Old Questions and Answers

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QUESTION: 
I'm just confused because according to newton's third law of motion, " any action has an opposite but equal reaction", hence forces in nature are balanced, and balanced forces means bodies are at rest, so how come in reality, we can observe motion?

ANSWER:
You do not understand the third law. If there are two objects, A and B, and A exerts a force on B, Newton's third law says that B exerts a force of equal magnitude and opposite direction on A. So, you see, the two forces are not on the same object and are therefore not "balanced".

QUESTION: 
If a man was to be proportionately increased in size, would it be more difficult for him to perform a pull-up? All his body percents are exactly the same including his muscle, fat, ability, ect. The pull-up bar is also high enough that his feet would not touch the ground in either situation.

ANSWER:
Scaling of biological organisms has been a topic of interest for a long time. I think Leonardo de Vinci was the first to think about it. Anyhow, I think the answer to your question is that it would be harder to do a pullup. Here is why: The weight of something is proportional to the cube of its dimensions. So, if you doubled all the dimensions of a man, his weight would increase eightfold. But, the strength of bones or muscles is dependent on their cross sectional areas, a bone or muscle doubled in size would have about four times the strength.

QUESTION: 
I am confused if I put a book from ground to a table at height 'h' ,note that the book is at rest before and after putting , total work done on book is 0, as change in k.e. Is 0 ,therefore total transfer of energy on book should also be 0 but after putting the book on the table at height 'h' the book gain a p.e. Of 'mgh'.how?

ANSWER:
Yes, you are confused! The idea of a potential energy function is that it keeps track automatically of the work done by a force always present in a particular problem. The gravitational potential energy represents the work done by the weight mg of an object. So, if you use potential energy, you do not count the work weight does, that is ΔK+ΔU=W_ext where W_ext is the work done by all forces other than gravity, in your example that is you. So, to lift the book you must exert a force upward of magnitude mg for a distance h and so the work you do is W_ext=mgh; since kinetic energy has not changed, ΔU=mgh.

QUESTION: 
Potential Energy + Kinetic Energy = 0. However Potential Energy = negativeKinetic Energy. Could please explain?

QUESTION: 
if you where able to make a hot air balloon that could contain a vacuum would it rise like one containing He or would the air pressure pressing from all side cancel it out ?

ANSWER:
The reason a hot-air balloon (or a hydrogen- or helium-filled balloon) rises is buoyancy. The buoyant force is equal, in magnitude, to the weight of the air which is displaced by the volume of the balloon. So, if you create a vacuum in a rigid balloon which has the same volume and weight as one filled with hot air, hydrogen, or helium, it would have a greater lifting force because you would not have the weight of the gas. The problem, though, is that it would be hard to make it sufficiently rigid that it would not collapse when you evacuated it; atmospheric pressure, acting on the outside of the balloon, is about 2000 lb/ft².

QUESTION: 
Of the four formally recognized fundamental interactions, which is most responsible for causing elements from the environment to be drawn into living cells (a reduction of entropy) and, when the cell dies, appears to cease, allowing a reversal of the organization and decay back into the environment (an increase in entropy)?

ANSWER:
Anything involving microbiology is basically chemistry. Anything involving chemistry is basically atomic and molecular physics. The only important interaction in atomic and molecular physics is electromagnetism.

QUESTION: 
If two beams of light pass one other, moving in opposite directions, at what speed are they moving away from each other? I believe the answer is at light speed but I guess the real question is, and briefly if you can. Why? ...or why not twice light speed?

ANSWER:
The velocity addition formula for special relativity is v_ab=(v_ag+v_gb)/(1+(v_agv_gb/c²)) where v_ag is the velocity of a with respect to the ground, etc. For your question, v_ag=c and v_gb=c so v_ab=(c+c)/(1+(c²/c²))=c.

QUESTION: 
I understand the part that all matter stays constant in a confined space, none can be lost no matter what happens to it. But I have trouble with the energy part. It is stated that if matter is changed into energy then that energy must be able to change back into matter for the whole of the conservation of matter & energy theory to work. My question is: has a scientifically qualified experiment ever, at any time, been done in the lab to show this? I mean taking pure energy of any type & changing it into matter of any type? I can find nothing on this on the net. Do you have the answer?

