason. First, the conditions for breaking depend on properties of the object other than its mass or its density; for instance, a glass ball would be more likely to break than a steel ball and not because of any density difference. Second, there is no physical difference whether ball A strikes ball B at rest or vice versa. All that matters is the relative velocity.


QUESTION: 
If a horse pulls on a calesa at rest, the calesa pulls back equally as much on the horsse. Will the calesa be set into motion?

ANSWER: 
I learned a new word—calesa; never knew that word before. Anyhow, what determines how an object moves is the forces on it, not the forces by it. Therefore if there is not some other force in the horizontal direction (like maybe friction from brakes being set), the calesa will accelerate in the direction the horse is pulling. What the calesa does to the horse is irrelevant in the motion of the calesa.


QUESTION: 
A hammer falls off a rooftop and strikes the ground with a certain KE. If it fell from a roof that was four times higher how would its KE of impact compare?its speed of impact?

ANSWER: 
Since gravitational potential energy is proportional to height, it would have four times the kinetic energy. However, since kinetic energy is proportional to the square of the speed, the speed would only be twice as great.


QUESTION: 
If you accelerate at a constant 1G, how long would your trip last ship time, for 100 years to pass on earth?

ANSWER: 
As I explained in an earlier answer, constant acceleration is not possible in special relativity.


QUESTION: 
If all objects are attracted by the force of Gravity to the center of the earth, why aren't air moloculesd sucked down to the floor?

ANSWER: 
They are, but they don't just go there and stay. Since the molecules are flying around with large velocities because of their temperature, they don't all just lie on the floor. However, there are more near the floor than there are near the ceiling. And, the higher you go, the fewer and fewer there are and eventually there are none—space. If there were no gravity at all, the air would all eventually leak out into space.


QUESTION: 
When you're riding in an elevator (or the space shuttle) you're accelerating away from the center of the earth. Gravity accelerates you toward the center of the earth. Acceleration is a vector quantity. So, why, when you're accelerating up at 9.8 m/s^2 do you feel 2g instead of 0g?

ANSWER: 
The way we do this kind of problem is to use Newton's second law (N2). When you are in an elevator, what forces are there on you? Only two, your weight mg, straight down and some force F which the floor of the elevator exerts up on you. N2 says that the total force on you equals your mass m times your acceleration a, so F-mg=ma.  If a=g, then F=2mg. Since F is how you judge your apparent weight, you feel twice as heavy as usual.


QUESTION: 
What causes the creation of plasma? Is it two or more nuclei being slammed together and heated to more than 200,000,000 degrees Fahrenheit or is it something else?

ANSWER: 
If you have a solid and heat it, it eventually becomes liquid. If you have a liquid and heat it, it eventually becomes gas. If you have a gas and heat it, it eventually becomes a plasma; to create a plasma, the average kinetic energy per atom must be larger than the ionization energy of the atom so that an inelastic collision can result in the removal of an electron from the atom.


QUESTION: 
With all the hassle that has been lately around 2012 - the end of mankind, I'm intrigated by the subject. I'm quite curios what is your scientific opinion regarding this subject: Will it be 2012 the end of mankind ? If yes why ? If not why ?

ANSWER: 
My scientific opinion is that it is astrological nonsense. See my earlier answer.


QUESTION: 
I’m having a problem determining the difference between Nuclear Fusion and Nuclear Fission. Listed below are some information that I’ve found. Could you clarify the major differences between the two, especially when it comes to matter that is heavier or lighter? Nuclear fusion - is the process by which multiple like-charged atomic nuclei (two or more joined together) join together to form a heavier nucleus. It is accompanied by the release or absorption of energy, which allows matter to enter into a plasma state. Nuclear fission - is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons (in the form of gamma rays).

