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?

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?

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.

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!

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?

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.

QUESTION: 
what is the speed of a bullet?

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).

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 ?

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?

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.

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

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?

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?

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 ?

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.

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?

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.

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?

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).

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.

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

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

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?

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)!?

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

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

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)?

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?

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.)

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.

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,

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?

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?

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"?

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.

• 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.

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.

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.

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?)

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?

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?

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?

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?

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

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

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?

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?

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?

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?!

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.

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 m_G. You cannot measure m_G 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 m_I and could compare it to m_G 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?

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.

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?

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?

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?

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?

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?

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

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

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

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?

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?

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?

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?

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?

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?

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?

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

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

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.

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 ?

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.

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.

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

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?

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?

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?

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 ?

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.

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?

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?

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)

QUESTION: 
Why is sonic boom audible and visible?

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

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

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.

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.

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.

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.

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?

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 ?

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

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.

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

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

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.

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

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

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

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?

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.

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?

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.

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.

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.

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.]

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

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.

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?

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?

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

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

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?

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.

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?

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?

QUESTION: 
How does heat cause wind?

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?

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

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.

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?

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?

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.

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?

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

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

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

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?

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

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

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?

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?

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

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?

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

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.

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?

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?

• 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.

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?

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.

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

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?

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?

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?

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?

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?

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?

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

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?

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.

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?

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

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?

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.

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.

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?

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?

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."

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?

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?

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?

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?

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

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?)

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?

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?

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????

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?

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!

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.

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

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

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 ?

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?

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?

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?

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?

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?

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)

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 ?

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.

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?

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?

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.

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

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 F_g=M_pM_eG/r² and electrostatic force is F_e=ke²/r². Hence F_g/F_e=M_pM_eG/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=(Rω)²/R=Rω². 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