ANSWER:
You understand wrong: matter can be created or destroyed. There is no such law as conservation of matter. See an earlier answer.

ADDED ANSWER:
I see that I did not answer the main part of your question! Energy to mass happens all the time. The best known example is pair production where a photon (no mass) creates an electron and positron. See an earlier answer.

QUESTION: 
What causes atoms to oscillate and do they oscillate infinitely or do they absorb energy from a limited source?

ANSWER:
Think of each atom in a solid to be attached to its nearest neighbors by tiny springs. This is a reasonable model which approximates much of the behavior of solids. Then each atom is a tiny harmonic oscillator. In classical physics, all those tiny oscillators could simply have zero energy, each would be at rest. However, quantum mechanically a harmonic oscillator cannot have zero energy, it has some minimum energy called the ground state. Therefore, it can oscillate forever even without any energy from outside. At room temperature all those oscillators are in higher states than the ground state and the energy to put them there came from the environment, that is the solid was heated up to the temperature of its environment.

QUESTION: 
I have a pressure washer that pumps 5 Gallons of water Per Minute and sprays water at 800 - 3500 Pounds Per Square Inch, at the nozzle. The Inside Diameter of my high pressure hose is 3/8". The diameter of the circle of the nozzle is 1/16". My question is: Is there a formula to convert PSI at the nozzle to Miles Per Hour (MPH) at the nozzle?

ANSWER:
If you give me the gallons per minute, you are essentially giving me the velocity in the 3/8" pipe—9.89 mph. Now, if the diameter drops to 1/16", the velocity increases by the ratio of the cross sectional areas, ((3/8)/(1/16))²=36, so the speed at the nozzle is 36x9.89=356 mph. Wow, there is a surprise! Surely it does not have the same flow rate for different pressures?

QUESTION: 
I was doing some water sports and was wearing a wet suit and a 50N (50 Newtowns) rated buoyancy aid. I weigh 93Kg. Cliff jumping into the sea, after resurfacing I bobbed gently on the surface with no swimming effort. Later wearing same equipment on fresh water river, fell of a kayak and promptly sank! To get to and stay on surface I had to swim or actively tread water. The question is what value N rated buoyancy would I need to "bob gently" in fresh water?

ANSWER:
You do not weigh 93 kg, your mass is 93 kg and your weight is 93x9.8=911 N. Suppose that you just float with your head above water in salt water. Then the buoyant force the water is exerting up is 911-50=861 N. Archimedes' principle says that this is the weight of the displaced water, and let's say salt water has a density of 1030 kg/m³; so, the volume of your body less head must be 861/(1030x9.8)=0.085 m³. So, the buoyancy you can expect in fresh water if just your head is up is 0.085x1000x9.8=836 N because the density of fresh water is 1000 kg/m³. So, to have identical buoyancy to the salt water situation, you need an additional 25 N (861-836) buoyancy.

QUESTION: 
.. is there a theory/postulate/law that explains how the strong nuclear force keeps the like-charged particles in a nucleus from flying apart - maybe some sort of virtual exchange particle?

ANSWER:
There simply is the strong interaction which is responsible for holding nuclei together. Is there a field theory which attempts to model how the force "works"? Yes, the virtual particles exchanged are various mesons. The force itself is much more complicated than the relatively simple electromagnetic force. But, you should simply say that, empirically, there is a fundamental force in nature which is far stronger than the coulomb force at small distances. A theory is itself more of a model to explain what you observe.

QUESTION: 
When you face a wall, place your toes against it, and try to rise up on the balls of your feet, why can't you?

ANSWER:
When you stand up straight, your center of gravity is above a point between your heel and toes and you do not tip over because there is no net torque which would tip you over. When you go up on your toes and are unable to lean forward, your center of gravity now exerts a torque about your toes which rotates your body backward. Try standing on your toes when you are not against the wall; in order to remain standing you will find that you must lean forward so that your center of gravity will be directly over your toes.