ANSWER: 
The important concept here is that if you find a reaction which results in less mass after the reaction than before the reaction, you will release energy because E=mc2 and energy is conserved in an isolated reaction. Hence, if you lose mass, that energy must appear elsewhere. Where it usually appears is in kinetic energy, that is thermal energy of the reaction products. Iron is the most tightly bound nucleus, that is the mass of the average nucleon (a nucleon is a generic term for neutron or proton) is smallest. As shown in the figure, average nucleon mass steadily increases if you get heavier or lighter than iron. Hence, splitting a nucleus twice the size of iron or heavier results in mass loss and therefore energy gain; this is called fission. Similarly, fusing two nuclei half the size of iron or smaller results in mass loss and therefore energy gain; this is called fusion. Fusion is the source of energy for stars and hydrogen bombs. Fission is the source of energy for nuclear power plants and conventional nuclear bombs.


QUESTION: 
Can you please explain how physics is used with character animation and animated films (disney, pixar, etc)? I am just wondering as I want to be an animator, but am struggling with physics.

ANSWER: 
The object (one, anyway) of animations is to make the scenes look real, believable. So description of motion (kinematics) is important to understand, I would think. The way things move and respond to forces in the real world is determined by the laws of physics and therefore those same laws apply to a simulated real world (animation).


QUESTION: 
I read an answer of yours that ended with "Because water is essentially incompressible, the density of the water does not change as you go deeper and therefore there would be no depth at which a sinking object would stop sinking." I've never really thought about it, but now I'm wondering "If not from density, where does the increasing pressure (like what you feel in your ears) come from?"

ANSWER: 
The pressure in a fluid is due to the weight of all the fluid above it. Atmospheric pressure in the air is due to the weight of all the air above it; because air is compressible, the higher the pressure, the higher the density. Similarly, deep under water the high pressure is due to the weight of all the water above you and the deeper you go the bigger it gets; however, since water is essentially incompressible, the increased pressure does not result in increased density.


QUESTION: 
Once an artificial satellite is placed in orbit above Mars it stays in orbit, why does this happen or does it simply follow the same principles that apply to it for staying in orbit above earth?

ANSWER: 
The laws of physics are exactly the same in the vicinity of Mars as they are on earth. The mass of Mars is different and so the periods of stationary orbits will differ from what they would be around earth, but it is all determined by classical gravitation and classical mechanics. Mars has two natural moons as well, Phobos and Deimos.


QUESTION: 
I am a high school physics teacher and am having some difficulty with part of the Compton Effect. I am trying to come to grips with whether the collision between the x-ray and the electron is elastic or inelastic? From what I have been able to find on the subject, when the x-ray collides with the 'whole' atom it results in an elastic collision and the x-ray leaves with the same frequency with which it came in with. On the other hand, when the x-ray collides with something closer to its own mass (an electron), it results in an inelastic collision and the x-ray is ejected with a lower frequency and energy. Any help you could provide would be greatly appreciated. I hate to think that I am not teaching it correctly and sending my students out into the world with misconceptions imparted to them by me.

ANSWER: 
The Compton effect is elastic scattering of photons from some mass. Elastic does not mean that the energy of the incoming particle remains constant, it means that the sum of the energies of the incoming particle and the target remain constant. Assuming the target is at rest before the collision, after the collision it will recoil and carry away some of the energy which the photon brought in and the only place it can get this energy is from the photon. It is maybe easier to see this by thinking about classical particles. If a BB (photon) hits a bowling ball (whole atom), the bowling ball is almost at rest after being hit and therefore the BB has approximately the same energy (and speed) after the collision. If the BB (photon) hits a marble (electron), the marble will be moving after the collision so the BB must have lost energy (and speed). Both processes are elastic. An inelastic collision is one in which the total energy before and after are not equal.


QUESTION: 
why do you hear your friends talking in he hallway even though you can't see them around a corner?

ANSWER: 
Two reasons: reflection of sound from walls and diffraction of sound around the corner. See this link for more details.


