Old Questions and Answers

Here is a history of questions and answers processed by "Ask the Physicist!".

QUESTION: 
Is that true that a single atom can be in two places at once? If it's true, why?

QUESTION: 
is it possible for the internal energy of a system to be larger than the kentic energy of the molecules and the atoms making up the system?

ANSWER:
The internal energy is the sum of kinetic and potential energies. So, the internal energy certainly can be greater than the kinetic energy. This is not an issue for an ideal gas, but certainly is for a solid.

QUESTION: 
If a bird is packed in a box, and it's carried by a boy, then if a bird flies inside the box only will the weight less or more or equal?

ANSWER:
This question has been previously answered. Your question is a little different, though, since you do not specify that the bird is hovering. If the bird accelerates upward the box+bird will weigh more and if it accelerates down it they will weigh less. If you want to get much deeper into this, see another earlier answer.

QUESTION: 
When you fire a gun in space (say a rifle of some sort), what will happen? If I were to fire a projectile into space, what would be the net reaction? Would the bullet and rifle travel the same distance, just in opposite directions?

ANSWER:
The principle which governs this situation is conservation of momentum. The momentum of an object is its mass times its velocity. If a system is isolated (like your gun+bullet in space), its total momentum never changes. So, before you fire the gun, the momentum is zero. After you fire, the bullet emerges with a speed V_bullet and the gun recoils in the opposite direction with speed V_gun. If the masses are M_bullet and M_gun, we must have M_bulletV_bullet=M_gunV_gun since the total MV must be zero and they move in opposite directions. For example, a 40 gram bullet with a speed of 500 mph when fired from a 2 kg (2000 grams) gun: the speed of the gun recoil would be 5 mph. Of course, if you are holding the gun we would add your mass to the gun's. (Incidentally you refer to each as "travel[ing] the same distance" which implies you think they will stop. They will never stop but the bullet in my example will travel 100 times farther than the gun in a given time.)

QUESTION: 
what is the diffrence between following through a hit/kick/swing against a sports ball and stopping at impact

ANSWER:
Take a look at a recent answer and see if that addresses your question.

QUESTION: 
according to the law of universal gravitation, what would happen to earth if the sun were somehow replaced by a black hole of the same mass?

ANSWER:
Things would get very cold and dark! But the earth would go happily on its way in the same orbit. To see the effects of the black hole you need to be inside the Schwartzchild radius which, for the mass of the sun, would be 3 km.

QUESTION: 
As I can presume you would sympathize with the frustration one can feel at 'bad/inaccurate science' being used to move works of fiction along, I ask the following questions in order to avoid said pitfall. In the latest theories regarding controlled nuclear fusion for potential power purposes, is helium-3 a potential reactant, or is it more practical as we currently understand the science to use hydrogen isotopes? What kind of radiation hazards exist with nuclear fusion as a potential power source?

QUESTION: 
I am currently taking a physics class in high school and we where discussing nuclear physics more specifically fusion and fission. My questions is a HBomb fuses hydrogen isotopes to create helium isotopes and large amounts of energy by means of using a traditional u238 atomic bomb for initial energy,could you create a device that creates hydrogen form helium exact opposite of nuclear fussion so basically fission of helium atoms to create hydrogen isotopes and take in a large amount of energy instead of burning and vaporizing every thing it would freeze or some thing similar( I'm not quite sure if something else could or would happen) my idea of how to get such a reaction to start would be a particle accelerator like the big one in sweden and just bash helium together at near the speed of light. So I was just wondering if such a thing could theoretically be possible.

ANSWER:
Whenever you split a light nucleus you must put energy in. For example, to cause one He⁴ nucleus to break into two H² nuclei, you must add energy somehow. However, you will not be able to get it to happen on a big enough scale to see anything get cold.

QUESTION: 
Does the pound (lb) measure mass or force?

ANSWER:
In everyday life the pound is often used to represent either the weight (force) or mass of something; similarly, in non-US countries, the kilogram is often used to measure weight (force). In physics, a pound is unambiguously a measure of force and a kilogram is unambiguously a measure of mass.

QUESTION: 
We know that gravitational lensing is due to light being attracted by matter, as predicted by Einstein theory of special relativity. Has it been experimentally shown that Newton's third law applies here too? I.e., has anybody detected the light - matter attreaction, in the reverse direction? It seems that Heisenberg's uncertainty princile muddles this extremely small movements. So does Newton's third law also to be modified by the uncertainty equation?

QUESTION: 
What is the difference between a "principle" and a "law" i n physics? Why do we have "Archimede's Principle" and "Newtow's Law"? I could never understand when one uses one or the other....

ANSWER:
Philosophy of physics may have very precise definitions of terms like these, but I think it can be ambiguous in physics. Here is my take on it. A physical law is a statement of an experimental measurement which is correct as accurately as you can measure it and for any condition under which it can be experimentally observed. Newton's second law is a very good example; it states that an object's acceleration is proportional to the net force on it and inversely proportional to its mass. The only way to find this out is to measure the acceleration while you vary the force and mass. Archimedes' principle, however, does not require that you go to a laboratory and measure the buoyant force, you can derive it using Newtonian mechanics. Sometimes, laws are misnamed from this perspective. Hooke's law, which states that the force a spring exerts on an object connected to it is proportional to its stretch (or compression) and opposite the stretch (or compression), is only approximately true and over only a limited range of stretches.

QUESTION: 
It is generally accepted in the firearms world that a longer barrel equals greater accuracy, but if repeatabilty and predictabilty are the hallmarks of accuracy then why wouldn't a short barrel be as accurate as long barrel. The short barrel has the same input or impact on the bullet shot to shot everyhing else being equal. I am not a student and hope this dose not fit into the "off the wall" catagory. I' m just and old man trying to decide on a project. It comes down to- If the imput is the same shot shot the results should be the same shot to shot, right?

ANSWER:
My brief research on the web reveals that there is considerable controversy over whether or not this is true. It will depend on other properties of the gun besides length, particularly the thickness of the wall of the barrel and the material from which it is made since these will determine the flexibility of the barrel. It appears that one important factor is that the firing will cause the barrel to vibrate like a reed; although this will be very small in amplitude, it may very well be large enough to make a significant difference in accuracy because of relatively large distances traveled by the projectile. One experimenter actually found that reducing the barrel length to 18" from 26" increased the accuracy of the rifle. Apparently one of the reasons for the belief that long is better is that if you rely only on mechanical sights on the barrel (as opposed to, say, a telescopic sight), it is the distance between the notch and the post which is responsible for the improved accuracy, not necessarily the effect of a long barrel on the projectile.

QUESTION: 
This question pertains to the penetration potential of two arrows having : -The same outside shaft diameter - different weights - traveling at different velocities - impacting into an identical target material. In effect, the arrows are identical in every regard with the exceptions noted as follows: Arrow "A" weighs 350 grains and leaves the bow at 212 feet/second; Arrow "B" weighs 650 grains and leaves the bow at 170 feet/second The arrows each travel 20 yards before impacting an identical target material. My question is, which arrow would penetrate deeper into the target; -The lighter (350 grain), faster travelling (212 feet/sec) arrow; or -The heavier (650 grain), slower travelling (170 feet/sec) arrow. In this "experiment" everything is the same except the weight and speed of the two arrows.

QUESTION: 
Why does gravity affect on the water when it is solid or liquid and does not affect it in its gaseous ?

ANSWER:
If gravity did not affect water vapor, it would escape into space. The reason the earth has an atmosphere is because it has gravity. On the moon, the gravity is not strong enough to hold gas molecules from escaping and hence has no atmosphere.

QUESTION: 
As time can be measured, what is the shortest duration of time? How many such periods would fill a second?

ANSWER:
See an earlier answer.

QUESTION: 
I've been reading about physicists that need to shield sensitive experiments (neutrino detection and the like) using "old" lead because the radiation from traces of lead-210 (half-life 22.2 years) in recently manufactured lead interferes with the detectors. My question: when and how is a metal "born"? I mean, any sample of lead-210 has been around for same length of time and so will be decayed to the same degree, surely? The implication is that when you create a new ingot of lead, it is virgin lead, and the radioactive isotope has yet to decay. Can you explain how that is so?

ANSWER:
²¹⁰Pb is a naturally occuring isotope of lead. It is a decay product of ²³⁸U which has a half life of more than 4 billion years. Therefore, if there is any uranium in the ore from which lead is extracted, there will be 210Pb in the lead which is refined from the ore. Since the refining removes only lead, the ²¹⁰Pb will eventually decay away.

QUESTION: 
If the rest mass of a photon is zero,then it should not be affected by gravity.But black hole shows that light is effected by gravity! how does it happen?

ANSWER:
See earlier answers.

QUESTION: 
Is it at all possible that just as photons are observable quantum particles (because of the huge quantity that are produced by things such as light bulbs and stars), there is some sort of observable quantum particle for gravity? It would make sense if these quantized particles were able to self-organize into different structures, causing different behaviors, when there are more or fewer of them around (maybe some kind of gravitational Lego block?).

ANSWER:
These quanta are called gravitons; they are hypothetical since they have never been observed and there is no successful theory of quantum gravity. See my earlier answers about gravitons. I do not see how their self-organizing would make sense. Do photons self organize? What they might or might not do needs to wait until a theory of quantum gravity is found.

QUESTION: 
why the sound not travel on moon whenever there is a surface of moon as a solid medium ?

ANSWER:
Sound will travel through the material of the moon, just not through the vacuum above it.

QUESTION: 
I was wondering, why do we say that electrons in an electric current move from positive to negative eventhough they move in the opposite direction? When physicists found out that electrons move from negative to positive, why wasn't the principal changed?

ANSWER:
Whether the electron is positively or negatively charged does not really matter. The charge was arbitrarily chosen to be negative (by Benjamin Franklin) before it was known that electrons are the charge carriers of most electric currents. Electric current is rate of charge passing through the wire, so that means positive current flows in the direction as positive charges would.

QUESTION: 
Sands drops on a conveyor belt from a stationary hopper. The belt was acted on by an external force to keep moving with a constant speed. I find out the power supply by the force is twice that of the gain in kinetic energy of the sands. I cannot figure out where the other half of the power is dissipated ?

ANSWER:
Friction between the sand and the belt must exist. Another way of saying it is that the collision between the belt and the conveyer is not elastic. The lost energy will show up as heat.

QUESTION: 
Can you point me in the direction of a formula that I can use to calculate the effective weight in pounds of 200 lbs dropped 1 meter? I want to understand (if I can) the relative load of doing a body weight squat (load equals body weight) vs the load of dropping down off a 1 meter high bench (load equals body weight x X).

ANSWER:
There is no way to determine this because the force which a falling object exerts on something depends on the time it takes it to stop. I do not know what a "body weight squat" is and do not have a picture of what you are trying to do.

QUESTION: 
I am a 8th grade physical science teacher and I want to be sure about my understanding of action/reaction forces before explaining it to my students. My students recently completed a ‘balloon rocket activity’ where they calculated speed and identified and described the action/ reactions forces involved. Here are my two possible explanations. Which one is correct?
#1 action force-the air in the balloon exerts a force on the air in the room as it is released, reaction force-the air in the room pushes on the air coming out of the balloon causing motion
#2 action force-balloon pushes on the air in the balloon (forcing it out), reaction force-the air in the balloon pushes back on the balloon causing motion.
I do understand that action/reaction forces involve two objects and do not cancel. I tried to relate the balloon’s behavior to that of a rocket when launched to space.

QUESTION: 
Can one gravitational field be made to reinforce another gravity field, like sound waves at the right frequency can reinforce each other?

ANSWER:
Interference of sound waves results from the wavelike properties; waves can interfere constructively or destructively. Gravity is not really analogous to this because a gravitational field is not a wave. If you have two gravitational fields, they just add like vectors. For example, if the moon is between the sun and the earth, it feels a smaller force than when the earth is between the sun and the moon. But, there is nothing analogous to interference.

QUESTION: 
If neutrons have only the attractive nuclear strong force to bind them together and no repulsive electromagnetic forces to push them apart, why isn't it possible to form large homogeneous "clumps" of neutron matter on earth? Or is it?

