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Questions and Answers

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

QUESTION: 
I'm a science teacher (bio and chem mostly) teaching middle school physics and I want to get this right. For the simple machine of an inclined plane, is the amount of work done to lift an object to the height of the top of the inclined plane less or the same as the amount of work done to slide the object up the plane, and why? I assumed they were the same, the plane just made it easier by decreasing force and increasing distance. If I want to calculate the work in each situation, what distance do I use? The height of the plane or the horizontal distance travelled?

ANSWER: 
If there is negligible friction, then the work to push it up the incline is the same as the work to lift it straight up. The reason is that work is the product of the distance moved times the component of the force along that distance. To lift it straight up you must exert a force equal to the weight of the object, mg, so the work is mgh where h is the height. To push it up the incline (angle A with the horizontal), you must exert a force mgsin(A) but the length of the incline is h/sin(A) so the total work is still mgh. So the distances are the vertical distance for straight up and the length of the incline for going up the incline (never the horizontal distance).

QUESTION: 
what would happen if, as the Bible states occured, "the sun and the moon stood still in the sky?"

ANSWER: 
If the sun stood still, the implication would be that the earth had stopped rotating on its axis; this would imply a large tangential acceleration during the short time when the earth stopped rotating during which catostrophic things would happen (like big buildings falling over). Since the main observed "motion" of the moon is also due to the earth's rotation, the answer would be the same for its stopping. If we meant the moon stopped in its orbit, then it would fall to the earth.

QUESTION: 
the national yellowstone guyser shoots every 1 hour 40meters high. with what velocity does water leave the ground. (no air resistance) gravity is 9.81

ANSWER: 
About 28 m/s which is about 83 mi/hr. However, I doubt if air friction is negligible.

QUESTION: 
Is there any way to use magnets to repel metal instead of attract metal?

ANSWER: 
Some materials exhibit a property called diamagnetism. Unlike ferromagnetic materials (like iron), these materials are repelled when placed near a bar magnet. However, the effect is so small that it is almost impossible to observe without doing a very careful experiment.  Actually, all materials are diamagnetic but in most materials ferromagnetism or paramagnetism (another attractive type of magnetism but also much weaker than ferromagnetism) overwhelm its effects.

QUESTION: 
is there a law that states two object of the same mass and density, one hot one cold, while in a vacuum , the hot object will transfer its heat to the cold, and the cold one will even become hotter than the origanal hot one.

ANSWER: 
No. There is a law, however, that, for isolated systems, heat always flows from high temperature to low, so the two must end up at the same temperature which must be lower than the hotter one starts at.

QUESTION: 
If I know the force of the motor and the weight of the vehicle, How do I determine the height of the Rocket? Example: Motor burns for 2.2 seconds Average Thrust of motor is 23 Newtons/Sec Weight of the rocket is 380 grams I know gravity and acceleration come into play.

ANSWER: 
I presume you mean the height to which the rocket will rise. There is insufficient information to do the calculation because as fuel is burned the rocket gets lighter and therefore will have increasing acceleration. So you need to know the weight of the fuel which will be burnt over the burn time and whether the rate of burn is constant.

QUESTION: 
When inflating a tire tube outside of the restriction of a tire, the tube will expand greatly. The question is will the hole in the center of the tire tube get larger or will the tube "expand" and cause the hole to get smaller. I believe the hole must get bigger as the tube expands in three dimensions. A tire "expert" says the hole will get smaller. As we don't have the means of testing this, can you give us your view as a ohysicist, an explination of the behaviour of the whole in the center of the tube.

ANSWER: 
If the hole got smaller, that would mean that the rubber inside the hole would be compressed, not stretched. That is unlikely to be the case for an unconstrained tire. Why can you not do an experiment? It would only require gettnig an old tube from a tire shop.

QUESTION: 
I have always wondered about the 2-dimensional representation of lightwaves (similar to sine/cosine waves, etc.). Do physicists really believe that the photons move up and down while traveling linearly? Is that phenomena of movement seen anywhere else? It's always compared to water waves, yet water is subjected to gravity and forced to have a 2-dimensional motion. My thought is that photons move in a cork-screw pattern (think of a single photon orbiting a central reference point and then move that photon linearly as it orbits the reference point. It will circumscribe a corkscrew pattern - a 3-dimensional pattern that viewed from any angle will look like a sine/cosine wave.

ANSWER: 
There is nothing two-dimensional about light waves. They are composed of electric and magnetic fields which are perpendicular to each other and both these fields oscillate, usually represented as sinesoidal oscillations as you state. The photons move in the direction the light wave moves, perpendicular to both the fields.

QUESTION: 
When you race a hollow steel wheel and a solid wooden wheel down an incline why does the wooden wheel win?

ANSWER: 
There are several ways to prove this. Perhaps the easiest way is to do energy conservation. At the top the energy is all potential, mgh, and at the bottom all kinetic, �mv²+�Iω², translational plus rotational. So, mgh=�mv²+�Iω², and using ω=v/R for the angular velocity (R is the radius of the ball or wheel) and I=�mR² for the wheel and I=2mR²/3 for the ball, you may show that v=√(4gh/3) for the wheel and v=√(6gh/5) for the ball; so the speed of the wheel is larger for the wheel at the bottom and therefore it wins. Note that the mass cancels out and so the metal or wood has nothing to do with it.

QUESTION: 
I believe my question has to deal with moments of inertia, but I'm not quite sure. Imagine a see-saw. If I have a given weight on one side and the other side balances, does it take the same amount of force to move the see saw irrespective of how much actual weight is placed on the other side? That is, will it take x amount of force to move the see saw if a single weight is placed on the other side which is equal to the counter weight - and will it also take x amount of force to move the see saw through the same amount of angular distance if more weight is placed closer to the fulcrum but still balances the same counterweight? The force would be measured at the same distance (at the end of the lever) each time. Let me try again. I have two see saws A and B both 10 feet long with a fulcrum in the middle of each one. Both of them have a 1 lb weight at one end. See saw A has a one pound weight at the other end which balances it and see saw B has more weight closer to the fulcrum but balances similar to see saw A. Does it take the same amount of downward force exerted at the end of the seesaw to move both seesaws through 5 degrees of rotation?

ANSWER: 
Wow, what a ramblin' question! You have a basic misconception�an object in equilibrium as you describe requires no force for it move through some distance or angle. The tiniest force will start it moving slowly and then it will go on its own to 5⁰  The question you really want to ask is how much torque (it is not just force which matters, but also where you apply it) it takes to give each the same angular acceleration. For example, how much torque to move it through 5⁰ in 10 seconds.

QUESTION: 
in quantum mechanics what is meant by a bound quantum state and why must a particle bound in it have non-zero kinetic energy?

ANSWER: 
A bound state is one where if you watch it forever you will never see it "come apart". For example, if an electron is bound in an atom you will never see it come out of the atom. The reason a bound particle cannot have exactly zero kinetic energy is that it would have to have exactly zero linear momentum, so momentum would be exactly known. If the momentum is exactly known, the position must be completely unknown; this is because of the Heisenberg uncertainty principle. But, if a particle is in a bound state you are not completely ignorant of its postion�e.g., an atomic electron must be somewhere inside the atom.