QUESTION: 
I am curious about something that I just read in an earth science book about the shape of the Earth. Since the polar circumference is less than the equatorial circumference, the Earth is an oblate spheroid. The authors attribute this shape to centrifugal force caused by the Earth's acceleration. However, I thought that centrifugal force was not a real force, but was instead due to a mass's inertia resisting the acceleration. Am I misunderstanding the concept? Thanks for helping me resolve this conundrum.

ANSWER: 
(This is sort of a long-winded answer, but I want to have an explanation of centrifugal and fictitious forces I can later refer to.) Centrifugal force is what we call a fictitious force. In an accelerating system, Newton's laws are not true. For example, if you are inside a big rotating drum you feel a force pushing you into the wall, right? Well, actually you do not; what you feel is the wall pushing you toward the axis of the drum. Your brain is trained mostly in nonaccelerating systems (moving with constant velocity) and so it wants to use Newton's first law which says that if an object is at rest the forces on it must add to zero; to make Newton's first law true (you are at rest in your system which is accelerating), your brain invents a force equal and opposite the centripetal force. This is called the centrifugal force and it does not really exist. A fictitious force is one we invent to force Newton's laws to be true in an accelerating system. Just because it does not exist does not mean that we cannot do physics with Newton's laws using it. If the accelerating system is a centrifuge, it works, doesn't it? The heaviest stuff is "pushed out" by the centrifugal force is true even if centrifugal force is just something we made up to make calculations easier. The rotating fluid earth is a little complicated, but I think that it is pretty intuitive: a ball of putty (or pizza dough) made to rotate faster and faster will stretch out into a pancake and you can say it is the centrifugal force which is causing it. A simpler case of "centrifugal stretching" is the following. A spring is attached to a nail at one end and a mass at the other. If you make it rotate about the nail, the spring will stretch out just the right amount so that the force of the spring will provide the necessary centripetal force. But if you make it go faster, a larger force will be required and so the spring will have to stretch more to do that. That example is easy to see without centrifugal force, but if you view it from the perspective of the rotating mass, you will say the centrifugal force pushes you out.


QUESTION: 
I would like to know if the protons and neutrons of metals such as nickel move about and vibrate relative to each other, or to they stay put (relative to each other)--while the atom vibrates and moves based on heat, chemical and magnetic affects, and other factors. If no one knows the answer to my question, if it's not known by the scientific community, then this is okay. Just say, "no one knows...the models we have today don't tell us."

ANSWER: 
If you mean do the atomic nuclei (wherein reside all the protons and neutrons) move in response to the actions you note, then, yes, absolutely.


QUESTION: 
More quantum entanglement! The measurement of one side of the pair is said to "put" the other one into the same state (say spin up or spin down). But I don't see that the spin being indeterminate (prior to a measurement) prohibits the entangled pair from having "synchronized" spins, that occurred at the event that produced the entangled pair. How do we know that is not the case as opposed to the "put" action at a distance?

ANSWER: 
I think you have the idea right. The use of the word "put" seems to be bothering you. The point is that the "entangled" particles are just that, what one "is" is determined by what the other "is"; call that "synchronized" if you wish. The pair is really a single system so making a measurement on one "collapses" the system to the required "synchronized" state.


QUESTION: 
I noted in some physics books that centrepital acceleration is inversely proportional to R (radius) while in others (and sometimes in the same text) that it is directly proportional to R. In the first case the book uses (a=v2/R) while in the second case the book refers to the formual (a=w2R). I feel something is wrong in this logic, as what relates a to v and R is basicaly a definition of the centrepital accelelration. Can you help?

ANSWER: 
The "catch" here is that both velocity (v) and angular velocity (ω) depend on R, v=Rω. So the two equations you quote are the same: a=v2/R=()2/R=2. It is incorrect, therefore, to say that the centripetal acceleration is proportional to R (or 1/R) unless ω (or v) is constant.


QUESTION: 
why is current a qualitative concept

ANSWER: 
Do you mean electric current? It is not a qualitative concept. Electric current is the amount of electric charge per unit time. 1 Ampere=1 Coulomb/second.