ANSWER:
The nuclear force is not just a simple attractive force as you suggest. It is much more complicated and, in fact, a bound system of neutrons has never been observed. A neutron star is a "clump" of neutron matter, but it is held together by gravity, not the strong interaction.

QUESTION: 
Why two protons stuck together have less mass than two single separate protons?

QUESTION: 
Is 2012 real?

ANSWER:
Yes, 2012 will come in a little more than a year. Prepare for the end! Seriously, read an earlier answer.

QUESTION: 
NASA Says "Gravity is a force of attraction that exists between any two masses" and relates this to why we don't fall off the earth. I am confused- as I am now watching NOVA, etc. on TV. As I understand it, gravity is the warping of the fabric of space-time. Please relate space-time to not falling off earth. Is it because the fabric of space-time of the earth is pushing on us, and the earth is equally pushing back on us? Does it have anything to do with the fact the earth is traveling at 67,0000 miles per hour around sun?

ANSWER:
Gravity is a force between objects with mass. But the origin of this force was a mystery for hundreds of years. Einstein's theory of general relativity is a theory which does an excellent job of solving this mystery: it views mass as warping the space around it such that the space becomes non-Euclidean, that is the shortest distance between two points is not a straight line. Hence, for example, light passing by a massive star is bent in its path because it is following a straight line which is curved if we project it onto our Euclidean space of everyday experience. None of this has anything to do with the motion of the earth. The easiest way to get a feeling for why we are attracted to the earth is to imagine a large mass sitting on a trampoline, warping it down. Then objects placed on the trampoline will be "attracted" to the large mass because of the warping of the "space". Be sure to note that this is not meant to be rigorous, just a simple cartoon to help us imagine the effects of warped space. You might want to read some of my earlier answers on general relativity and gravity.

QUESTION: 
Glockenspiel problem: From redistribution of lycopodium powder I have verified that there are 2 nodal lines which for a 790Hz G are about 14 cm apart. My understanding is that a standing wave has been set up in the bar with a wavelength of twice the nodal separation, and therefore the speed of the waves in the steel bar is 790 x 0.28 which gives about 220 m/s, 20 times too slow for steel. Indeed if you take the speed to be 6000 m/s the nodal lines should be 3.8 m apart! Can you help me work out what I've done wrong?

ANSWER:
Sound waves are longitudinal. But the bar is not vibrating longitudinally, it is vibrating transversely. Think of it like very heavy string vibrating; if you have a vibrating steel string, the speed of waves on the string have nothing to do with the speed of sound in steel.

QUESTION: 
I know there are nuclear processes that provide heat and energy, (the sun, power plants, etc...) but are there any nuclear process that mankind can harness that absorbs heat and energy and locks it away in matter.

QUESTION: 
If I were to roll a bowling ball off a 10ft-high ledge at 20mph, and another bowling ball from a 1000ft-high ledge at 20mph, would the bowling ball from the 1000ft ledge land farther away from the ledge than the other ball? In other words, since the height is the only difference, does that change the distance away from the ledge the ball will land?

ANSWER:
When the ball leaves the edge of the cliff, it has a horizontal component of its velocity of 20 mph and a vertical component of zero. As time goes on, the downward component of the velocity gets bigger and bigger but the horizontal component stays the same as 20 mph. Therefore, since it takes longer for the ball to fall from the high cliff, it will travel farther in the horizontal direction. (All this assumes that air drag on the ball is small.)

QUESTION: 
You stated in a previous answer, "The equivalence principle says that there is no experiment that you can perform which can distinguish whether you are in a gravitational field or an accelerating frame of reference." OK, what would happen if you were in an enclosed room away from any gravitational sources, and were accelarating at 1G? I understand that it would feel the same as being stationary on the earth. But, what if you stayed in this room for a very long time? At 1G accelaration, you would hit light speed in about a year (which, of course, is impossible). How could you remain in this room for many years, still feeling like you were in a 1-G field, and not exceed light speed?

ANSWER:
As I have emphasized in earlier answers, acceleration does not play the important role in relativity that it does in Newtonian physics. Acceleration is not an invariant quantity (i.e., unlike classical physics, all observers do not measure the same acceleration of an object). Newton's second law no longer may be written as F=ma but must be written in the form F=dp/dt. Anyhow, acceleration in the way you are thinking about it, such that you experience an apparent force equal to the earth's gravitational field, is not measured by an outside observer but is your acceleration relative to an inertial frame which has the same speed you do. An outside observer would see your acceleration get smaller and smaller as you approached the speed of light. I encourage you to read the earlier related answers.

QUESTION: 
First of all you are lucky, you're a physicist, I want to be one two :( but too bad I'm still 15 :( Anyway, I wanted to ask about E=MC2, I'm a big fan of the great Albert Einstein and his theory is the best, I'm trying to understand everything about it , Einstein said that M=E and E=M, but why did he have to put the speed of light2 in his theory I mean to me it's not strange that E can turn into M and M can turn into E but why did he have to say C2?

ANSWER:
Mass is a form of energy, that does not mean that you can have one kilogram of energy. In physics, all physical quantities must have the appropriate units. The appropriate unit we use for energy is kg-m²/s² which is called a joule (J). Therefore, mass does not have the same units of energy. But since c has units of m/s, c² has units of m²/s² and so mc² has the units of energy. Of course, that does not explain why E=mc²; that is a whole other kettle of fish. I expect you are not ready for the math, but I gave a brief derivation in an earlier answer.

QUESTION: 
My friend's brother was working on a science project, determining the angle which best optimises the distance traveled by a ping pong ball when launched from a catapult. He found 60 degrees above the horizontal to be the optimal distance, which really confused him because he was looking for 45 degrees. My friend and I explained to him that it was because of air resistance. Then we spent the last hour trying to get an equation for the distance a ping pong ball would travel in the x direction given the initial velocity and launch angle when considering air resistance. Basically, given Vo and theta, we are trying to solve for x, the distance the ping pong ball would travel in the x direction. How would you solve for this?

ANSWER:
This is a very difficult problem and one which can only be solved approximately. You can get an idea about how difficult this can be by reading some old answers involving air resistance. The textbook 45⁰ is seldom appropriate for the real world. I would suggest that you keep this project restricted to an experimental one in which you determine the angle for maximum range. Here are a couple of suggestions to make it more interesting:

1. Change the initial velocity of the ball. For no air friction the maximum range is independent of initial speed, but if air friction is important it is not.

2. Maybe repeat for different balls. Perhaps a golf ball would be interesting because it has much more mass. The more massive ball would have a much smaller effect from air friction (although it experiences about the same force).

3. If possible, measure the terminal velocity of the ping pong ball (the speed it achieves after falling until it reaches a constant speed). There might be an interesting correlation between the results of #1.

QUESTION: 
In a chemistry class I took recently, my professor shared an interesting anecdote. He had taken a class in college where the final exam was a single question: how fast does a train measuring two miles long have to be traveling in order to fit entirely into a tunnel measuring one mile long? If I'm not mistaken, as an object approaches the speed of light, it will shrink (or shorten, I'm not sure of the terminology). I posted the question on Facebook, and some of my friends became interested. Unfortunately, none of us have the know-how to answer it. Could you please answer this question? The idea that there is a single quantitative answer to this is extremely intriguing.

QUESTION: 
What prevents a particle from being perfectly at rest?

QUESTION: 
I work in the medical field and I do not understand how it is that the larger a syringe, the lower the pressure is on injection. If the diameter of each syringe opening is the same (which I would assume to be the case since they all fit into the same needles, etc.) and you use a different volume of fluid through the same diameter opening, wouldn't the pressure increase as the volume increases? I cannot understand why this is not so and am hoping that you can help me see the answer through math. Here is the info:http://tomfangrow.com/Syringe%20Pressure.html and they are listing the diameter of the of the syringe, not the diameter of the opening. So with a larger diameter syringe you have a larger volume of fluid.

QUESTION: 
I understand that we have achieved temperatures in the lab of a few billionths of a degree kelvin (pK). Following E=mc^2 as the temperature (E) of the particles is reduced their mass should be reduced by deltaE/c^2. Does this mean that if the temperature of the particles could (hypothetically) be reduced to zero then they would have 0 mass and no weight in a gravitational field. How can matter have no mass and still be defined as matter?

ANSWER:
You misunderstand what E=mc² means. When absolute temperature is approached, the kinetic energy of the atoms approaches zero, that is they stop moving. But, they still have their mass energy when they cease to move. (Actually, you know, absolute zero cannot be achieved and a particle cannot be perfectly at rest.)

QUESTION: 
If someone discovers a unique, undiscovered, physics principle do they have the right to name it... Like stars?

QUESTION: 
So I understand the concept that based on the theory of relativity, the faster speed a traveler goes, the faster time occurs for that traveler. So, why can't we test this theory with some crazy fast planes? Or with a simple rocket with a clock and a radio? Or have we (that you know of)? I realize that this would require ridiculously fast speeds (ludicrous speed!) to give a significant result, but why couldn't we just measure with precision?

ANSWER:
Well, I guess you do not understand! Actually, moving clocks run slower, not faster. For the traveler himself, time progresses at a perfectly normal rate; you seem to imply that if I were going fast and looked at a clock I would see its hands spinning. Not so. Anyhow, time dilation, as this phenomenon is called, has been tested innumerable times. Scientists have taken extremely accurate atomic clocks on trips around the world and found the clocks to behave as relativity would have it. An everyday example is the GPS. Here the device is dependent on timing of signals from satellites which, of course, are moving. If corrections were not made for time dilation on the moving satellites, GPS systems would not work to an accuracy better than a few miles. To learn a little more about time dilation, read the following question.

QUESTION: 
do you know" why" time slows down if you are travelling near the speed of light? relative to someone who is not.

ANSWER:
It is not hard to show, mathematically, why moving clocks run slow. I have always found a simple example to be more instructive than a mathematical demonstration, though. An earlier answer describes the light clock which makes the fact plausible, I think.

QUESTION: 
When converting energy to usable energy, the portion of usable energy which is actually available after the conversion is less than energy used to create it. Does this hold true for nuclear fusion/fission?

ANSWER:
Actually, there is no such "law" that only part of energy can be usable. You are thinking of the second law of thermodynamics which states that no engine operating between two temperatures can be 100% efficient. But some machines are 100% efficient. For example, if I lift a stone I impart increased potential energy to the stone. If I then drop it, that energy is 100% converted to kinetic energy of the stone. In fission or fusion, mass is converted into kinetic energy of the reaction products, 100% efficiently. If you then try to build an engine to take that thermal energy and use it to, say, drive a turbine, only part of that energy can be thus used. This, however, is absolutely no different from trying to use the thermal energy from burning coal to drive the turbine.

QUESTION: 
Although theoretically impossible. if the Sun's mass suddenly ceased to exist how long before the lack of its gravity would be felt on earth and the solar system. We know light traveling at 186,000 miles per second would take seven minutes before we would notice its loss. Any hypothesis on a highly theoretical question? Isn't it true that gravity is the least well understood of the four forces? Is there any connection with the speed of light?

ANSWER:
See an earlier answer. Regarding "least well understood," it all depends on what you mean by well understood. In some respects the theory of general relativity, the theory of gravity, is considered to be one of the most successful physics theories. On the other hand, nobody had been successful in devising a theory of quantum gravity.

QUESTION: 
what is the difference between an object at rest and the same or identical object in motion. Does the application of a force physically change the object or if the object itself is not changed then something else must be changed.

ANSWER:
The questions you ask are answered by Newton's first and second law. Newton's first law states that an object with no net force on it will remain at rest or continue moving in a straight line with constant speed. There is no difference between the two states of moving or rest, there is no meaning to absolute rest; whatever you see to be at rest may be seen as moving by another observer. Newton's second law says that if there is a net force on an object the result is that the object experiences an acceleration in the direction of the force and proportional to the force; so, what is changed by a force is the velocity.

QUESTION: 
Are there any effects on the body if traveling 30 miles per hour or more? My therapist says no and I say yes there has to be some effect. Who is right?

ANSWER:
Your therapist? Hmmm. The earth has a speed at its surface of about 1000 mph because of its rotation, a speed of about 67,000 mph due to its orbit. Do those have any effect on you? Speed is not what you feel, acceleration is what you feel. Accelerations greater than a few times larger than gravity can have bad effects on your body.