QUESTION: 
How does a neutron detector work? The neutron's lack of charge must make it very tricky.

ANSWER: 
The main idea is to use a material rich in hydrogen. When a neutron collides with a proton, the proton exits with considerable momentum (think of a cue ball colliding with another billiard ball at rest). Then the proton is detected.

QUESTION: 
Why neutrinos oscillate in other forms of neutrinos and other particles not?

ANSWER: 
Neutrinos are not the only particles which experience oscillation. See http://en.wikipedia.org/wiki/Neutral_particle_oscillations

QUESTION: 
why do meteors buen up when they fall through the atmostphere while skydivers don't?

ANSWER: 
Because meteors enter the atmosphere with very large velocities and skydivers enter it with as much smaller speed, that of the airplane from which they jump.

QUESTION: 
Suppose we have a permanent magnet and a ferromagnetic object rested on a frictionless table. The distance of the magnet from the object is �D�. Normally the object will be accelerated and will hit the magnet. Now my question is if the object is again dislodged from the magnet and again placed at distance �D� from the magnet then will it be accelerated same way as the first time or will there be any decay in acceleration? In general sense is there any decay in the magnetic property of the permanent magnet if it is subjected to attraction to an object repeatedly.

ANSWER: 
When you place the ferromagnetic material, which I will assume is initially unmagnetized, in a magnetic field it becomes magnetized. The amount of magnetization it has depends on the intensity of the field, so as it accelerates closer to the magneti it becomes more polarized. Now, when you take it away, it has some residual polarization (you know that when a nail is near or on a magnet, it becomes a magnet itself). Now, the second time it starts out magnetized so, it seems to me, it will experience a greater initial acceleration than before because of its magnetization (you must orient it in the same way as the first time, otherwise the acceleration could be smaller). Note that the acceleration will not be exactly uniform because as it gets closer to the magnet it gets more magnetized and so its acceleration will increase.

QUESTION: 
Can you possibly figure out the following without more information on weather conditions? How long it would take a 22 lb. object to fall from the top of mt. everest (elevation 8848m), temp is 252K. Thanks for any help.

ANSWER: 
The short answer is no because you need to know more about the object than its weight in order to calculate the air resistance forces on it. The problem would be further complicated by the fact that over such a large change in altitude the air pressure changes significantly and the pressure is important in determining the air resistance at any point. To get some idea of how complicated this type of problem can be, you might want to look at an earlier answer.

QUESTION: 
i am doing GCSE physics coursework in england and i am struggling to show that an increase in velocity causes an increase in mass using the formula E = mc²/√(1-(v²/c²)). i was wondering if you could help? it really is rather important. much appreciated

ANSWER: 
Write E=√[p²c²+m²c⁴] and solve for p. You will find that the momentum p is not mv as it is classically, but rather [m/√(1-(v²/c²))]v, so if you think of momentum as being mass times velocity, the mass of a moving object is increased. Personally I prefer to not think of mass as increasing but of momentum being redefined to preserve the classical idea of momentum conservation; see an earlier answer to get more detail on that point of view.

QUESTION: 
I have to do a planning and designing lab based on the hypothesis that the velocity of a bead remains constant as it falls through a liquid (in this case, oil). the apparatus i must use is: measuring cylinder, oil, bead, stopwatch, and metre rule. Please help me as soon as you can!

ANSWER: 
You measure the average velocity if you divide the distance traveled by the time to fall that distance. First, measure the time it takes to fall 10 cm, then 9 cm, then 8 cm, etc. If all give the same result for average velocity, the velocity will be constant.

QUESTION: 
there are 8 masses at the corners of a cube of side 'a'. what is the moment of inertia about the z -axis which runs along the edge of the cube between two vertices (origin x,y,z is at a vertex)? i have tried using the parallel axis thereom. i first worked out the mom. of inertia through the centre of mass and have got I(cm)=(1/6)Ma^2. i then added this to the perpendicular distance to the z axis squared, multiplied by M^2 i.e (1/2)a^2 * M to get I(z)=(2/3)Ma^2. where M is the sum of the 8 masses (M=8m) do you agree with this result as i am not sure i have treated the mass distribution correctly. thanks

ANSWER: 
If I understand correctly, the z axis passes through two closest corners of the cube. Then the masses at those corners contribute nothing to the moment of inertia. There are 4 masses a distance a from the axis contributing ma² each. There are 2 masses a distance (√2)a from the axis contributing 2a² each. So I=8ma².

QUESTION: 
So i have to make a msuical instrument to make in which i have to use air colums?...right now i am in my sound unit.i am in grade 10. do u have any possible suggestions for an instrument i should make?

ANSWER: 
How about a row of soda bottles each of which is filled to a different level with water and then you blow over the top of each to make different notes.

QUESTION: 
what is the difference between special theory of relativity and general theory of relativity?

ANSWER: 
The special theory deals with transformations among systems which move with constant velocity with respect to each other. It is this theory which leads to the phenomena of time dilation, length contraction, E=mc², etc. General relativity is mainly a theory of gravity saying that gravity is the warping of spacetime by the presence of mass.

QUESTION: 
In an article about neutrinos in Wikipedia it says that every second 50 trillion solar electron neutrinos pas trough the human body every second. Do they pass trough the tiny spaces between atoms in our body, or they pass literally trough our atoms without interactions?

ANSWER: 
The latter.

QUESTION: 
Can a colission between matter and antimatter generate more energy than that of a fussion reaction of normal matter?

ANSWER: 
In a relative sense, yes. In fusion (or fission) a small percentage of the mass is converted to energy. In annihilation, 100% is converted.

QUESTION: 
I teach physics in Jackson, WY and my students asked me the following question: At absolute zero, what properties would matter exhibit given that 99.99% of all matter objects are empty space (space between nucleus & electrons)? Would it be possible to pass solid objects through solid objects if the individual particles were not moving?

ANSWER: 
It is not physically possible to achieve absolute zero. The reason is the Heisenberg uncertainty principle which says that if you know the momentum of a system exactly (zero in the case of absolute zero) then you cannot know anything about the positions of the particles (they could be anywhere in the universe!) Even if it were possible, why would stopping all motion allow objects to move through each other? There would still be the coulomb forces among the charges in the atoms which is the force which keeps objects from passing through each other at higher temperatures.

QUESTION: 
We are taught that at light speed, time (i.e., "ship clock time") stops. I am trying to get a handle on the nature of reality from the point of view of a photon (or any massless particle thus traveling at c), as it would seem to exist "outside of time", yes? So from its point of view, it crosses the universe in an instant...then what? In conventional language we are taught that particles that travel at c don't decay, but isn't more accurate to say that they don't experience time, and therefore the notion of decay has literally no meaning for them?