QUESTION: 
My question is about Vacuum. it is the absence or air or any matter? there is a vacuum particule or something like that?

ANSWER: 
Normally vacuum refers to a volume from which all gas has been removed. If there is a solid object inside this volume, we would still say there is a vacuum but the object would not be part of it. There is nothing called a vacuum particle. However, our modern understanding of a vacuum is that there are virtual particles (like particle-antiparticle pairs) popping into and out of existence, existing only for as short a time as allowed by the uncertainty principle; this is referred to as vacuum polarization. So, in some sense, there is really no such thing as a completely empty space. Maybe you have heard the old saying "nature abhors a vacuum".


QUESTION: 
I was standing at the bottom of the stairs in my house (which has a downstairs and upstairs heating zone) and felt cold air gently falling down from above (we have not yet turned on the upstairs heating zone). Then I thought of the phrase "hot air rises" and it did not ring true. Almost the opposite! Cold air "sinks". I was wondering if the answer really had to do with the idea of displacement instead of convection.

ANSWER: 
This sounds more like an issue of semantics than of physics. Warm air rising displaces the colder air and forces it down, the only place it can go. So, does cold air sink? Sure, but it is doing so because it is displaced by rising warm air.


QUESTION: 
Can you tell me what sort of voltages can be achieved by a thermocouple when the temperature difference is only about 100° C ?

ANSWER: 
Typically a few millivolts. Read the Wikepedia article on thermocouples.


QUESTION: 
If I run my computer with a 300W power supply, it will produce a lot of heat (as well as doing useful work). Will it be just as efficient at heating my house as running a 300W electric heater? Or will the 300W electric heater somehow produce more heat per unit of energy put into it? If the computer is less efficient, then where does the extra energy go?

ANSWER: 
A 300 W power supply means that it can supply up to 300 W if called upon to do so, not that it is continuously consuming 300 W. Usually, it supplies much less than that. Not being 100% efficient, not all power it consumes is supplied at its output; rather some power is wasted in heating itself up. Therefore, if a 300 W power supply is running at full power, it is consuming more than 300 W of power from its source. In any case, it will certainly supply less heat than a 300 W electric heater which converts nearly all of its input power to heat.


QUESTION: 
Does a planet that is spinning on it's own axis exert more or less gravity (on its inhabitants) than a planet, of the same mass and distance from the sun, that is still ? What would happen to people and matter on earth if it stopped rotating on its own axis? Would we get heavier?

ANSWER: 
As I have said repeatedly, the weight of something is the force the earth exerts on it and therefore independent of its motion. However, your apparent weight will be changed if the earth rotates on its axis. The apparent weight is the weight which would be registered by a scale you are standing on but it is not a measure of your true weight if you are accelerating (as you are in a rotating system). For example, if you were in an upward accelerating elevator you would appear to weigh more but would not. It depends on where on earth you are located; at the poles there would be no effect (true=apparent weight) and at the equator you would have maximum effect. For more details about how to calculate this see an earlier answer. As you will see there, if the earth stopped rotating you would appear to be about 0.35% heavier on the equator.


QUESTION: 
if you push on a tennis ball and a bowling ball with the same force which ball with roll faster? Why?

ANSWER: 
Newton's second law states that if a force F is exerted on a mass m, its acceleration a will be given by a=F/m. Therefore, pushing with equal forces for equal times will result in a greater final speed for the less massive ball (the tennis ball) because it had larger acceleration.


QUESTION: 
Why is it more difficult to spin a metal cylinder around an axis perpendicular to its length than parallel to it?

ANSWER: 
Why is a more massive object more difficult to get moving than a less massive object? Because of inertia. In translational physics (moving along a line) inertia depends only on mass. In rotational physics, inertia is determined not only by the mass but also by the geometry. The quantity that plays the role of mass (that is the quantity which quantifies resistance to rotation) is called moment of inertia. The farther the mass is from the axis around which you are trying to rotate it, the harder it is to rotate it. For an axis along the symmetry axis of the c