QUESTION: 
Why when we throw an object vertically upwards to reverse gravity and stops at a certain height, say that the speed of that object at that time equal to zero does not say that the weight of the object may become less or greater than the force of gravity ??

ANSWER:
The weight of an object is the force which the gravitational field exerts on it. Provided that the height is very small relative to the radius of the earth, the weight is totally independent of how the object is moving or where it is.

QUESTION: 
does the outside temperature change when a mass passes through the atmosphere. I imagine energy is released by the moving body creating a rise in the temperature......what happens in space , where there is no atmosphere?

ANSWER:
When an object moves through the air it experiences a drag force. This force does negative work on the object, that is it takes energy away from that object. What is happening, microscopically, is that the object is colliding air molecules which bounce off with more kinetic energy, that is the temperature of the air will increase. Hence we can say that energy lost to air drag is converted to thermal energy. Under normal circumstances, the increase in temperature is too small to notice.

QUESTION: 
is it true that while a rock is still rising its velocity and acceleration are both downward?

ANSWER:
No. A thrown rock always has its acceleration downward but its velocity on the way up is upward.

QUESTION: 
Equally massed negative electrons and positive positrons annihilate each other when they collide. What if positrons are replaced by heavier but singly charged positive protons in the experiment? Do they annihilate each other? If not and form a neutron or hydrogen, why that route? And come to think of it, why does not hydrogen collapse to form a neutron?

ANSWER:
No, an electron and proton do not annihilate each other because they are not each others' antiparticle. And a neutron is not simply a composite composed of an electron and a proton. When a neutron decays there is a third particle, a neutrino, which is emitted; this is called beta decay. An electron and proton may come together to form a hydrogen atom.

QUESTION: 
if you shoot a gun towards the back of the train and the bullet goes 2000 miles and hour in a train thats going 2000 miles does the bullet stop? what would happen?

QUESTION: 
What prevents something from getting colder than absolute zero? I have ideas, like "you can only lose so much heat", but nothing I can really put together. Nothing scientific.

ANSWER:
Temperature is a measure of the average kinetic energy per particle. At absolute zero all motion ceases. You can't get any "stoppeder" than stopped!

QUESTION: 
I'm working on a project involving gravitational fields, and I came across something interesting. If you calculate out the gravitational fields of the Sun and the Earth relative to the Moon, the Sun's field is much larger. So why is it that the Moon revolves around us instead of assuming a planetary orbit around the Sun?

ANSWER:
Interesting question. If you think about it for a minute, you will realize that the moon does orbit the sun also.

QUESTION: 
Why does crockery make such a loud sound when banged into another piece of crockery such as when stacking dinner plates?

QUESTION: 
If your riding a motorcycle, going 55 miles per hour, and it is 75 degrees outside, what is the temperature of the air that you feel on your skin? I ride a motorcycle and I know that there is a formula but I just don't know what it is and I hope that you have the answer.

QUESTION: 
What happens if a spinning object is released from the axis of rotation? In what direction will it move?

ANSWER:
If it is spinning about an axis which passes through its center of mass (COM), nothing happens, it just continues spinning. If it is spinning about an axis not through its COM, its COM continues moving with a constant velocity equal to the velocity it had at the instant the object was released. It will also be spinning but about its center of mass now. (My answers assume there are no other forces or torques acting on the object.)

QUESTION: 
I am a biologist/teacher and I have a science question that came up as my son was doing a project that has me scratching my head. He was supposed describe how the work done my several natural events changed the environment by the work they did. One of the natural events was fire. Now, as a biologist...I can't lie...I am not great at physics. However, since work=force * distance, do fires do work? Fires may "travel", however, they do not have mass for forces to act upon (fire is energy, not matter).

ANSWER:
For starters, it is not true that fire is not matter; a flame consists of hot gas released by the chemical reactions going on in the burning. Certainly fires contain energy, so where is the work being done to create this energy? To understand this you have to look microscopically. There are chemical reactions going on and that is where the work is being done. Interestingly, the energy is really coming from E=mc². The chemistry law which we all learn, that the sum of all the masses after a chemical reaction is the same as before is technically incorrect. For example, combining carbon and oxygen to form CO₂, a simple form of "fire" releases energy and the mass of the CO₂ would slightly smaller than the mass of C+O₂; this difference is really tiny, too small to measure with any lab balance; what that means is that chemistry is really a crummy (by which I mean inefficient) source of energy if compared to nuclear energy which converts a much more significant fraction of the mass to energy. Thinking of work purely in terms of Fd is really a mechanical idea which does not give a bigger picture of energy. Of course, fire can do work because we can (and of course often do) use the energy it releases. If you want more details about the chemistry of fire, a good short article can be found on HowStuffWorks.

QUESTION: 
Why does the center of mass of a bomb follow a parabolic path if it explodes in the air?

ANSWER:
It is rather complicated to derive the equations which show that the center of mass is an important location for a body or collection of bodies. The basic idea is that, because of Newton's third law, all the forces on something (like the bomb) which originate inside the system cancel each other out if you observe the entire system. Also, Newton' second law says that the acceleration of the center of mass of a system is equal to the sum of the external forces on the system divided by the total mass of the system (this is what is complicated to prove). The bomb experiences only gravity as an external force; all the forces which caused it to explode are internal forces and sum to zero if you look at the whole system. If you really want, I can provide the derivation, but you probably do not really care!

QUESTION: 
barring gravitational forces, can a spacecraft, achieve the volicity of it's own thrust?

ANSWER:
Whenever you talk about velocity you need to specify with what it is relative to. If you are on the spacecraft, you will always see the exhaust coming out with some constant speed depending on how you create that exhaust there on the ship. If you are standing on earth, watching the spacecraft accelerate away from you, there is no reason why the ship could not be moving faster relative to the earth than the exhaust was moving relative to the spacecraft.

QUESTION: 
Does cooling a substance (e.g. a freezer pack) below its freezing point, take its temperature lower? or is the freezing temperature the lowest it can go?

ANSWER:
The only limit to how cold something can get is absolute zero, about -273⁰C=-460⁰F.

QUESTION: 
This maybe in quantum physics... couldn't find the answer(s) in previous ques/ans listing here. As an electron joins with an ion to complete an orbit/shell; does it impart any spin? Magnetic? does it impart any energy (Exo or Endo)? And the reverse Electron separating from an ion; spin... magnetic? energy (Exo/Endo) Is there any information concerning where a electron leaves an atom/ion? as an atom/ion has magnetic poles (North & South) Would this effect the spin/rotation?

QUESTION: 
We are studying momentum. We are taught that when delivering a karate chop to a board (in order to break it) one must pull back on the chop and not follow through. This delivers more force. Why, then, when you hit a baseball doesn't that same rule apply? Why are you taught to follow through?

ANSWER:
I guess I do not believe your basic tenet about "pulling back". Suppose that we simplify the situation and throw a ball at the board. If the ball sticks to the board it has transferred its momentum P to the board and if the collision lasts a time t then the force experienced by the board is P/t; but if the ball bounces back with the speed it came in with, the momentum transferred is 2P and the force is thus twice as great. But, here is my problem: your hand is not analogous because your arm is what is responsible for pulling back your hand, not the collision. Because of the addition of your arm to the equation, there is no guarantee that momentum will be conserved. I do not profess to know much about karate, but if this technique works it may be something like "letting" your hand bounce back, not "pulling" back, that is quit pushing when you collide. Then your hand would be more analogous to the bounced back ball. (What I am trying to say here is that the technique may be right, but the reason is not simple momentum conservation.) Regarding the baseball, the situation is very different because the baseball, unlike the board, is not at rest when the collision happens. If you looked at the ball-bat collision from the frame of reference of the incoming ball, the bat would bounce back.

QUESTION: 
I know that if two carts (same mass and velocity) stick together when they stop because the momentum is conserved. But, why, if they do not stick together, do they bounce off eachother? How come the momentum is not conserved then?

ANSWER:
Your question is incorrectly stated because what you describe happens if the carts have the same mass and opposite velocities. Momentum is a vector quantity and under the conditions you state, the momentum is zero before the collision and must also be zero afterwards. If the carts both stop, obviously the momentum is zero. But, if the carts have equal and opposite momenta after the collision they also have zero momentum.

QUESTION: 
do all electrons travel at the same speed? If they do what percentage of the speed of light do they move?

ANSWER:
No, all electrons do not move at the same speed. They can move at any speed less than the speed of light, just like anything else.

QUESTION: 
I am interested in blocking radiation for health reasons from cell phones, tv's, computers and surroundings in general. There is in the market different products you can carry around your body for this purpose. I would like to know your opinion on the most effective-cost effective method. One option is to carry a magnet but what kind and which side goes in which direction, does it need to touch the skin, and how much can it block? I would like an option that protects as much as possible of my entire body and also for my child who is 5. Please be as detail as possible, I want to do this right.

ANSWER:
You are setting yourself up to be preyed upon by people who take advantage of people's irrational fears. There is no credible scientific evidence that any of these devices is a health hazard. And one thing is really true: there is absolutely no way that a magnet will protect you from any of the radiations from any of these devices because the only radiation reaching you is electromagnetic waves (radio waves, light, maybe some x-rays) and all are totally unaffected by a magnet. If you are really worried about this, your only option is to get rid of them. Trust me, the dangers are minimal if not nonexistant.

QUESTION: 
My question is about the sound waves produced by a guitar. What happens to the sound when you touch the string of the guitar very lightly?

ANSWER:
A vibrating string does not just vibrate with the frequency which you "hear", the fundamental, but with all the harmonics of that frequency as well. If you were to touch the string in the center you would kill all vibrations of the fundamental and all even harmonics. If you touch it elsewhere, you kill all vibrations which do not have a node at that point. (A node is a point on the string which does not move at all as the string vibrates.) This is a technique used by violinists to excite higher frequencies than one normally can play on a violin.

QUESTION: 
I understand the equivalence between gravity and acceleraton. (Someone in a spaceship accelerating at just the right rate, would feel gravity as as he were on the earth). So then, if we feel gravity we must be accelerating but I don't understand what is accelerating. It can't be the earth so what is it?

ANSWER:
The equivalence principle says that there is no experiment that you can perform which can distinguish whether you are in a gravitational field or an accelerating frame of reference. It does not follow that if you are in a gravitational field, you are accelerating.

QUESTION: 
A” metre”rule measures 1 metre in its rest frame (surprise surprise). How fast would another observer need to travel past it in order to see it as a “12 inch” rule instead

QUESTION: 
If you're on a spaceship with rotating searchlight on top, traveling just a snail's pace slower than the speed of light, what happens to the light beam? When pointed forward, does it creep out at the snail's pace to make up the difference? And what about off the side or pointed behind?

ANSWER:
If you shine it forward, you see it receding at a speed of c. But an observer watching you go by will also see it going forward with a speed c. Amazing, huh? But true. At extremely high speeds things do not behave like you expect them to. If you shine the light out the side with speed c, you will see it going sideways with speed c. But, if somebody sees you going by, the light will come out of your ship sidways but also with a component forward in your direction. But the speed of this light will still be c.

QUESTION: 
recently i got an answer on why the speed of light 'c' has such high value. but from where did we calculate this speed? is there any mathematical calculations to find c = 3 x 10^8 m/s?

ANSWER:
See an earlier answer.

QUESTION: 
If I put a fly in a sealed container and place the container on an analytical balance, when the fly takes off and begins to hover in the container, will the reading on the analytical balance go down or stay the same?

ANSWER:
It will stay the same. The fly hovers because the air exerts a force up on him equal to his own weight. But, by Newton's third law, the fly exerts a force down on the air which is equal in magnitude to his weight; the scale registers this downward force. You might be interested in the answer to an earlier question similar to yours in which I go into much more detail.

QUESTION: 
If a hummingbird is hovering in a car, not in contact with any surface in the car, will it travel at whatever speed the car travels? And if so, what force is causing the hummingbird to travel forward in the car since it is not actually in contact with the car? Additionally, what happens to the hovering bird if the car is suddenly stopped? Will it lunge forward with the car or remain in place?

QUESTION: 
If it is impossible to reach the speed of light, and the nearest solar system is over 4 light years away, does this mean that the human race is pretty much stuck in our system?