ANSWER: 
Well, photons do not have a "point of view" and since no material object may travel with the speed of light, and certainly a photon cannot carry a clock along with it, there is really no answer to your question. However, we can look at the limiting case of a very large velocity (just not c). Suppose that there is a spaceship going by at 99.999% the speed of light. Then we will measure that the clocks on that ship are running extraordiinarily slowly. However, a person on the spaceship would see the clocks running at a perfectly normal rate. When the person in the spaceship looked outside he would see a nearly unrecognizable universe: nearly all the stars (even those "behind" it from our point of view) would collapse into a very tight cone right in front of the spaceship, nearly a point; and, because of length contraction, the distances to these stars would be greatly reduced relative to our measurements. So, your idea that you would cross the universe in a very short time is essentially correct in the limit of very large speeds. Regarding your related question of decay, we believe that photons are the only massless particles and therefore travel with speed c, and certainly an isolated photon is stable. (Again, no particle with any mass can go the speed of light.) A photon may be perturbed by an electromagnetic field and turn into a positron-electron pair, but I guess you would not call that decay. We now believe that neutrinos, until a few years ago assumet to be massless, have a very small mass and therefore cannot travel with the speed of light. Gravitons have been hypothesized to move with speed c but have never been observed.

QUESTION: 
When physicists talk about light, the speed/velocity are often used interchangeably. Velocty is a vector, while speed is a scalar. Since light is not accelerating, is it properly a vector (magnitude & direction...velocity), or scalar (magnitude only...speed)?

ANSWER: 
It is technically always incorrect to use the two terms interchangeably. Speed, a scalar quantity, is the magnitude of the velocity, a vector quantity. The speed of light is, in relativity, the same in all frames of reference but the velocity is not; i.e. the direction of a beam of light may be different in a different frame.

QUESTION: 
My name is Daniellka, age 13. I want to make a perpetual motion machine, but my dad says it is not possible. Why is it not possible? My idea is to use natural earth magnets to suspend the rotating shaft, so the machine will not have any friction and can generate electricity. Is there a way?

ANSWER: 
Your father is completely right, your idea cannot work. First, let's deal with one basic misconception you have. If you suspend your device by levitating it with magnets you will greatly reduce friction but you will not eliminate it, so if you spin your device it will spin for a long time but not forever. Secondly, you want to use your device to generate electricity. But the resulting electricity will contain energy; where did that energy come from? In a power plant the energy being supplied (burning coal, falling water, etc.) is not just used to overcome friction and keep the thing going; it is used to create the electrical power being generated. There are two important laws which are well established in physics:

• The first law of thermodynamics is simply conservation of energy: the total energy of an isolated system must remain unchanged, that is you can never get something for nothing.
• The second law of thermodynamics is much more subtle than the first but, for your purposes it may be simply stated: you can't even break even!

QUESTION: 
When particles escape the nucleus in spite of the strong force, why is it an alpha particle (2 protons, 2 neutrons) instead of some other configuration of these two particles? I understand that as atoms become large (or unstable due to unfavorable geometry or energies) some particles will, at random, "tunnel" and escape the nucleus in spite of the effect of the strong force. I guess I'm baffled as to why it must be an alpha particle.

ANSWER: 
One of the ingredients in a theory of heavy particle decay is the probability of the particle actually being formed inside the nucleus. The alpha particle is, on a per particle basis, the most strongly bound nucleus and therefore has a much higher probability of getting together than other possibilities. In some sense, alpha decay is a special asymmetric case of nuclear fission in which a heavy nucleus splits into two heavy pieces.

QUESTION: 
Are the constellations we see from earth in our galaxy?

ANSWER: 
Constellations are made up of whatever is in that portion of the sky; some are stars in this galaxy, others are other galaxies.

QUESTION: 
what happens (theoretically of course) if you set of a nuclear device in a vacuum ?

ANSWER: 
It explodes essentially just the same as in the atmosphere but there would be no sound and no heated air.

QUESTION: 
Does time dilation and the universal limit of the speed of light mean that it will be impossible for us to ever explore space beyond our own solar system?

ANSWER: 
I do not see why these would prevent it. The problems I see are more of a practical nature:

• You could take advantage of time dialation but to accelerate to such high speeds in a reasonable amount of time would require accelerations so huge that a human body would be crushed.
• To get to a very high speed would require a very large energy input and the source would be difficult.
• Assuming you did get to a high speed, the huge flux of interstellar gas would cause serious radiation hazards difficult to shield against.
• Again assuming that you could get to a very high speed, the visible sky would collapse to almost a bright point in front of you making navigation very difficult.

Finally I should note that there are already a couple of old space probes which have left the solar system and still send signals back to earth, so we are already "exploring" but without any passengers.

QUESTION: 
what exactly are the type of protons being accelerated at CERN? Are they from a metal, gas etc?

ANSWER: 
I am quite sure the original source is an ionized hydrogen gas. However, there is no such thing as a "type of proton", all protons are identical regardless of where you get them.

QUESTION: 
Why does blue light travel more slowly than red light in transparent media?

ANSWER: 
See the following question & answer.

QUESTION: 
According to Maxwell's Electromagnetic theory, refractive index of a dielectric nonmagnetic medium is given by, n= ��K, where K is the dielectric constant of the medium. But, refractive index of a medium depends on the wavelength of the radiation used. Hence,

1. Does dielectric constant of the medium depend on the wavelength of the radiation present?
2. If no, then how and why does the refractive index of the medium depend on the wavelength?
3. If yes, then does the electric field in a region change if the wavelength of the radiation present in that region is chenged?

ANSWER: 
Yes, dielectric constant depends on the wavelength of the radiation. Technically, the important quantity is the frequency of the radiation rather than the wavelength. The frequency (f) is something which is the same both inside and outside the medium but the wavelength (λ) is not the same because the speed (v) is different inside and outside; these are related by fλ=v. The reason for the frequency dependence of the dielectric constant is that if you think of electrons in the material as being little oscillators (mass on a tiny spring) they respond differently to different frequencies. Beyond that qualitative explanation it gets too complicated to fully describe here.

QUESTION: 
what are the qualitative effects of thickness, tension and lenght on the frequency of a vibrating string

ANSWER: 
The frequency (f) of a vibrating string is determined by two things, its length (L) and the speed of waves (v) in the string. If the string is clamped at both ends (as in most stringed instruments) then the frequency is f=v/(2L). The velocity is determined by two things, the tension in the string (T) and its mass density (m=m/L), v=√[T/m]. So, finally, f=[1/(2L)]√[T/(m)]. That is quantitative and you asked for qualitative. Varying the three things (tension, mass density, and length) separately while keeping the other two constant results in:

• a shorter string has a higher pitch,
• a higher tension has a higher pitch, and
• a lower mass density has a higher pitch.

QUESTION: 
When the suns rays shine down on Earth is the volume measured in any particular unit? Energy? If so, can it be found out how much of this unit is found per square foot on the Earth? Lasty, can this unit of energy (I'll call it for the time being) be compared to say electricity if so what is that formula?

QUESTION: 
At school, I recently completed a lab in which we heated a soda can, then quickly stuck it upside down in a bucket of water. The soda can imploded. Why does such rapid temperature change happen?