ANSWER:
Well, if we could achieve speeds comparable to the speed of light (c), four years is not such a long time. And, at such high speeds, the clock of the travelers would show much less time, maybe only a few months. But the problems of attaining such speeds and the stresses large accelerations would put on the travelers would be such that, in my opinion, the only practicable way of traveling such distances would be to have a colony in a large craft with no intention of ever coming back. (Of course, that is not without its problems either!)

QUESTION: 
How did Chicago Pile 1 achieved a chain reaction? I know that they used purified graphite as a moderator and VERY pure uranium in the pile (reactor). The yellow cake was obtained from the Eldorado plant in Port Hope Ontario, which went through an ether process at the Mallinckrodt Chemical Works in St. Louis. Later some of the Mallinckrodt uranium was sent to the University of Iowa at Ames to be cast. Both the cast product and the Mallinckrodt product were used in the CP-1 matrix; the cast product, being purer, being placed closest to the center. During the testing, building up to the pile, they used a beryllium/radium neutron source, both in New York City and, later, in Chicago, to test the graphite as a moderator (as well as initiators for the atomic bombs). That I understand. However, when it came to the actual pile there is no mention of a beryllium/radium neutron source. It certainly appears that they relied on the uranium itself to initiate fission. But, how did they get the first neutron(s) to begin the chain reaction? Does U-235 undergo spontaneous fission? If so, it must be at a VERY slow rate and with a good moderator (graphite or heavy water). I've heard about spontaneous fission and the Flerov-Petrzhak discovery of sontaneous fission in 1940. Fermi must have known about this. So, did they use a radium/beryllium source or rely on spantaneous fission to start CP-1?

ANSWER:
It is indeed true that spontaneous fission is a rare event. On the other hand, there are one heck of a lot of atoms there and even very improbable events are quite possible at reasonable rates. Indeed, the first reactor, in Chicago, had no external neutron source but relied on spontaneous fission. Spontaneous fission can also be triggered by external radiation like cosmic rays. It took me a while to find a source which explicitly said this (see page 23).

QUESTION: 
we know that light is travelling at a very high speed. what makes the light (or photons) to travel at such speed. is there any force that is cating on these photons??????

ANSWER:
The most basic of physical laws, Newton's first law, states that an object upon which no force acts moves with constant speed in a straight line. Hence, nothing needs a force acting on it to continue at a constant velocity. The reason that light has the speed it does is because of the properties of electricity and magnetism of which it is composed. The speed of light in a vacuum is a fundamental physical constant.

QUESTION: 
What is power measured in kwh?

ANSWER:
kWh (kilowatt-hour) is not a measure of power, it is a measure of energy. Power, measured in watts (W), is the rate of energy consumption. One Watt-second is 1 Joule (J), a unit of energy. For example, a 100 W lightbulb consumes 100 J of energy per second. Since a kW is 1000 W, and one hour is 3600 s, 1 kWh=3,600,000 J.

QUESTION: 
Does the frame of reference concept carry over into the interaction between electrons and magnetic fields? Specifically, does a magnetic field only result from an electron moving relative to an observer? For example, would an observer not moving relative to an electron not see a magnetic field and an observer moving relative to an electron see a magnetic field?

ANSWER:
If the electron were a charge alone, it does not have a magnetic field if at rest. However, since the electron has a magnetic moment (i.e. it looks like a tiny bar magnet), it does have a magnetic field when at rest. If it is moving, there is an additional magnetic field due to the moving charge.

QUESTION: 
If there were two people on opposite ends of a metal rod that was one light-year long, and one person pushed their end of the rod one meter, how long would it take for the other end of the rod to move one meter?

ANSWER:
This question has been asked many times and has been answered. The bottom line is that it would take much more than one year.

QUESTION: 
Say you started moving straight upwards from the ground, how high could you go before the air becomes unbreathable?

ANSWER:
People climbing Mt. Everest, about 30,000 feet high, usually carry oxygen because the air becomes so thin at that altitude. It is not that the air becomes "unbreathable" but that there is not enough of it.

QUESTION: 
If the amount of gravity speeds/slows time, Then by that logic, would a person living on the moon age faster then a person who lives on the Earth?

ANSWER:
Yes, but the time rate would be so small as to be almost impossible to measure.

QUESTION: 
If you fired a gun in space, would the kickback send you flying in the opposite direction at the same speed as the bullet?

ANSWER:
No, you would fly off with the same momentum as the bullet. Momentum is mass times velocity, so if your mass is 1000 times larger than the bullet's, your speed would be 1000 times smaller.

QUESTION: 
I just watched a program on History last night about time travel. My Dad was really into it and I learn a lot from him. He told me back then that if you put a clock on a space ship and have a clock with the same time back on earth, and that ship travels close to the speed of light (actually he said "very fast") when the ship gets back to earth the clock on the ship will have run slower than the clock on the earth. They said that if we could hit warp, we could get to Alpha Centauri (sp?) in about 45 months. However, in actuality, it would take years. Here's the question: Does that not blow Star Trek and Warp Speed out of the water? Let's say that Kirk gets a distress call from a planet light years away. He hits warp. On his vessel it takes minutes. On the other planet it takes years for him to get there. By that time - well, it's all over.

ANSWER:
You are absolutely right with regard to what we currently believe to be possible for time travel. You may go to the future but you can never exceed the speed of light and you cannot go to the past. Star Trek is fanciful in that anything greater than warp 1 is greater than the speed of light. Warp 9, for example, is about 1500 times the speed of light; at that speed it would take 15 years to get to the center of our galaxy! (I mined these "facts" from Lawrence Krauss's fun book The Physics of Star Trek.) So Star Trek authors have to imagine another means of time travel than just going fast, things like worm holes which let you (in principle) burrow from one point in space-time to another. (Don't ask me about worm holes because I really do not know that much about them!)

QUESTION: 
In outer space, there isnt gravity to hold back and since space is a vacuum, there is almost nothing to cause drag or resistance or resistance on an object. If we kept applying a force on the object, wouldn't it keep accelerating? To and even past the speed of light? (provided we had a strong enough propulsion device)

QUESTION: 
I understand that a moving charged particle creates a magnetic field. If you have two parallel wires with electrons moving the same direction, the magnetic fields cause the wires to attract. If the same two wires have electrons moving one way on one wire and the opposite way on the other wire, they repel. Now, what about the case where the electrons are traveling in a vacuum. When two electrons are moving opposite each other, they will repel. But what about when they are moving parallel to each other? Their velocity relative to each other is zero. Do they see a magnetic field and attract, or do they see only the electric charges and repel?

ANSWER:
The two situations (current in a wire and current in an electron beam) are very different as you seem to realize because there is no electric field in the case of the wires. There will still be magnetic forces in the beam cases, but the electric forces will be so much bigger that you would be hard-pressed to see any magnetic effect at all. If you try to look at two single electrons moving parallel, they will see no magnetic force in their own rest frames, but they will in the laboratory frame of reference. This is where releativity comes in and transformation of the electromagnetic field is probably beyond the scope of this site.

QUESTION: 
I just recently had an argument with an engineering friend of mine and was hoping you could help settle it as we tend to end up at an impasse when it comes to disagreements like this. The argument involves human body falling at terminal velocity and landing back first on various materials. He is of the opinion that landing on cement will ultimately cause less damage to the body than landing on soft loosely packed soil. He bases this on the idea that the speed of sound through something as dense as cement is faster than the speed of sound moving through something as dense as flesh and that this somehow imparts a greater ability to absorb an impact and therefore will do less damage to the body than loosely packed soil. I am of the opinion that the soil is able to shift its position under pressure and able to be compacted into a denser form than its current state and that this allows it to better absorb the impact (than concrete would) which reduces the damage done to the object hitting it. He argues that this is only the case in low speed impacts and that in a high speed impact at terminal velocity the concrete would be the better material to land on. So, which material would you expect to do the least amount of damage to an object with roughly the same density as water that hits it at terminal velocity and why?

QUESTION: 
Is there a formula which could be used to calculate the following scenario: How long would a single point on the floor be in contact with a disc brush having a surface area of 126 sq. in., rotating at 300 RPM, and traveling forward in a line at 2.5 mph?

QUESTION: 
I have a question about traveling around 99.9% the speed of light and communications at those speeds. If a ship were to travel away from Earth at 99.9% the speed of light would communications with Earth be possible? The second part is what effect would time dilation have on communications? Would it be possible to respond to a question that in relative time has not been asked yet?

ANSWER:
In an earlier answer I discussed how communications would sound, but communication is certainly possible. As I note in that earlier answer, you could not have a conversation type of communication because of the large distances and hence long travel times of signals. You certainly cannot respond to a question which you have not received.

QUESTION: 
Does the inverse square law apply to laser beams?

ANSWER:
No, because the inverse square law applies if the waves spread out in all directions uniformly. It does not apply to a flashlight either.

QUESTION: 
When you jump off a high building, your velocity changes drastically to zero as you hit the ground. Since your mass is constant, the impulse is also constant. How does a soft mat cushion the fall?

QUESTION: 
in a current carrying wire ,is there a potential difference between any two points ,in between which there is neither active or passive elements

ANSWER:
All real-world wires have a nonzero resistance and so, if a current flows, a potential difference exists. (Superconducting wires will not have a potential difference.)

QUESTION: 
I am doing a report on breaking the sound barrier, achieved first by the United States Air Force (And Assistance with NASA!)! I was wondering how fast is the speed of sound in ranking? I know light would be the first, but would sound be the second fastest?

ANSWER:
Do you mean second fastest wave? Light, you must appreciate, is in a class by itself because, unlike all other waves, it can propogate through empty space. However, light propogating through a medium is somewhat slowed and therefore far faster than sound. Regarding sound: sound waves through a solid, like a steel rod, travel much faster than sound in air. There is really no point in trying to "rank" wave speeds because there are so many possibilities.

QUESTION: 
My daughter is working on a physics project that includes making a rollercoaster that must include 7 elements (loops,cork-screws,camelbacks) but the marble must complete the loop and stop at the original beginning. Is it possible, being pulled along by gravitiy and falling that it could be built to return to the original starting height? Without motors or mehanical propulsion?

ANSWER:
No, it is not possible. If she had a frictionless marble, she could do it but some energy will have to be lost to friction and the real therefore could not return to its original altitude without an external push somewhere.

QUESTION: 
why does a half empty bottle of sand (or half full lol) not roll?

ANSWER:
Whether or not it rolls depends on lots of things. If the sand is real dry and powdery, it will sort of behave like you have half filled it with a fluid and roll just fine. If it is real wet and sticky, it will behave as if you have half filled it with concrete; but this will still roll if you get it going fast enough, just in a jerky fashion. If it rolls too slowly, the fact that the center of gravity of the whole thing is not at the center of the bottle means that it will rock rather than roll. Think of the limiting case where the bottle has no mass, at least negligibly small compared to the sand. It will rock like a half cylinder.

QUESTION: 
I am looking for a physics answer to why it is bad to have too much positivity. That is, I am looking to describe - it in somewhat lay terms - how a basic structure of an atom has protons and neutrons... yada yada yada As you might have guessed, I am not a physicist but am looking for an answer that is indisputable and based in fact.

ANSWER:
It is not at all clear what you are asking. Since you mention protons and neutrons, maybe you mean the nucleus of the atom, not the atom (which also includes electrons, "negativity"?) There are two competing forces in a nucleus, the electrical force which tries to blow it apart because protons repel each other electrically, and the nuclear force by which nucleons (which is what we collectively call neutrons and protons) attract each other. If you try to make a nucleus with just protons, it will not be stable because the electrical force wins out. But, if you add some neutrons (which do not feel the electrical force) to the mix, you add some nuclear force without adding any electrical force, so the nucleus may become stable. This also has the effect of having the protons farther apart because there are more particles in the nucleus, and this means the electrical force is weaker. Atom heavier than around calcium tend to have more neutrons than protons in their nuclei.

QUESTION: 
An iron rod 1cm in diameter and 1 light year in length is located in outer space far from any astronomical body. Sufficient force is applied to one end (end A) of the rod the direction of its opposite end (end B) to move it one meter. An observer at end B will wait to observe the rod move one meter and then apply sufficient force on the end (end B) of the rod to move it one meter in the direction of end A. The final position of the iron rod along the axis of the forces applied from either end will be where it began. How long would an observer and end A wait between the initial force applied at end A and end A returning to it's original position?