QUESTION: 
Is there any polarity for strong and weak nuclear forces, just like +/- and N/S poles exist for electro magnetic force?

QUESTION: 
A pendulum clock uses an oscillating rod with an adjustable weight to keep time. During an earthquake, the weight becomes stuck in a position 1.0cm farther down the rod than its position for proper timekeeping. A person who wants to return the clock to proper timekeeping can:
a. add another equal weight at the original position of the first weight
b. move the point of suspension of the rod up 1.0cm
c. move the point of suspension of the rod down 1.0cm
d. add another equal weight 1.0cm above the original position of the first weight
e. not obtain the original moment of inertia with any of the suggested methods
My gut tells me that it's e, because if you change the rod at all you are changing the center of mass - which will not achieve the results needed - but I am not sure.

QUESTION: 
Discounting air resistance, does it take more energy to run outside than on a treadmill? Does the motion of the belt pull you along? Are the mechanics of running somehow different on the treadmill? This subject is discussed a lot among people looking to get outside to run this time of year. Many people say that you are only picking up and putting back down your feet on a treadmill, whereas on ground you have to propel yourself forward, or that the belt pulls your feet under you for you, or that you don't have to lean forward because you have no momentum. This doesn't really make sense to me.

ANSWER: 
In principle it makes no difference whether it is you moving forward or the ground moving backward (neglecting wind effects). As long as you do not hold onto something while you run on the treadmill, the exercise you get should be the same as running on flat ground.

QUESTION: 
Pretend you are sitting in an airplane and holding a ball, and the airplane is moving at a constant speed. Assume that no windows are open and there are no other sources of air flow (e.g. fans, etc.). If you drop the ball straight down, will it land directly beneath your hand (i.e. lose absolutely no momentum on its way down)? If so, is it because pressure from the rear of the plane is causing the air around the ball to move at the same speed as the airplane, or is there more to it than that?

ANSWER: 
The simple answer is that it is the principle of relativity which insures that the ball falls "straight down" (which I put in quotes since an observer on the ground would not see it fall straight down). The principle of relativity states that the laws of physics are the same in all inertial frames of reference. An inertial frame of reference is one in which Newton's laws of motion are all true; what this means is that the frame in whcih you do your experiments is not accelerating. So, the earth is (approximately) an inertial frame of reference and the ball falls straight down because there are no horizontal forces on it. The airplane (moving with constant speed in a straight line) is also an inertial frame (approximately) and so, since there are no horizontal forces on it, the ball will also fall straight down in accord with Newton's laws. Your idea about the "pressure from the rear of the plane�" is wrong.

QUESTION: 
When kittinger jumped from 103 thousand feet he reached a speed of 614mph. did he slow down to 125mph when he got closer to the earth. my question is, if i jump out from that high do i continually gain speed or would I slow down to my terminal velocity because of the upwards force of drag?

ANSWER: 
First of all, it is not at all certain what his speed was when he deployed his main parachute. You may read a lengthy analysis and discussion in a previous answer to a question about this event. Regarding your question, if you are injected into a region with speed larger than your terminal velocity in that region, you will slow down until you reach that speed.

QUESTION: 
What exactly causes a magnetic field? I know that a magnetic field requires a moving charged particle and that magnetic fields are a relative effect. But specifically, what is the mechanism that causes magnetism. Also, how was the electric force and the magnetic force combines to form the electromagnetic force? I have a theory, it seems to be correct, but I want to make sure that there is nothing else I am missing. Thank you for your time.

ANSWER: 
You have answered your own question�the source of magnetic fields is a moving electric charge. If you want to get deeper into it you should think of the electromagnetic field not as electric and magnetic fields as being separate things but rather as a single entity. The electromagnetic field is a tensor which has 16 components but only 6 independent quantities (which may be expressed as the three components of the electric and magnetic fields). If you have a static charge distribution, all the components of the field tensor which can be identified as being "magnetic" are zero; that is there is only an electric field. But if you move past that change distribution (or put it in motion), you have to transform the tensor into another frame and, given the properties of the field tensor, magnetic components appear.

QUESTION: 
if there were a conductive hollow cylinder with a negatively charged ring of the same diameter as the cylinder attached to one end and a positively charged ring attcahed the other end, would the passing of current from one ring to the other generate a magnetic field within the cylinder?

QUESTION: 
If I fire a gun straight into the sky, would the bullet fall back to earth, or carry on to space?

QUESTION: 
Say you have two balls with the same mass (1 kg). You lift one of them to a hight of 1km above ground, and the second one to 2km above ground - and then let go and let them fall. Will the second ball achive a higher velocity than the first ball right before they hit the ground or will they both achive a certain maximum velocity and continue going with it untill they hit the ground?

QUESTION: 
we know that mass is a state of energy, but can energy exist without mass? I mean energy itself, not the energy being transferred.

QUESTION: 
The Coulomb static force law represents the electromagnetic force when a charge is at rest. When the charge is moving, the magnetic force component is included (relativitstic effect). The gravitational force law which is identical in form to Coulomb's Law applies to mass. So is there a magnetic analogy for gravity when mass is in motion? If so what is the term that is analogous to qv X B. Also playing off this question... there are electric and magnetic constants (permit, and permeab}... Why is gravity not treated this way (instead of G, 1/4pi*constant) ... something like a gravity permittivity?

to name a few. Regarding your second question, the factors of 1/4p are for convience in electromagnetic theory to make some other equations come out in simpler form; there is never any real physical significance to how one chooses to write a proportionality constant.

QUESTION: 
A common source of neutrons is the deuterium-tritium reaction using a beam of deuterium impacting a tritiated foil. The information I've found on this topic states that the resulting neutron is emitted in a random direction. I would think that the conservation of linear momentum would dictate that one of the reaction products (the neutron or the helium nuclei) would be travelling in the same direction as the incident deuterium ion.

ANSWER: 
This is a fusion reaction and, for a high probability of the reaction happening, the deuteron should come in with relatively small energy. Much energy (more than 17 MeV) is released in the reaction which ends with a ⁴He nucleus and a neutron; the reason for this big energy release is that the alpha particle is very strongly bound. You are certainly right, momentum must be conserved, but the ⁴He can carry it as well as the neutron, so the distribution of neutrons, while not perfectly isotropic, will be nearly so.

QUESTION: 
What is electron spin. apparently it is to do with gyroscopic motion, but i don't understand what. Is it simply the rotation of [certain] fundamental particles around two orthogonal axes?

ANSWER: 
This question has been previously answered.

QUESTION: 
Why is the equation of translational kinetic energy KE=1/2mv^2 but the famous equation E=mc^2 does not have the 1/2?

ANSWER: 
First of all, they represent different things so why would you expect them to be the same? One is kinetic energy and the other is rest mass energy. Furthermore, the expression you give for kinetic energy is only an approximately correct equation at velocities very small with respect to the speed of light. At high speeds this is not the kinetic energy.

QUESTION: 
It is said that an ANT can lift something like 10 times his own weight, so if it were human sized, it would be able to hold a truck or something in his jaws. BUT I believe the the smaller the object, the less effort it needs to lift something heavier but I'm not sure why I feel like that. Its as if the laws of gravity for a 100 pound person are different than for a tiny little fraction of a half an ounce of a bug and the little tiny leaves it carries. Its like from the perspective of an ant, its no different than us living on the moon with the same laws of physics or gravity.

ANSWER: 
The physics is simple: to lift any object you must exert an upward force equal to the weight of the object being lifted; there is no scale issue as you suggest. So now you must ask why the ant can exert an upward force relative to its own body weight than we can. That is a question for biology, specifically physiology, not physics.

QUESTION: 
I understand that most heavy elements (above ~ #104) are not found in nature but only manmade in colliders, and that all elements we find in nature (carbon, iron etc) were made from a supernova before our solar system formed. Could some of these manmade elements have been formed in other more powerful supernova and be abundant in other parts of the universe? If so could they be detected?

ANSWER: 
There is no lack of energy in any supernova. The problem is that the heavier elements are very short lived and so promptly decay after their creation. Also, the likelihood of their being created in the first place is very small. In accelerators where they are created we used very focused high intensity beams and end up with very few of the final products.

QUESTION: 
In Einstein's thought experiment, he showed that an accelerating elevator in space would be indistinguishable from a stationery elevator in a gravitational field. What happens if we conduct this experiment for a long time? A person could stand in a stationary elevator for a long time (assuming he is fed, etc.) with no change. However, doing the same in an accelerating elevator for approximately a year at 9.8m/sec/sec would result in the speed of the elevator approaching light speed. What would happen after 2 years? Has anyone ever taken the thought experiment this far?

ANSWER: 
It makes no differrence how large the speed is; this is the principle of relativity, the laws of physics are the same in all frames of reference.

QUESTION: 
The earth is spinning at a fairly high rate of speed. Why don't we have 1000mph+ winds whipping around the equator?

ANSWER: 
Because the earth drags the atmosphere around with it rather like the skin of an orange rotates with the orange.

QUESTION: 
Is it possible to make a primitive compass by rubbing a needle/piece of iron with a silk or "statically charged" cloth? This was used in a recent "Man vs Wild" segment and in the movie "The Edge" with Anthony Hopkins. Is this possible..or is it "urban myth"?

ANSWER: 
To work, the needle must be a magnet, that is be magnetized. So the question is how do you get it magnetized? I did some research and some experiments on your behalf. I can see no reason why rubbing silk should work since electric charge does not cause magnetic fields. However I found a web site devoted to survival techniques which said that this is how to do it. So, I rubbed a needle with silk (50 times in the same direction as directed) and then floated it on a thin slice of cork in water; this "compass" did not work! Next I found a website which said that the needle should be heated to red hot and then allowed to cool down pointing north-south. This makes sense because by heating it up you raise the temperature above the Curie point where all magnetization is destroyed and then when you cool it below the Curie temperature the magnetic domains in the needle tend to align with the external field, that of the earth itself. This "compass" did not work either, maybe because the earth's field is so weak that the needle became magnetized, but not strongly enough to be sufficiently sensitive to work. Next, I found instructions to rub the needle with a magnet 50-100 times in the same direction (not back and forth); this seemed silly to me since I know from prior experience that simply hanging a nail from a magnet results in a magnetized nail. So I simply took my needle and hung it from a pair of my pliers which I happened to know are magnetized. This compass worked like a charm. If you can magnetize a needle by rubbing it with silk, I don't know how and I see no physics reason why it should work.

QUESTION: 
The earth is constantly gaining weight from space dust. I read something like 60 million tons of space dust per year. Supposedly, the maximum size animals can reach is determined by gravity. Millions of years ago, we had much larger animals. Has the weight(mass) gain of the earth over the millions of years been enough to cause any significant increase in the gravitational force exerted on the surface?

ANSWER: 
Actually, the number I find is about 40,000 tons. This is like 4 x 10⁷ kg. Now, the mass of the earth is like 6 x 10²⁴ kg, so the fractional gain per year is like 10^-17, essentially zero. Over a million years the earth's mass would have increased by 10^-9 %.

QUESTION: 
A radio station broadcasts at a specific frequency-----i.e. a specific wavelength. If I put the radio station on a very fast moving platform---say 500 miles an hour, would the wavelength compress---become shorter---in the direction that the station is moving and would it elongate---become longer---in the opposite direction?

ANSWER: 
Yes. However, the differences would be extremely small because 500 mi/hr is really a very slow speed for radio signals which travel at the speed of light, 180,000 mi/s.

QUESTION: 
there are two circles traveling at the same exact speed (not rotations per second, but actual speed like miles per hour) one circle is twice the size of the other (therefore the dot on the circumference of the circle that is measuring each rotation has twice the distance to travel) each continue for the same amount of time. would either make more rotations than the other?

ANSWER: 
I assume that you mean the the centers are moving with the same velocities and the wheels roll without slipping. So each will travel the same distance in a given time. If the distance is, say 100 circumferences of the larger wheel then the larger wheel will rotate 100 times. But the smaller wheel, having a circumference only half that of the larger wheel, will have to rotate 200 times in the same distance.

QUESTION: 
Does the effect of gravity have to "travel" and does it do so at the speed of light? For example, if the sun suddenly ceased to exist, it would take several minutes (I think) for us to notice the light disappear. Would the earth's orbit begin to change at that time also or would the orbit change have occurred when the sun first disappeared?

ANSWER: 
I have previously answered this question/

QUESTION: 
Why exactly does the speed of light (c) come in to play in Special Relativity? It seems to be rather arbitrary in the sense that the Lorentz Transformation would (from what I can gather in my admittedly novice research) yield the same results using any speed that is accepted as being constant, whether it be the speed of light, sound, or the 4:17 to Charleston. Is light specifically used for any reason other than it's bearing on optics?.

ANSWER: 
Good question. I know the postulate that c is a universal constant seems very ad hoc. In fact, that postulate is not needed. Here is the reasoning: the cornerstone of special relativity is that the laws of physics must be the same in all inertial frames of reference. Maxwell's equations, the laws of electromagnetism, predict the speed of light to be exactly what it is and to depend only on two fundamental constants of nature (constants which quantify the strengths of electric and magnetic forces). Since Maxwell's equations are laws of physics, they must be the same in all frames of reference and hence the speed of light must be the same in all frames of reference.

QUESTION: 
A compression spring balance will give a different mass reading on the Moon from that on Earth. A beam balance, however, will give the same reading. Explain why.

ANSWER: 
A spring balance measures the force on the balance and so it measures weight, not mass. A beam balance compares two different weights, but their weights are proportional to their masses.

QUESTION: 
The Doppler Effect changes the apparent frequency observed but is that apparent change measurable in the energy of each photon or is the change in color due to the source just a matter of perception?

ANSWER: 
The photon has a different energy in the moving frame and therefore a different frequency. The frequencies of photons are not independent of the frame of reference.