ANSWER:
I have previously answered almost exactly the same question. The answer is that he would wait much much longer than two years.

QUESTION: 
Have tachyons been empirically proven to exist? If not, what evidence is there to suggest they might exist?

ANSWER:
Not to be flip, but the answers to your questions are "no" and "none". The Wikepedia entry on tachyons is pretty readable.

QUESTION: 
A friend says that changing electric and magnetic fields generate one another, and this gives rise to visible light when the frequency of change matches the frequencies of light.

ANSWER:
Your friend has it right. Electromagnetic waves are caused by radiating electric charges; charges which are accelerated radiate. An example of this is the broadcast antenna of a radio station which has electrons oscillating back and forth to create the radio waves. For visible light, atoms act like tiny antennas.

QUESTION: 
Why do balls bounce? Also because non-bouncy balls travel back up with less energy, does that mean more energy has been transferred to the surface upon which it was bounced (and a more bouncy ball has transferred less energy to the ground?) AND if that is true what would be the best way to measure the difference in energy transferred (throwing balls at scales, throwing them at Styrofoam and observing imprints, etc.)?

QUESTION: 
As I understand it, when a photon is created, say in a light bulb, it is instantaneously moving at light's speed. Its acceleration is therefore infinite. Since it's massless, perhaps that level of acceleration is acceptable, but the entire picture seems somewhat muddy to me. Is there a physical description of the event, or is it in reality no more than a mathematical result of equations those of us not in the field cannot grasp?

ANSWER:
Let's just stop thinking about photons for a second. Think about water waves; do you worry about their accelerating up to their speeds instantaneously? No, they just begin at the source and propogate out. Or sound waves? There is something beating against the air and the waves just move out. The same is true for electromagnetic waves. A radio antenna has electrons accelerating back and forth creating time varying electric and magnetic fields which result in electromagnetic waves. Acceleration is never an issue in these examples, waves are created going as fast as they go.

QUESTION: 
With my understanding of relativity an objects speed is determined by how fast it is moving in relation to another object. Also no object can pass the speed of light (C). So how would it be possible for an object to travel at 75% C in one direction while another object travels 75% C in the opposite direction?

ANSWER:
See earlier answer. The bottom line is that things do not behave like you expect if the velocities are not very small compared to the speed of light.

QUESTION: 
I came up with a thought experiment described below. I setup a potential difference of say V volts between two horizontal parallel plates kept fixeda certain distance apart.I have given the upper plate +q C charge & the lower plate -q C charge.This system of course has some energy that depends on the charge in the plates and the plates separation. Now what i do is I pass a negatively charges particle with some initial kinetic energy, say Ei and velocity component purely horizontal , halfway through the two plates . As the particle emerges out of the electric field, it has gained some velocity along the vertical direction and thus has a kinetic energy Ef, greater than Ei. The difference dE = Ef - Ei is obviously positive implying it has apparently absorbed some energy from the field in course of its passage through it. As far as I believe, this brings about no change in magnitude of charge in either of the plates and the separation between plates has been held constant. So in no way is there a change in the energy of the parallel plates system. It follows that by doing this I have managed my little particle to gain some energy without any other system having lost it!!

QUESTION: 
I know that sub atomic particles are believed to have spin due to the internal angular momentum and magnetic field but i heard that if two sub atomic particles are created at the same time they both have to have opposite spins, but until the spins are observed they both have both spins.. how can this be? If it is true, how does this work to a pair of sub atomic particles where the twin of the particle is X number of light years away and we observed one and found it went clockwise tthis means the one x number of light years away instantly starts going anti clockwise which means the information travelled fast than the speed of light which ofcourse is impossible.

QUESTION: 
A physics teacher told me that velocity is the rate of change of displacement with respect to time. It seems to me that displacement is a vector, which is the difference between two vectors that describe a body's position relative to a fixed reference point. Velocity represents the change in these position vectors over a certain time period. So velocity is the rate of change of position with respect to time or it is the rate of displacement with respect to time. Either of these definitions is OK with me. But to say it is the rate of change of displacement with respect to time is very confusing to me. Is it actually correct to say that? If so, please explain how I am wrong.

ANSWER:
It is, I think, redundant to say rate of change with respect to time. "Rate of change" implies "with respect to time". Velocity is indeed the rate at which the displacement vector changes. The average velocity over a time interval is the change in displacement vector divided by the elapsed time. The instantaneous velocity is the average velocity over a vanishingly small time interval which, unless you have studied calculus, will likely not mean much to you.

QUESTION: 
In geometric optics, when we have two lenses in series, and we wish to calculate the position and size of an image of an object... we can take the image of the first lens as the object for the second lens. But it is not clear to me why we are allowed to do this in cases where the placement of the second lens prevents the image of the first from forming. For example, if we have a convex lens with on object to its left creating a real inverted image on the right... and now we place a concave/diverging lens between the convex lens and the real inverted image.... why are we allowed to use the hypothetical real image of the first lens, which doesn't exist now, as the object for the second lens? It doesn't seem obvious to me at all, that we should be allowed to do this. Can you explain this?

ANSWER:
In a case like you describe, if the object of the second lens never actually forms, then it is called a virtual object and the corresponding object distance must be entered as negative in the lens equation. I have chosen an example to show that it can be done both analytically and with ray tracing. There is a converging lens (blue) with focal length f₁=1 cm, an object at an object distance p₁=2 cm. The image distance is found using the lens equation, 1/f=1/p+1/q, so 1/1=1/2+1/q₁, so q₁=2; the magnification is m₁=-q/p=-1 and the image is real since q₁>0. Now, as you suggest, insert a diverging lens (red) with f₂=-3  cm (f₂<0) 1 cm to the right of the first lens so that the first image never actually forms. Take as the object distance p₂=-1 cm so the lens equation is now 1/(-3)=1/(-1)+1/q₂, so q₂=1.5 cm, m₂=-1.5/(-1)=+1.5; so the image is not inverted (compared with the object which is itself inverted on the ray diragram) and magnified by 1.5 and real. In my ray diagram I have only drawn two rays (in red) to locate the final image, the central ray and the ray coming in parallel to the axis; you could draw more but only need two to locate the image.

QUESTION: 
I am a artist currently studying in London! I was going to make some work about the Sun, more specifically the sunset. I did some calculations and tried to work out the difference in sunset times if you were on the ground and to each other floor in a building. Working with the measurement of one floor being 10ft I worked out the time, PER INCH, the sunsets are 0.05625 seconds in difference. I was wondering if you could confirm or replace this figure as correct.  

QUESTION: 
If an object with mass greater than that of the earth (say 1.5X) was to be placed on the surface of the earth, would the earth remain stationary or would it travel on a new path? To develop on what I mean; because the gravitational field of the earth would of course act towards it's center, and the mass's weight would therefore be acting towards the center of the earth, would the object simply sit on the surface or invoke movement due to being of greater mass?

QUESTION: 
i just took up Table Tennis again after a long layoff and as you may know friction (imparting spin to the ball) is a large part of the game. I was surprised to read in one book that the amount of spin imparted to the ball from the paddle during the stroke was not a function of the SPEED at which the paddle "brushes" (almost tangentially) the ball, but of the ACCELLERATION while the paddle is in contact with the ball. They suggested approaching the ball slowly and accelerating through impact . I wrote to an "Ask the Coach" forum and the coach said he thought it was just a function of speed but asked if anybody knew the physics of the problem. I think i understand the difference between static and dynamic friction, but since the paddle isn't starting from rest, just going a constant high speed in one case vs. slow to fast in the other, i didn't think this would apply.

ANSWER:
Here is all I can think of. Because of inertia and limited frictional force, hitting the ball with a surface moving very rapidly may result mainly in slipping of the two surfaces which would not be so good for imparting spin. So, starting slow to allow the paddle to "grasp" the ball more effectively might result in more transfer of spin.

QUESTION: 
How do you explain Boyles Law in terms of molecules and atoms and what assumtions do you make?

ANSWER:
Boyle's law is just a special case of the ideal gas law (for constant temperature), PV=NRT. To derive the ideal gas law you assume the gas is in thermal equilibrium with the container and that the volume of all the molecules is small compared to the volume of the container.

QUESTION: 
What is the force called that makes items in a container go to the sides of the container when you spin the container around? Tony - 9yrs old

QUESTION: 
Can you please break down Schrodingers equation for me? What each variable stands for and can you please explain to me the theory behind his equation. I'm just a curious English professor and I'm having a difficult time understanding the equation as well as Heisenberg's Uncertainty Principle equation. If you can put the breakdown in layman turns I will forever be grateful.

QUESTION: 
When salt is poured on a table it couses a mound, why does gravity not couse the salt to spread out on a flat layer?

ANSWER:
Because of friction, the same reason a block will not necessarily slide down an incline if there is adequate friction. Actually the study of such phenomena (often called "sand pile physics") can be pretty complicated and is a serious study. The angle the pile makes with the ground is called the angle of repose; interestingly, the angle of repose for a particular material is independent of how high the pile is.

QUESTION: 
Doing an outside loop, why does the normal decrease with an increase in tangential speed?

QUESTION: 
The more I read the more confused I get....I was asked What kind of light would be emitted by atoms if energy were not quantized?

ANSWER:
If an atom could be in any energy state and decay to a lower state, you would have a continuous spectrum of light. If you had lots of atoms being excited and the deexciting you would just get white light.

QUESTION: 
In a standard textbook example, a charged particle moving with a constant moderate velocity perpendicular to a constant magnetic field is deflected. If an uncharged observer is traveling parallel to the charged particle with the same velocity, it would see the charged particle as having zero velocity at first and then motion in the direction of the deflection observed by the stationary observer. If the magnetic field is "constant", how does the co-moving observer explain the force on the charged particle? I guess another way to ask the question is if charged particle motion in a magnetic field produces a force on the particle, and motion is relative (to the field), then is it possible to have "moving" field produce a force on a "stationary" particle? Or more fundamentally yet, if the field is constant, how does the particle "know" it's moving through the field?

ANSWER:
What a good question. The thing is that electric and magnetic fields are not really different fields like all elementary texts make it look. There is one field, the electromagnetic field and the "mixture" of how much of each is electric or magnetic depends on the frame of reference of the observer. If, in a particular frame of reference, there is a pure electric field, then if you view this field from a moving frame, a magnetic field will appear. Similarly, if, in a particular frame of reference, there is a pure magnetic field, then if you view this field from a moving frame, an electric field will appear. The situation you describe is the second and it is then the electric field which produces the force on the particle "at rest".

QUESTION: 
Lately, in physics class, I have been studying how some substances will heat up (less calories) faster than others. I have a question pertaining to this on the atomic level, rather than the molecular level as we have been learning. If some substances heat up faster than others, then wouldn't certain elements heat up faster than others when changing states (say from a liquid to gas)? For instance, if we take water, with both hydrogen and oxygen, why wouldn't one element separate from the other (assuming they have different boiling points, or require less calories for heat of fusion)? Why doesn't one element reach the boiling point and not separate from the other element?

ANSWER:
This is really chemistry, but chemistry is really physics! It all depends on the molecule you are working with. Some molecules will dissociate before the solid melts or the liquid boils and then you have a mixture of the constituent elements. But most often, particularly with common inorganic compounds, the molecules are sufficiently strongly bound that heating them does not supply the necessary energy to dissociate them and the properties are determined by the molecule, not its constituents. (Your particular example is one in which the atoms are too tightly bound to be broken apart by adding some heat. But, if they were not, when they separated you would already be well beyond the boiling points of either because both are gases at normal pressures and temperatures. The bottom line in answer to your question is that it takes energy to break a molecule apart and often supplying heat is insufficient to do that.

QUESTION: 
It's my understanding that as velocity increases, so does mass. Now, let's say we have two spaceships accelerating away from Earth at the same rate parallel to one another. To observer's on Earth, their velocities are increasing and therefore so is their mass. However, relative to one another, they are motionless and therefore their mass would not increase. Does this mean that the mass of a particle is relative to the observer? From Earth's perspective, the ships would need more and more energy to accelerate, but from the ships' perspective, they would not. What's the explanation for this apparent contradiction?