QUESTION: 
Why do certain colors affect the amount of heat that it can absorb? For example: Why do black colors get hotter than lighter ones? Does it have to do with the specific heat source? If I were to test this with different glasses of water with a specific color using food dye, would I be better off using a heat lamp or sunlight to heat the water?

ANSWER: 
Light carries energy. When something absorbs light the energy must go somewhere. Where it goes is to heat up the absorber. Something black absorbs most of the light (all colors) impingent on it whereas something white reflects most of the light which strikes it. Therefore the black heats up more.

QUESTION: 
we did an experiment in chemisty with radiation vs distance we were still getting readings how can we account for this. i believe that is cosmic rays because they are always in the air is that why

ANSWER: 
You are right, there is always cosmic radiation. Also, there are numerous naturally occuring radioactive nuclei which are everywhere, for example radioactive potassium in concrete.

QUESTION: 
How do we stand on the ground, or a better way to ask it might be "Do we really touch it?" What is actually happening at that level?

ANSWER: 
"Really touching it" does not really have a meaning on a microscopic livel The thing that allows the floor and your feet to exert "contact" forces on each other is the electric forces between the atoms in your feet and the floor. Essentially the outermost electrons in atoms on one surface repel those of the other surface.

QUESTION: 
If there is "less" gravity on the moon, why do astronauts walk slower? Shouldn't their movement be "less" restricted? Shouldn't their muscles have more reign?

ANSWER: 
Several reasons. First, since acceleration due to gravity is smaller, anything which involves falling (for example running) will be slower. Second, the space suits are cumbersome and not good for quick, graceful movement. Third, the unfamiliarity of the situation, much less gravity, would make you more tentative and careful moving around.

QUESTION: 
What replaces air in a vacuum? If the theory that even "empty" space is made of matter (antimatter) is true, then where could the antimatter come from if you suck all of the air out of a sealed container? Can there actually be "nothingness"?

ANSWER: 
A vacuum, classically, is totally void. However, quantum mechanically a vacuum need not be empty and, under the right conditions, particle-antiparticle paris can pop into existence and then pop back out of existence. This may seem like it violates energy conservation, and it does, but that is ok provided that energy is nonconserved for a short enough time to be consistent with the uncertainty principle.

QUESTION: 
I really need you to weigh in on the notion of string sustain vs. string gauge. For example, a couple of weeks ago a banjo friend in Washington State posed the following question to me: Once the inertia of a heavier gauge string was overcome with X amount of Force, would it vibrate longer (momentum) than it's lighter gauge counterpart that was excited with the same amount of Force?

ANSWER: 
I think there is no way to predict this. What is it that determines the sustain of the note? In physics we look at the simplest case first to begin to understand more complicated situations. Ideally, the string vibrates forever! It will stop vibrating because it loses energy in some way or other. This is usually due to friction, either friction moving through the air or the bending friction as the string flexes; I suspect air friction is relatively negligible. The flexing friction will be determined by the properties of the string, what it is made of; e.g. if a rubber band and a piano wire are both tuned to the same frequency, the rubber band will stop vibrating first because of greater damping due to its composition. Another possibility is that sound is coming out and it carries energy away from the string, so even in the absence of any frictional damping, the sound carries away energy. Here things will get complicated again since if you have two strings with the same amplitude and frequency, the more massive one has more energy to start with. Thus, if you have no friction and the two strings each are radiating the same sound intensity, the lighter one will die out first since it had less energy to start with. Of course, if you give the lighter string more amplitude it will start with the same energy as the heavier one but it will also probably radiate more sound energy. Finally I should note that, as a further complication, the instrument itself will probably play an important role. If you simply take the string between two points, not connected to an instrument, you will hardly be able to hear it at all. The resonant connection to the "box" causes the whole instrument to make sound which is loud and carries energy away. So if you repeat experiments with instruments of different designs, say a Stradivarius and a cigar box, you will likely get very different results.Note all the "ifs" in this answer! This is real-world physics where you can examine lots of possible pieces but there are so many variables that it becomes very difficult to make a clear prediction. But, wait�physics is not just a theoretical science, it is also experimental! That is why Galileo gets much more respect than Aristotle as a scientist�you have to make measurements to understand the world, you can't just think about it. So the thing to do here is get an electric guitar with a pickup, connect it to an oscilloscope (a lot of computer-based recording software can do this) and actually measure for various strings the time it takes for the amplitude to die off to, say, half its original amplitude. When you have the answer, you can use the theoretical discussion above to begin to ask why.

QUESTION: 
It seems that E = MC^2 is a remarkably simple formula - so much so that I'm amazed it would turn out to be so simple. Why not E = 2.7MC^2, or E = MC^2.3, or something else not so elegant? Is there a reason that it is so simple?

ANSWER: 
Well, Einstein would have answered that nature's fundamental laws are inherently simple, that is the beauty of nature. When developing the theory of special relativity there comes a time when we want to ask what is the energy of a particle, that is what changes when we do work on the particle. One simply calculates the work done by a force and defines that to be the change in energy of the particle. Mc² just falls out naturally. You could probably get a book on special relativity and follow this relatively simple derivation yourself.

QUESTION: 
Gravity is one of the four fundamental forces. However, as I understand it, gravity is the result of the bending of space-time. Not only that, but I've been told that part of the theory of gravity involves gravity particles which have yet to be discovered. Other forces don't have particles, do they? I mean I've not heard of electromagnetic particles or weak or strong nuclear particles. A recent show on the History Channel showed Dr. Neil deGrasse-Tyson saying that there is no force of gravity, referring to the idea of space-time warping. So what is gravity? Is it a force or is it just the shape of space-time or is it both?

ANSWER: 
This question has been answered more than once before. Whether or not you want to call it a force I consider to be a case of semantics. Classical gravitational theory, i.e. Newton's, was enormously successful for centuries. Modern gravitational theory, i.e. Einstein's general relativity, has provided us with understanding of the roots of the force which we perceive, namely warping of spacetime which you note.

QUESTION: 
When energy is used to do work like move something or break something, is all the energy still conserved in another form or is some of it used up in the work? If, for example, you hit a concrete block with a hammer and shatter it, where does the energy from the blow end up. Is it just converted to heat, or is some converted to heat, some to sound, and some lost? Or, is it somehow tied up in the broken bonds of the material? If it is "conserved" it seems like none can be used up, but when, for example, a powerful ocean wave breaks on a shore it does not seem to heat things up much. And if none is lost, but simply converted to another form, is the work done for free?

ANSWER: 
Total energy of an isolated system is always conserved. But, work is part of the energy equation. Work done on or by the system on the environment changes the energy of the system. So to answer your question you must carefully define your system. Essentially, choose your system. Take a simple situartion where a baseball is approaching a vase with some kinetic energy. Consider this an isolated siystem, nothing else interacts with it. After the vase breaks there is the same amoiunt of energy as before it breaks. This will show up in all the forms you suggest: sound, increased temperature of parts of the system, kinetic energy of all the fragments and the ball, and increased potential energy due to the broken bonds previously holding the vase together. Kinetic energy of the wave goes into heat mostly; but you don't really notice it since there is so much sand and water and the temperature increase will be immeasurably small. Work done for free? Never!