ANSWER:
Just as the case of your earlier question, the resolution of the apparent contradiction is length contraction. Think of there being a stick attached to the earth and pointing out to the spaceships' destination. As the spaceships go faster and faster this stick gets shorter and shorter so that each burn of fuel results in a smaller and smaller acceleration.

QUESTION: 
This is a pretty basic question, but what exactly is force? Newton's second law states that F=ma. Mass, distance, and time are easy, and velocity and acceleration are obvious to anyone who can ride a bike. Momentum is a little more difficult, but it has an intuitive definition (how hard it is to stop something, or how much it would hurt if you crash your bike at various speeds). I can't define force without using the terms "mass" or "acceleration." Given that F=ma is a law and not a definition, can you give me a definition of force? Or am I misunderstanding "law" and velocity=distance/time would also be a "law"?

QUESTION: 
A rollerblader leans into a turn as he moves in a circle. The net torque about any point in the rollerblader must be zero if the rollerblader is not to fall over. If the torque is computed about his center of gravity, then the torque caused by the normal force of the ground up on his wheeels is balanced by the torque due to the centripetal force of static friction of the ground on his wheels. If these torques are computed about the contact point of the wheels on the ground, however, there seems to be a non-zero, net torque due to his weight since the normal and static friction forces act through the contact point and would not contribute to the torque about this point. The moment of inertia of the wheels negligable. How can there (seemingly) be a net torque about the contact point and none about the cg? P.S. I submitted this question to the "Mad Scientists Network" and the engineer who answered it said there was an outward, "righting" force that acts through the cg. He did not state the nature of this force but it sounded suspiciously like he was thinking of centrifugal force. This didn't seem right to me. I want to know how a physicist would explain this.

ANSWER:
Hey, guess what: the engineer got it right! He just didn't give you a complete enough explanation, and you deserve one! Ask the Physicist goes where other Q&A sites only dream to go! Enough silliness, here it is. Newton's laws are not valid in accelerating frames of reference and you are attempting to apply Newton's first law (N1) (the sum of all torques equal zero is the rotational equivalent of N1) to the skater who is accelerating because he is moving in a circle. If he were not accelerating, i.e. moving in a straight line and leaned over he would fall. The best way to deal with accelerating systems like this is to do a trick which inserts forces which do not exist to force Newton's laws to be true; these are called fictitious forces. In this case you must add a fictitious force which points away from the center of the circle, acts at the cg, and has a magnitude equal to the centripetal force; this is called the centrifugal (Latin root center fleeing, fugo is I flee) force. Then everything works out and he is in "equilibrium" in his accelerating frame.

QUESTION: 
what happens when a rubber ball is thrown in a square room in outer space?

ANSWER:
Every time it collides with the wall it will lose a little energy, but between collisions it will move in a straight line. So, if given an initial velocity parallel to one of the walls, it will bounce in a plane, going slower after each collision. If there is air in the room, it will also slow down as it goes on its path.

QUESTION: 
I have read that in the early formation of the universe photons collided to create electron. How can two massless particles (photons) create a particle with mass (electrons)? What is the process?

ANSWER:
Mass is not a problem, because photons have energy so it could be converted into mass, another form of energy. The real problem is electric charge. Photons have no charge and so charge would have to be created. I do not know where you read this, but I think you must have misunderstood.

QUESTION: 
I understand that nothing with mass can travel the speed of light because an infinite amount of energy would be required to accelerate the mass and there is not an infinite amount of energy in the universe. As a statement, it makes perfect sense. But I don't understand the math: ". . . 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." Can you explain without math why an infinite amount of energy would be needed?

ANSWER:
No, you really cannot do it totally without math, but maybe I can make it more explicit. Let's calculate the quantity 1/√(1-b²) for various values of b. If b is the ratio of speed to light speed, then this factor is what determines how big the energy of the particle is. For example, if the speed is 50% the speed of light, b=0.5, b²=0.25, and 1/√(1-b²)=1/√(1-.25)=1.15. Go faster, say 80% the speed of light, then b=0.8, b²=0.64, and 1/√(1-b²)=1/√(1-.64)=1.67. Go faster, say 99% the speed of light, then b=0.99, b²=0.98, and 1/√(1-b²)=1/√(1-.98)=7.07. Go faster, say 99.999% the speed of light, then b=0.99999, b²=0.99998, and 1/√(1-b²)=1/√(1-.99998)=223.6. Go faster, say 99.99999% the speed of light, then b=0.9999999, b²=0.9999998, and 1/√(1-b²)=1/√(1-.99998)=2236. Can you see where this is going? When (never) we go 100% the speed of light, then b=1, b²=1, and 1/√(1-b²)=1/0=infinity.

QUESTION: 
Why is it necessary to orbit Earth and reach an escape velocity to fly away from the Earth? Why can't a spaceship, say destined for the Moon, just fly in a (generally) straight line from the ground to the Moon without orbiting Earth?

ANSWER:
It is not necessary. You could, in principle, go directly to the moon. Planetary probes do not go into earth orbit on their way out.

QUESTION: 
My inquiry is regarding a vertical ascent away from the Earth towards space. I was in a discussion with my father recently and we ran into a disagreement. His stance was that in order to leave the Earth vertically one needed to achieve Escape Velocity consistently through the ascent. My argument was that if there were a source of constant or, maybe consistent is a better word, propulsion an object could rise vertically at any speed.

ANSWER:
Escape velocity is the minimum speed you need to give to a projectile to go up and never come down. If you had a source of propulsion, you could escape the earth at 1 mph if you wanted to.

QUESTION: 
If you were in a helicopter hovering above a single point on earth's surface for a long period of time, would your helicopter need to have any horizontal velocity to keep up with the rotation of the earth? Would the answer to this question change with the distance you are from the surface of the earth (i.e., boundary layer effects)?

ANSWER:
Certainly not. The helicopter flies with respect to the air and the air moves (more or less) with the rotating earth. Any lateral velocity component the helecopter has would be to compensate for wind, not motion of the earth.

QUESTION: 
Someone told me that if one body revolving around another is always facing that body which it is revolving around, that it is also rotating about its own axis, I don't understand how a body can be rotating if it is always facing the body it is revolving around, can you explain how this can be so please?

ANSWER:
To determine whether you are rotating, look a distant objects. If the distant stars all do not move you are not rotating, if they do, you are. A good example is the moon, the same side of which always faces the earth. Over the course of one month, it sees all the stars rise and set.

QUESTION: 
I just have a basic question regarding relativity. It's my understanding that for an object traveling at 99% the speed of light, time will slow down by a factor of approximately 7 (from an observer's perspective). So, if I were to travel 4.37 light years to Alpha Centauri at 99% light speed, from the perspective of an observer on Earth I would get there in about 4.41 years, but time would have slowed down for me and I would only have aged 0.63 years. Thus, from my perspective, I just traveled 4.37 light years in a matter of only 0.63 years. So, from my perspective, did it seem like I was traveling faster than light or am I thinking about this in the wrong way?

ANSWER:
Here is what you are missing: you see the distance between the earth and the star as moving by you at 99% the speed of light. Not only do moving clocks run slow, moving lengths get shorter. The distance you see between the earth and the star is approximately 1/7 of 4.37 light years, so there are no inconsistencies.

QUESTION: 
Since the Earth is constantly spinning do you drive faster in one direction (east or west) than the other?

ANSWER:
You have to ask, faster relative to what? Relative to the ground, 60 is 60. Relative to a stationary point outside the earth, your velocity would be dependent on the direction of your velocity relative to the ground.

QUESTION: 
To my understanding, the binding energy is defined to be the energy that must be added to separate the nucleons. (i) The binding energy of Uranium-238 is positive, that is energy is required to separate the nucleons of U-238, then why is it still radioactive? (ii) Hydrogen-2 has an average binding energy per nucleon less than that of Uranium-238, but Hydrogen-2 is stable, why?

QUESTION: 
A person is rotating a string with a mass at the end of it above their head. Let's say the plane of rotation is formed by the x and y axes. While rotating, the string (and therefore, the tension force causing rotation) makes an angle A with the plane of rotation. If you are spinning the mass with a constant freqency, the mass has no vertical (z axis) movement. I assume this is due to the z-comp of tension force balancing Fg. As the tension force increases, the angle A decreases. Question: Is it possible to have A = 0? Why can the mass never go above the plane of rotation?

ANSWER:
If A were zero, there would be no vertical component of the tension to hold up the weight unless the tension in the string were infiinite which is not going to happen. To move the mass above the horizontal plane where your hand is would require a net upward force and there is no source of such a force.

QUESTION: 
If you brought a flashlight into space and turned it on, would the light float away?

ANSWER:
The photons carry momentum, and so it would act like a little rocket to conserve the momentum; the flashlight would accelerate. However, the mass of the flashlight is so large that you might wait a long time to see anything happening. If there were an appreciable effect, you would feel the thrust when you held a flashlight.

QUESTION: 
The magnetic constant (permeability of vacuum) has an exact solution 4.pi/10000000 This solution is in SI units. I cannot find a comparable solution in imperial units. How could I convert to ft, lbs... the solution would be?

QUESTION: 
If Kinetic Energy is a product of mass and velocity does the massless photon in fact have kinetic energy?

QUESTION: 
I had a question about Torque, Internal forces & external forces. Here are 2 examples helping me to state my question:
1) The earth is rounding the sun with respect to the Sun. The forces which are acting on the system are 2 gravitational forces that are action & re-action. The Force vector & the Position vector will be in opposite direction; so their cross product is Zero & then the Torque will be zero.
2) In a system of particles, two objects are attracting each other with respect to a point (Q), which is located outside of the system. With respect to (Q), We say these two forces cancel each other out & there will be no Torque on the system in view of (Q).
Why in case 2, we say these forces cancel each other; but in the first case, we don't consider the re-action of Sun's gravitational force?

ANSWER:
Both cases are the two-body system. Usually, when doing solar system calculations, we approximate the sun's mass to be infinite (because it is so large relative to any of the planets). In that case there is one and only one force, the gravitational force which the sun exerts on the earth. There is no "reaction" force which is pertinent. There is also no torque on the earth as you note, because rxF is zero, so the earth has a constant angular momentum. The whole problem is reduced to a one-body problem. If we treat the sun and earth as a two-body system (your case 2), we need to include both bodies. If we consider the system as isolated, the earth and sun exert equal and opposite forces on each other and so the net force is zero. Similarly, the net torque is still zero. Thus both linear momentum and angular momentum of the pair of bodies are constant. The two-body problem may be reduced to an equivalent one-body problem and solved for the motion. What you keep referring to as the "reaction" force does not make sense. If the sun exerts a force on the earth, the earth must exert an equal and opposite force on the earth; those are referred to as a Newton's third law pair. If the sun's mass is very large compared to the earth's mass, you ignore the force the earth exerts on the sun because it has negligible effect on the motion of the sun.

QUESTION: 
considering that two identical shaped objects of different masses hit the ground at the same time why if I add extra weight (as paperclips) to a paper spinner does it fall quicker when air resistance has not changed?

QUESTION: 
If you had a large rock's density but then smashed the rock into smaller pieces. How could the smaller piece's density compare to the larger rock's density...?

ANSWER:
It depends on whether the rock was homogeneous (all made of exactly the same stuff) or not. Density is mass divided by volume, and any particular stuff has the same ratio of mass to volume regardless of the volume. For example, if you had a 3 kg rock of pure quartz and broke into three pieces of equal volume, you could be sure that each piece had a mass of 1 kg. The density is determined by how heavy the individual atoms are and how they are put together and this is the same regardless of the size of the rock. [One proviso which you may not be interested in: if you break a material into small enough pieces, nanoparticles, a cluster of say 100 atoms might not have the same density as the bulk material. One of the things of interest to nanophysicists is how small a particle may be and still have the same properties as the bulk material.]

QUESTION: 
is it correct (or at all meaningful) to say that Planck's constant is the quantum of momentum?

QUESTION: 
When discussing angular rotation, which is the correct phrase to use "the body is rotating about its axis" or "the body is spinning about its axis" or "the body is revolving about its axis"?