QUESTION: 
a solid cylindrical tube of, say steel, or wood, or anything that has sufficient regular crystalline arrangement to support a ridgid structure. This tube or rod is one light year long and traverses a distance in open space. At the terminal of each end of the rod is a beeper that beeps everytime the rod is pushed into it. I'm on one end and you are on the other. I can communicate to you by tapping one end on the rod Morse Code style, and when I am finished with a statement you tap back your reply. Please explain how this does not result in "instantaneous", or more importantly, faster-than-light speed information flow. I've pondered it (although not really long and hard) and while I could easily be missing a simple and obvious fact that would make the entire scenario useless as an experiment I can't think of one now and am currently looking for answers.

ANSWER: 
The crux is that when you push the end of something the other end does not start moving instantaneously. You compress the crystal structrue near you and it pushes on the next layer and compresses it, etc. until the displacement reaches the other end. So the displacement reaches the other end with the speed of sound. I have previously answered this question in somewhat more detail.

QUESTION: 
In the soup can race, two cans of soup, having the same mass and diameter are placed on an incline and allowed to roll down the incline. One can contains only Broth [ I have used Broth and chicken noodle [ CN ] in my demos in class ], the other can was cream of mushroom. Of course, the Cream of mushroom comes in last in all cases. Explanations for the results that I have found state that the I [ e.g. mom. of inertia ] of the Broth is less, because it behaves as a hollow cylinder , and so it wins the race, or that the internal friction of the Broth is less, resulting in a lower I value, and again it wins the race. Question: What is it about the Broth that gives it a lower I value ? [ I don't see how it can have a lower I value if it behaves as a hollow cylinder ...a thin walled cylinder has I = 0.5 M ( R_i² + R²) = MR², where R_i = inner radius, R = outer radius [ since R_i is approx = R ]. The solid cylinder has I = 0.5 M R². Shouldn't the Solid Cylinder win this race ? And does the can of Broth [ and CN ] really have less internal friction ? I would have thought it had greater internal friction, as it sloshes around, and the Cream of mushroom behave as a solid unit, with no internal friction.

ANSWER: 
What happens, approximately, is that the broth does not rotate at all as the can rolls down the incline and the mushroom soup rolls right along with the can. But, the moment of inertia of the can includes only the mass of the can, not all the broth as well, so MR² is not the moment of inertia; also, the moment of inertia of the soup and can must be calculated separately since it is not a uniform mass distribution. So to do an analysis of this problem you must separate the masses. Let m be the mass of the can and M be the mass of the soup/broth. Then the moment of inertia of each can is mR², the moment of inertia of the soup is �MR², and the moment of inertia of the broth is zero. If you do energy conservation, the initial energy of each is (m+M)gh and the final energy of each is �(m+M)v²+�I(v/R)² where I is the net moment of inertia for each, mR² for the broth can and mR²+�MR² for the soup can. If my algebra was right the result is v_broth/v_soup=√{(4m+3M)/(4m+2M)}, a number greater than 1.

QUESTION: 
Do Helmholtz cavity resonators pertain to gases only, or can liquids also be employed if the excitation frequency is adjusted to compensate for a change in the speed of sound in the liquid medium? In other words, instead of blowing out a candle with a current of air emenating from a cavity resonator, would Lord Rayleigh have been able to create, say, a water jet to do the same?

ANSWER: 
I think the compressibility of the gas is an important component.

QUESTION: 
what is the specific heat of zero ?, and also is there a negative specific heat?

ANSWER: 
I recommend that you read this.

QUESTION: 
Is the specific heat of a gas a fairly well defined quantity like that of a solid or liquid? why? and could you please explain? Thanks, have a great day.

ANSWER: 
I do not know what you mean by well defined. Specific heat is the amount of heat needed per unit mass to raise the temperature by 1 degree C. Gases are a little different from solids or liquids in that they can appreciably expand when heated, hence losing some of the energy to work done by the expansion. So there are usually two different specific heats specified for a gas, one measured while keeping the volume of the gas constant and one measured while keeping the pressure of the gas constant.

QUESTION: 
Graphite, which is pure carbon, burns in an open flame until the graphite completely disappears. What happens to the carbon?

ANSWER: 
It is released as CO and CO₂.

QUESTION: 
Doesn't the center of a mass that is spinning itself (planet, LP, frisbee, etc.) move - so that there is really no unmoving center? Even at the very center, even to the atom, don't these atoms still spin and thus move?

QUESTION: 
Is the density of gases a constant or variable quanity?

QUESTION: 
I'd like to pass on this student's question from my 8th grade Science class. We had been talking about how meteors heat up from friction while traveling through the earth's atomosphere. The question was : "Do bullets heat up while traveling through air?"

QUESTION: 
How does a hydrogen atom give off 4 different colours of light on a line spectrum although it has only 1 electron?

QUESTION: 
what composes a magnetic field, not what generates it, what is it constructed of ?

QUESTION: 
What is gravity? Where does it come from? I realise it is relative to mass and that we have a realy good system for meauring it, but how can two items attract each other without expending energy? Can we measure energy loss between two particles that attract each other gravitationaly? Do we realy understand gravity or is it simply a highly developed and very practical theory?

QUESTION: 
Can you explain to me the significance of an infinite specific heat ?

QUESTION: 
For a long time now I've theorized that there is a relationship between magnetism and gravity. More specifically in the matter that reacts/causes them. The movement of electrons (current) produces a magnetic field, perhaps the movement of neutrons through space do the same? Is it possible to conduct an experiment involving neutrons or protons? Such as creating a material with total internal refraction of neutrons (much like how fiber optics work) and coiling it around. Then measuring any effects it may have. Perhaps doing the same for protons (proton exchange membranes?). I've read many of your replies to questions, including the one from physorg about the relationship of neutrons and gravitons. Perhaps gravity is produced by the movement of protons or neutrons through space. If we can concentrate that effect (like an electromagnet), we could essentially form a gravity inductor.

• Magnetism is not a separate thing, it is just one part of the electromagnetic field. For example, suppose you have a proton sitting still; then there is just an electric field, but if you observe the proton from a moving frame a magnetic field "magically" appears.
• It is well known that gravity is a static field, so in that sense it is more like an electrostatic field than a magnetic field.
• Neutrons, unless bound inside nuclei, have a mean life of only about 15 minutes; so they would not be a good source for your gravity theory.
• Gravity is an extraordinarily weak force. It is only when you have very large amounts of mass around that it is noticeable at all. So protons or neutrons on their own aren't going to have measureable gravitational fields.
• There is already a theory, the general theory of relativity, which tells what gravity "is"; it is the warping of space time by the presence of mass. Einstein, after completing this theory in 1917 spent the rest of his life trying to connect gravity to electromagnetism to no avail.