ANSWER:
This is really semantics, not physics. I will give some common usages which are by no means rules. Revolve usually means move around an orbit, not about an object's own axis; however, the dictionary lists this as one possible use of the word revolve. Rotate and spin are essentially synonymous. In atomic, nuclear, and particle physics spin is usually used as a noun meaning the intrinsic angular momentum a particle or system of particles has which is a lot like rotating about its own axis but not to be taken too literally because spin is a nonclassical quantity. Orbital motion in atomic, nuclear, and particle physics is usually referred to as orbital angular momentum rather than revolution. In astronomy, revolve normally refers to orbital motion and rotate normally refers to spinning on an axis.

QUESTION: 
If time is affected by both mass and speed (ie. the faster you go and the closer you are to a large mass the slower time goes realtive to others), does this mean that a planet that is smaller than ours and is orbiting its sun slower than ours experience time faster than us relativly? And thus if life is capable of existing on this planet could it have evolved quicker?

ANSWER:
First I need to note that gravitational time dilation in the vicinity of a mass the size of the earth is incredibly small; it is observable and used by GPS software, but really tiny. Similarly, the speed of earth in its orbit is very small with respect to the speed of light, so any time dilation due to that motion is also tiny. But the real crux is that time dilations are the rates of clocks as observed by observers other than in the clock's frame, the time for any observer in his own frame proceeds at the same rate as your clock. If you observe a clock at a lower altitude or one that is moving relative to you, it runs slow; to an observer riding with that clock, it runs perfectly normally.

QUESTION: 
My understanding of physics is that matter can neither be created no destroyed. Rather, it can only be changed. (ex. if a building is burnt down, all the matter would still exist, but just in different forms, such as smoke, dirt, etc.) If this is true, then where did the matter that was exploded in the 'Big Bang' come from? (i.e. it had to exist in some form BEFORE the Big Bang, since matter can neither be created nor distroyed)

ANSWER:
Your "understanding of physics" is wrong. Matter can be destroyed or created. Mass is a form of energy and it is energy which cannot be created or destroyed. We consider the total amount of energy in the universe to be constant. Your notion of the constancy of mass is from chemistry where, although mass is not really conserved in chemical reactions, it is so close to being constant that you can do chemistry assuming it is. So, where did the energy to create the universe come from? Nobody knows.

QUESTION: 
I saw a commercial for a new type of car tire designed to increase fuel efficiency by reducing the amount of friction between the tire and the pavement. This makes it easier to get the car moving which means the car is using less fuel to make the car move. All this I understand. My question is, if we reduce the friction that keeps us from moving, do we also reduce the friction which helps us stop moving? Is braking ability being sacrificed for the sake of a better fuel efficiency?

ANSWER:
The friction used to drive you forward or to brake is static friction, friction which keeps two objects which are in contact with each other (tire and road here) from slipping. Sometimes, it is kinetic friction (if you are "peeling out" or skidding). But there is also friction associated with the deformation of the tire; because the wheel slightly "squishes" when it rolls, energy is lost to this kind of friction. Also, there is something called rolling friction which is related to the tires sticking to the road so you are continually doing work "unsticking" the tires, thereby losing energy. The "improved tire" addresses, I presume, the latter two types of friction.

QUESTION: 
I am a volunteer in scouting. I was wondering (although I have my own theory) what may happen if an individual had a magnesium block on their person and was electrocuted or struck by lightning. What do you think would happen

ANSWER:
I don't think the electricity would have anything to do with what would happen, it would be the associated heat. Magnesium is easy to ignite in little flakes but hard to ignite in bulk. So it might or might not ignite under the circumstances. If it did, it would be very hard to put out. That would likely be the least of the victim's worries. What's your theory?

QUESTION: 
This relates to time dilation. I've learned that if a person is passing a motionless observer at the speed of light, he will be observed to be traveling faster (as far as measuring time) than he would observe himself to be moving. My question stems from this notion. If you're in space - is motion relative? Do you have grounds to say one person is traveling at the speed of light whereas another person is motionless verses the opposite? Who is to say - in a perfect, hypothetical environment - which person is moving? If so, would they both observer the other to be moving faster than they observe themselves to be moving?

ANSWER:
As I seem to have to say several times a day, no person (or any object with mass) can travel "at the speed of light". You can get close, but not at that speed. See an earlier answer. That said, the notion of absolute velocity has no meaning. If one space ship has a speed of 100,000 mph relative to another, the laws of physics are such that there is no difference if we consider the speed of the second ship to be 1000,000 mph relative to the first. See another earlier answer.

QUESTION: 
I'm watching the Empire Strikes Back right now and The Falcon's Hyper Drive is broken right now. Meaning it cannot go in to light speed. My question is, hypothetically of course, can a space ship enter hyper space with in the atmosphere of a planet?

ANSWER:
Physics of 2010 says no space ship can "go in to light speed".

QUESTION: 
http://en.m.wikipedia.org/wiki/File:Beta_Negative_Decay.svg
I found the above diagram while I was reading parts of the wikipedia article on a nutron. After a little thought am confused to how a down quark can decay in to an up quark, if quarks are elementary particles and by definition have no substructure.

ANSWER:
A particle with no substructure is not eternal. A simpler example, and electron and a positron, each with no substructure, can annihilate and create two photons. "Elementary" particles can participate in reactions.

QUESTION: 
When an acoustic wave travels through a medium of particular velocity, does it change its frequency? In other words, which is fundamental property for a sound wave, frequency or wavelength?

ANSWER:
Wavelength changes, frequency does not.

QUESTION: 
does a baseball and tennis ball have a different speed when thrown with the same force?

ANSWER:
Once again we have the question "how much speed does a force cause?" Read my earlier answer. The answer is that knowing the force is not enough. But, if you stipulate that you push on each with the same force for the same time, or that you push on each with the same force over the same distance, the tennis ball will have the larger speed because it has a smaller mass and will therefore have a larger acceleration.

QUESTION: 
how were physicists able to determine that vanishingly tiny particles have the property of spin? is it really possible that the spin of such small objects is observable or is "spin" simply a term used to denote some sort of mathematical abstraction? is it correct to think of an electron "spinning" the way a top or frisbee does?

QUESTION: 
If time is affected by travel at the speed of light, (i.e. time travels more slowly for someone traveling at the speed of light). What would happen if you took two people (or clocks), and sent them in opposite directions on an elliptical course that will make them eventually pass by each other. What would the time difference be when they met in the middle? Say, subject A travels "west" and subject B travels "east" both leaving at the same time, traveling at the speed of light. Would both of their clocks be equally offset to normal time when they pass by each other going opposite directions? Does the direction you travel have anything to do with the time dilation?

ANSWER:
You must not say "at the speed of light" because nothing can go the speed of light. For relativistic effects to be important, the speed should be comparable to the speed of light, say 50% or more the speed of light. You do not need an "elliptical course" for your question, you just need to send the two in opposite directions and then bring them back. This was worked out in an earlier answer; the bottom line is that each ages the same as the other but less than if they had not made the trip.

QUESTION: 
Two pitches are thrown. Assuming all other variables remain constant such as batting power, wind, pitch, contact point, etc., which pitch would travel further? A 95 MPH pitch or a 85 MPH pitch.

ANSWER:
The important quantity is the impulse which is the average force times the time that force acts. I found that a typical impulse is about 13 Ns for a batted ball. The important thing is that the change in velocity is proportional to the impulse. So the ball which comes in with the smaller velocity will go out with the larger velocity. Equal impulses will project the 85 mph ball faster. If you are interested, the 13 Ns impulse will cause the 85 mph ball to end up with a speed of 116 and the 95 mph ball to end up with a speed of 106 mph.

QUESTION: 
With reference to energy transformation, why is a car's breaking distance greater down an incline rather than on a horizontal plane? I know that the main energy transfer would be of kinetic energy to heat energy [why heat energy?].

QUESTION: 
What would happen to the volume of a basketball left outside on a sunny day? Which gas law applies to this phenomenon?.

ANSWER:
The ideal gas law will cover this question; it combines all the other usual gas laws like Boyle's law, Charles' law, etc. It is PV=NRT wher P is pressure, V is volume, N is the amount of gas, R is a constant, and T is absolute temperature. In the case of your basketball, N is constant, so as the temperature increases the product PV has to increase. Basketballs I have seen are really not very stretchy, that is you can increase the pressure quite a bit without changing the volume very much, so mostly the volume would stay about the same and the pressure increase. If the ball starts out not fully inflated, then the volume would increase more.

QUESTION: 
how to compute the amount of power is needed to keep an object floating in the air. (For example how much power is needed to keep an electric helicopter floating)

ANSWER:
It is much more complicated than you think. At first blush, it would take no power because if just hovering there is a vertical force equal to the weight but it does no work. But in order to supply the necessary force you need a mechanism and energy must be put into that mechanism to supply the required force. For example, the force might be provided by a moving airfoil (the helicopter propeller blade) and, as you suggest, this might be driven by an electric motor. But the required energy input is determined primarily by the engineering design of this motor-propeller system, i.e. how efficient it is. So the bottom line is that there is no way to answer your question in general.

QUESTION: 
a weight lifter stands on a bathroom scale. she pumps a barbell up and down. what happens to the reading on the scale? SUPPOSE SHE IS STRONG ENOUGH TO ACTUALY throw the barbell upward. how does the reading on the scale vary now?

ANSWER:
When the barbell is accelerating up (which means going up and speeding up or coming down and slowing down) the scale reads more than the combined weights. When the barbell is accelerating down (which means going up and slowing down or coming down and speeding up) the scale reads less than the combined weights. If she throws the barbell up and it is not in contact with her, the scale will read her weight. (I assume that the scale can respond very quickly so it is really measuring the force her feet exert down on it at any instant.)

QUESTION: 
I was watching a documentary on Einstein, and contemplating his theory of relativity(and the recent debates on the speed of light being constant) and came up with a question, which while I am sure it is non-sense, still begs to be asked. Has/Can the speed of light in a vacuum, free of electromagnetic/gravitational anomalies ever been measured by two items that are truly at rest? This would seem, by its nature, to have to occur in space, with two items that are not moving with our solar system relative to the universe. It would also seem to need to occur between two man made items since all other objects in the universe are inherently in motion. Feel free to correct me if my assumptions on the requirement of the experiment are wrong. It would seem, though, that this would be the ultimate test of the speed of light being constant.

QUESTION: 
I know about Einsteins theory of relativity and how nothing (even an affect like gravity) can travel faster then the speed of light. Does that mean, when I look up at a star at night, if i was able to instaneously travel to the star, it wouldn't actually be there because its light has taken us so long to reach us?

ANSWER:
No, it just means that it would not look like what you are seeing here on earth. Of course, there is no way to "instantaneously" travel anywhere.

QUESTION: 
A show about space stations were talking about a problem with communication with ground control because radio waves travel in a straight line. I wanted to know why do they travel in a straight line? I thought all EM waves spread spherically?

ANSWER:
How a radio wave is transmitted depends a lot on the design of the antenna. A point source (very small and more or less spherically symmetric) will send out waves that spread spherically, but since we generally do not wish to beam in all directions, an enormous amount of energy put into an antenna designed to look like a point source would be wasted. If instead we beam out in one direction by using a "dish" antenna (like the headlight of a car, for example), we can put our money where our mouth is (put our power where the receiver is, that is) and invest very little energy to send a signal. Another advantage is that the strength of the signal falls off very rapidly for spherical waves (like 1/r²) but maintains much more of its strength over large distances for a dish antenna. The disadvantage would be that you have to aim it at the receiver.

QUESTION: 
Will the coefficient of kinetic friction remain constant for a system, even if the mass and acceleration change?

QUESTION: 
I seem to have a paradox. Suppose we have 2 independent waves of amplitude A and energy E and in phase and travelling in the same direction coming from 2 lasers which are wired together. If we combine these 2 waves, by superposition, we will get a resultant wave with the same frequency and wavelength and amplitude 2A. Since the power of a wave is proportional to the amplitude squared and the frequency of all these waves are the same, the energy of the resultant wave should be 4E. However, by conservation of energy, the energy of the resultant wave should be 2E. Doesn't this violate the conservation of energy? Can't we get free energy?

QUESTION: 
Why does a superconductor have no resistance to a current. I mean what is the quantum state of a superconductor that allows it to have the properties that it has both with current and repultion of any magnetic field. Thanks for your time and hope to here from you soon.

ANSWER:
The basic idea is that, at low temperatures, electrons can pair up and these Cooper pairs become the current carriers. The theory of superconductivity is called BCS theory for its authors Bardeen, Cooper, and Schriefer; they won the Nobel prize in 1972. Although discovered in 1911, superconductivity was not understood until the 1950s. The mechanism for the more recently discovered high-T_C superconductors is still not fully understood theoretically.

QUESTION: 
As an English major, you can imagine I've not done a whole lot of math. In movies and TV portrayals of mathematicians and physicists, we always see them working on blackboards with chalk or, more recently, on whiteboards using dry markers. These portrayal may well be stereotypes created by writers which have been emulated by subsequent writers. However, if these are true to life, why don't they use paper and pencil instead? I can think of a couple of likely reasons., but I'd like to know the real one.

ANSWER:
Sorry to disappoint you, but there is no "real one". Reasons to work at a board are that

• science is often collaborative and this is a convenient way to show and share a train of thought;

• it is easier to erase and revise than paper and pencil; and

• some folks think better on their feet and while moving around.

Often the only contact with scientists that you humanities guys get is seeing us teach a class and we couldn't very well do that on paper and pencil. Finally, most scientists probably actually spend more time with paper and pencil or at a computer keyboard than at a blackboard.

QUESTION: 
I know that as an object moves though space it gains mass. Is there an equation to find the mass gained.

ANSWER:
If the mass of a particle at rest is m₀, its mass m when traveling a speed v is given by m=m₀/√(1-(v²/c²)) where c is the speed of light.

QUESTION: 
my question is about which equation properly governs the situation where an electron is approaching an electric field.

ANSWER:
I am not sure what you are after here. The force F felt by a charge q in an electric field E is given by F=qE and knowing the force you can calculate the motion of the electron. Since the charge of an electron is negative, the force it feels is opposite the direction of the field.

QUESTION: 
I am trying to calculate the force involved in one American football tackling another, assuming both are travelling at full speed and hitting head on, will it be the sum of the momentum of both players ?

ANSWER:
As I have explained many times before, there is no way to calculate the force from such information. It depends on the following:

• What are the masses of each?

• What are their speeds?

• How long does the collision last?

• What is the nature of the collision? Do they stick together? Or, if they bounce off, how much energy is lost?

FOLLOWUP QUESTION: 

 2 object collide, 1 weighing 210 lbs travels 6 metres at 9.52 metres per second with acceleration of 5.66 metres per second squared. It's force is equal to 1188.6 lbs per metres per seconds squared and it's momentum is 1999.2 lb metres per second the other object is 268 pounds, travels 4 metres at 8.49 metres per second with an acceleration of 4.51 metres per second. It's force is equal to 1208.68 lbs per metre per second squared and it's momentum is 2275.32 lb metres per second the collision last 1 second and is non elastic. What is the Impulse caused by the collision and how do i calculuate the force of generated during the collision ?

QUESTION: 
Why do photons lack mass?

ANSWER:
Because, being the particle associated with electromagnetic waves which have no mass, they could not. Also, according to relativity, only a massless particle can go the speed of light.

QUESTION: 
Due to the length contraction, you notice that a passing train appears to be shorter than when it is stationary. What do the people in the train observe about you? If you are on a train that is going really fast, do the people on the ground look shorter, longer, or the same?

QUESTION: 
I am writing to you today to see if you can help me to understand the increase in velocity you are able to achieve by pumping a skateboard. I am not referring to pumping while inside of a pool or halfpipe, but instead pumping across a flat surface by carving back and forth left and right. How does this work?

ANSWER:
This is a bit involved, too much so for me to give a concise answer. The idea is much like roller skating, though, where you manipulate things so that the friction between the wheels and the ground is a force forward. There is lots of detail (including the physics) at silverfishlongboarding.com.

QUESTION: 
Re: The life cycle of matter. Does matter ever really disappear or does it simply change form? i.e - When something burns and produces heat, does the heat contain matter? Obviously heat dissipates but if it does contain matter, could that matter be reconstituted into something else later? Bottom line, is our universe a never ending cycle of matter into energy and energy into matter?

ANSWER:
The crucial idea is simply that mass is simply a form of energy and the total energy of an isolated system does not change. When something burns, the released energy is in kinetic energy of the reaction products (they move faster). This energy came from chemistry, specifically the binding energies of molecules. The root source of this energy is mass and a tiny amount of mass disappears when you burn something. It is very small because chemistry is really a poor source of energy. Nuclear reactions, fission and fusion, are a much better source of energy and there you can rather easily measure the loss of mass of a nuclear system which releases energy.

QUESTION: 
what percent of its hydrogen mass could the sun actually convert into helium?

ANSWER:
About 0.8%. That is how much mass is converted into energy in the fusion process.

QUESTION: 
does a single photon produce an interference effect or not?

ANSWER:
Yes. If you have a slit and a photon passes through it, the photon may end up being "deflected", that it will be diffracted. To see the single slit diffraction pattern, however, you need very many photons to "flesh it out", but you can send them through the slit one at a time so the interference is not with other photons. In essence, you must acknowledge the duality of the radiation, it is both a photon and a wave, and interference effects are manifestations of the "waveness".

QUESTION: 
Phobos, one of the moons of mars, orbits at a distance of 9378km from the center of the red planet. What is the orbital period of phobos? I would easily know how to do this if I was given the mass of mars, I am unsure if I am simply expected to look that up, or if there is a way to solve for this/ solve around this? T= 4pi²r³/GM

QUESTION: 
This is quite possibly a fizzics question rather than a physics question, nonetheless, I ask you: Why don't sodas retaining their fizz like they used to? Is it the recipe, something to do with the planet, the atmosphere or what? How on earth can we get back our fizz?

QUESTION: 
If I drop an object into a pool of water, is the wave generated proportional to the size of the object dropped? Example, if I drop a 1" and 4" ball bearings into a pool of water, will the the wave projected by the 4" ball bearing be 4X bigger than the wave generated by the 1" ball (keeping in mind that the 4" bearing is 4X heavier than the 1" bearing). Also, how does the height from which and the ball bearing is dropped affect the size of the wave?

QUESTION: 
I heard that you can die or pass out from an acceleration beyond 4 or 5 g's. Does this mean that if you are going, say 5 miles per hour, and you decelerate greater than 5 g's you could pass out?

ANSWER:
Your numbers are a bit low. Fighter pilots routinely experience around 10g in tight turns but they do wear special suits to help the blood being pushed from the brain causing loss of consciousness. Also, duration is an issue; you will not pass out immediately. Certainly the case you state will not cause you to pass out; the time to stop would be only about 0.05 seconds at an acceleration of 5g. Whatever was restraining you, though, might cause pain but you would not pass out from the acceleration. Finally, you will certainly never die from a 5g acceleration. You might read Wikepedia for more information on g-forces.

QUESTION: 
Why is kinetic energy a scalar even though it contains v in the formula which usually means a direction involved?

QUESTION: 
Why must a horizontally moving projectile have a large speed to become an Earth satellite?

ANSWER:
Because the harder you throw something horizontally, the farther it will go until eventually it will go all around the world. Newton realized this and discussed it in his famous book Principia. You might enjoy an applet which uses Newton's original sketch.

QUESTION: 
We are studying free-body diagrams. Say an object is tossed into the air and then reaches its highest point, does this mean that all the forces acting on the object are balanced?

ANSWER:
Absolutely not. You cannot tell whether an object is in equilibrium by knowing its velocity; its acceleration will tell you this. In this case, the object is at rest but an instant earlier it was not and an instant later it will not be. It is therefore accelerating and there must be an unbalanced force; this force is the object's own weight which does not disappear just because it stops for an instant.

QUESTION: 
Why does a bouncy ball bounce back up after it huts a surface?

ANSWER:
During the time the ball is in contact with the surface it is experiencing an upward force greater than its own weight. This force causes the ball to have an upward acceleration (Newton's second law) which causes it to acquire an upward velocity after the force has momentarily stopped it.

QUESTION: 
Does an electric arc generate an electromagnetic field??? Can a electric arc be be generated in complete vacum, for example in open space???

ANSWER:
Do three question marks make a question more pressing? Just kidding! Yes, an arc has electric charges in it and these charges are moving, so there will be fields generated. In fact, many of the electric charges have acceleration and so electromagetic waves are also generated. This is the origin of static on AM radio during thunderstorms. Also, Marconi, the discover of radio transmission, used spark gap transmitters to send his earliest radio signals. Since an arc is a plasma breakdown in the gas, it cannot happen in a vacuum. What can happen is corona discharge, where the electric field becomes sufficiently strong at the surface of a metal that electrons stream into the vacuum.

QUESTION: 
In two boys of unequal weights running towards each other with the same speed meet lead-on what is the result of the collision?

ANSWER:
It depends on how elastically they collide. For example, if they stick together after the collision, they will move in the direction of the heavier boy. In any case, the two will both move in the direction of the heavier boy after the collision.

QUESTION: 
"The laws of physics are the same for all observers in uniform motion relative to one another." But theoretically if one had a space shuttle that could travel at 2% the speed of light and on board it had a particle accelerator the could compensate for 99% the speed of light then relative to an observer out side the craft wouldn't the particle at times be moving at ~101% the speed of light. Why is this impossible?

ANSWER:
I have answered this question before. It is also linked to on the FAQ page.

QUESTION: 
Physics books and physics professors routinely explain gravity with the Einsteinian theory, that mass warps space. I have no evidence to dispute that and I have no reason to doubt it. My problem is with the model commonly used to illustrate it. A rubber sheet with a steel ball on it, hence gravity warped. If the universe were two dimensional then the analogy would make sense to me. Since the universe is at least three dimensional, then to be a valid model and become three dimensional, the warped rubber sheet has to be rotated through infinitely small arcs through 360 degrees at right angle to the plane of the rubber sheet before warping by the ball. When that is done the warping shape disappears. Remaining is possibly a model of density gradation but not of warping. If that is true then the model of the rubber sheet analogizes nothing, in my opinion. I would appreciate enlightenment if I am missing something.

ANSWER:
See an earlier answer.

QUESTION: 
A friend of mine and I have an argument over what is a faster sport tennis or badminton. The criteria is how fast it would take to serve a tennis ball/shuttlecock from one side of an olympic sized tennis/badminton court to the player waiting on the other side assuming that both are standing on the out of bounds line. We are assuming ideal conditions and that the players in both cases are equally strong and fast.

ANSWER:
You may not realize it, but your question is mostly about air drag on projectiles. I seem to get more questions about air drag than just about anything else except maybe variations of the twin paradox. Maybe that is because it is perhaps the most important phenomenon mostly swept under the rug in most elementary physics courses. There are several instances of earlier questions involving baseballs and lacrosse balls which are very similar to this one. For high speed projectiles, air drag is very important; e.g., a 100 mph baseball loses about 10 mph by the time it crosses the plate. Approximations have to be made to quantify the situation you are interested in, but I feel the results I will present are pretty close to what happens on the court. (The following is probably way more detail than you want, but I put it here for anybody who might be interested.)
The approximations are:

• I neglect gravity because the times involved are are sufficiently short that the ball/shuttlecock will not fall far or very much change its vertical speed.

• I assume that the drag is proportional to the square of the speed—twice the speed, four times the force of drag. This is an excellent approximation for these speeds, these objects.

• The form of the force I use is F≈¼Av² where A is the cross sectional area presented to the wind. Here A=πR² where R is the radius of the ball or the outer circle of the feathers. This probably slightly overestimates the force for the tennis ball (whose "hairs" have the function of decreasing the drag) and underestimates it for the shuttlecock (whose "feathers" are designed to increase drag).

• Data for tennis:

  • v₀=73 m/s=163 mph

  • R=0.032 m=1.26 in

  • m=0.057 kg=2 oz

  • back line to back line distance: 24 m

• Data for badminton:

  • v₀=92 m/s=206 mph

  • R=0.025 m=1 in

  • m=0.005 kg=0.18 oz

  • back line to back line distance: 13.4 m

QUESTION: 
What is the origin of the word "magnet"? I have heard that there are two different explanations. I have found a lot of information referring to the ancient Greek city of Magnesia and a Shepard named Magnes. Is there any other possible explanation?

ANSWER:
From the Online Etymology Dictionary:

magnet