QUESTION: 
The clock that works with light pulses is commonplace when we're learning about Einstein's Special Relativity. Unfortunately, I can't translate the reasoning to a real clock, with a real pendulum. How different would be the classical explanation, using a real clock?

ANSWER: 
Why is the light clock not a "real clock"? It keeps perfectly good time here in my reference frame, right? I just have to calibrate it so it says tick each second and it is a good clock. Now, here is the trick: take some other clocks (which you consider more "real") and put them next to the light clock. They all run just fine, right? And some of them could be biological clocks like a petry dish of growing bacteria or you growing older. Now, observe all these clocks from a moving frame. It would certainly be strange if another observer observed the clocks running at different rates wouldn't it?.

QUESTION: 
When a number not divisible by 3 is divided by 3 (ex. 10/3=3.3333 repeating ) it does not come out to an exact answer. How, then, theoretically speaking, could there be an exact point on a line segment 10 units long that would be exactly one-third in from the end point?

ANSWER: 
Your answer is both trivial and profound. This point exists because our understanding of geometry and mathematics certainly demands the existence of the number 1/3. In terms of a physical point 1/3 of the distance along a stick, you could get arbitrarily close to finding it but,not perfect because of having to use instruments which can never make an absolutely perfect measurement. It is a concept which nobody should have any trouble accepting. Or should we? One of the open questions in physics is whether space itself if continuous as we assume or is discretized, that is there is some distance which is the smallest possible distance. You might be interested in an earlier answer.

QUESTION: 
How fast was the cat falling that I threw off my 13th-story apartment? (10-foot stories, fairly big cat)

ANSWER: 
There is no way to know without significantly more data. All I do know is that cats often survive such a fall. There is an interesting web page which estimates a cat's terminal velocity at about 60 mi/hr, achieved after falling about 5 stories, but after that they can slow down again by spreading themserlves out! You didn't really throw the cat out, did you?

QUESTION: 
I have two friends that skydive. They got into an argument the other day about how they felt some time after they jumped, once they reached terminal velocity. One said he felt weightless, while the other said he didn't feel weightless. My question is this, what should they feel at terminal velocity? Should they feel 100% of their body weight? Does the force from air resistance become a pseudo normal force? And, just for fun, if they got crazy and jumped out of a plane with a large piece of ply wood so that they were laying on the piece of wood which was falling to the ground, would they feel their weight once they reached their new terminal velocity?

ANSWER: 
As my wife would say, there is no arguing with feelings. What it means to be "weightless" in the usual vernacular is that there are either no forces at all acting on you or else your weight is the only force on you but you have an acceleration equal to the acceleration of gravity. If there were no air at all your friends would be weightless as they fell; an astronaut in orbit has a centripetal acceleration pointing toward the center of the earth so, technically, he is free falling. It is important to bear in mind, however, that this use of weightless is, strictly speaking, incorrect because your weight is the force that the earth exerts on you and this does not go away if you happen to be falling. Now, I would say that neither of them should feel weightless by that definition because there is the weight force down and the air resistance force up which are equal in magnitude at terminal velocity so it is analogous to laying on the floor where the upward force equal to your weight is the contact force with the floor. Do you feel weightless when laying on the floor? I think you would not say so.

QUESTION: 
I saw the following question in a sample admission test booklet: Q. An object is moving in a circle with uniform speed. Which of the following quantities is conserved? A) linear momentum B) velocity C) angular momentum D) rate of change of linear momentum E) none of the above To me, it seems that both C and D are correct. However, according to the answer key, C is the answer. Why is D incorrect? Isn't rate of change of linear momentum equal to the centripetal force, which, in this case is constant because speed is not changing?

ANSWER: 
The reason D is incorrect is the same reason that A and B are incorrect: rate of change of linear momentum is a vector which has constant magnitude but is always changing its direction. An answer of speed, or magnitude of linear momentum, or magnitude of force would have been correct.

QUESTION: 
how fast will i fall down on the moon if i way 1000 pounds?

ANSWER: 
Your weight is not relevant. The acceleration due to gravity at the surface of the moon is about 1.6 m/s/s, so if I drop you, you will have a speed of 1.6 m/s after one second, 3.2 m/s after 2 seconds, 4.8 m/s after 3 seconds, etc.

QUESTION: 
We are studying "sound" in our elementary physics class. I wanted to find out the relationship of pitch to liquid density. I tried pouring an equal amount of Karo syrup, Canola oil, Rubbing Alcohol (99%) and water in four identical pop bottles and hit them with a ruler. Then I placed the bottles in order from lowest pitch to highest pitch. I found that with these four liquids there was a perfect inverse relationship between the two variables with Karo syrup having the lowest pitch when you hit the bottle, but highest density; and rubbing alcohol having the highest pitch, but lowest density. Is there always this relationship, or do these 4 particular liquids just happen to work out so well. I have informed my students that I am not sure if a greater density of a liquid will always produce a lower pitch. Unfortunately, I could find little on the internet that discusses this relationship.

ANSWER: 
I am not sure I have the picture of what you are doing but it is likely that what matters is the speed of sound in the liquid, not the density of the liquid. The pitch you hear is the resonant frequency of the system. Think of a guitar string which has a particular length. You can make the speed greater by increasing the tension in the string and, as you probably know, the result is that the pitch gets higher. A pop bottle is a little more complicated than a guitar string, but it will still have a resonant frequency determined by the geometry of the bottle and the speed of sound in the fluid. Since all bottles are the same, I believe your experiment shows that the speed of sound in alcohol is greater than in Karo syrup. The speed of sound in a liquid is determined by two things, something called the bulk modulus (B) (which measures how elastic it is) and the density (ρ), so speed of sound is related in some way to the density. The speed is equal to the square root of the ratio B/ρ so as the density decreases, the speed increases and the frequency also increases if B stays the same. This is in accord with what you found for your particular choice of liquids, but it was fortuitous because the bulk moduli are certainly not equal. While researching this I did find a science fair project abstract which compared speed of sound with density of six fluids and found them not correlated. Finally, try water; water has a density larger than alcohol but also a higher sound speed, so your pitch, if I have analyzed things correctly here, should be higher.

Bottom line: Pitch (frequency) is not a fundamental property of a liquid but speed of sound is; pitch depends on the geometry of the container. The speed of sound in a liquid depends on the density but not only on it.

QUESTION: 
If a ball is falling in the air, with no air resistance, is the object falling 9.8 m/s faster each second?

QUESTION: 
are you a real physicist?

QUESTION: 
I am 9 and have a question. I have been reading about how when ionised atoms recombine, they emit photons. If you have one Hydrogen atom in space, and you somehow separate its electron far (like one meter) from the proton, and release it; when the electron comes back to the proton, will it emit a photon? If it does, what wavelength photon will be emitted? Does it recombine to make the hydrogen atom, or make a neutron?

QUESTION: 
Marcia is a skydiver who jumped out of her plane. She falls very quickly for a couple of seconds. Then she opens her parachute and her speed decreases to a slower, steady rate until she reaches the ground. Explain what a force is and what forces were acting on Maria during her jump