QUESTION: 
If p=mv/(sq. root 1- v²/c²), does this mean massless photons have infinite energy?

QUESTION: 
Why is light attracted to black holes, if photons have no mass?

ANSWER:
One of the first verifications of Einstein's theory of general relativity was to observe that gravity bends light. This was done by looking at a star whose light had to pass close to the sun during a solar eclipse and seeing that it was not where it was supposed to be. Read more about this by looking at links on my FAQ page.

QUESTION: 
I write an article on car safety for my company. My topic this month is objects hitting windshields and the probability of cracking, breaking or injury to the windshield/driver. So if two cars are driving toward each other at a low speed (25mph) or speed limit on a highway (70mph) and a bottle, water balloon, egg (Halloween), cup filled with ice is tossed out of car A and hits car B's windshield. What is the damage to the windshield? I'm looking for the lowest speed/heavy object, highest speed/lightest object.

ANSWER:
I doubt that I can be of much help to you. There are too many variables for me to be able to focus on any physics principles. Here are a few things that you should be sure you understand

• Relative velocity, e.g. if cars are approaching each other at 70 mph, an object thrown from one would hit the other at a speed of about 140 mph, i.e. as if a 140 mph cup of ice (or whatever) hit a car at rest.

• Newton's second law should be understood to understand the collision itself (see faq page). In particular, the average force on the windshield will be approximately mass x striking velocity/time for mass to stop.

• If you have to work in English units (lb,ft,s), and do anything quantitative, the mass to use in calculations should be weight(lb)/32 and all velocities converted into ft/s from mph. That way, forces will come out in lb.

I have no way of knowing the physical properties of glass used and so cannot possibly know threshhold forces for damage to windshields.

QUESTION: 
what is weight ? explain why we feel lighter while travelling in a lift which is accelerated downwards by ''a'' m/s2

ANSWER:
What "weight" is is a semantic problem. (I say this because a textbook which I am familiar with defines weight to be what a scale would read which, to me, is wrong.) To me, weight means the force the earth exerts on me. When I stand in an elevator there are two forces on me, the force of the floor up and my weight down. I judge how heavy I feel by how hard I feel the floor pushing up on me. When I accelerate up, Newton's second law tells me there must be a net force up and so the floor pushes up with a force bigger than my weight, so I feel heavier than usual. When I accelerate down, the net force must be down and so the floor pushes up on me with a force smaller than my weight and I feel lighter than usual. Actually, our feeling heavier is a much more complicated thing. For example, your internal organs are supported by muscles and, when accelerating up the muscles must pull harder than usual and this is transmitted to your brain which it interprets as your being heavier all of a sudden.

QUESTION: 
My young son asked me what would happen if there were no more temperature in the world. It made me wonder what the theories are about what would happen inside an atom, if all movement ceased.

QUESTION: 
what is the relationship between pressure and density for a given amount of a gas at a constant temperature

QUESTION: 
I have a question about Einstein’s paper on The Special Theory of Relative (STR). In the introduction there are some references to phenomena that I have never heard of. Are there differences with what happens when moving the magnet verses moving the conductor? Is there a phenomenon where there is a difference in the electrical field around the magnet and a difference with an electromotive force without a corresponding energy? Or is Einstein saying that the customary view (I assume the Maxwell’s view) is wrong and that what is observable is the same for moving a conductor or moving the magnet?

ANSWER:
I believe that it was well known that it does not matter whether you move a coil toward a magnet or a magnet toward a coil, in either case an EMF around the coil will occur. But, it was puzzling because in the first case the electrons in the wire are moving through a magnetic field and therefore experiencing a force which propelled them around the coil and, if the coil was open, caused a voltage across the ends; in the second case, there was no apparent law of physics which said that if the field moves and the charges stand still the charges will experience the same force as if it wer them which were moving. Relativity takes care of this because there is no preferred frame of reference and the two should be identical.

QUESTION: 
Lets assume you have a charged particle that travels with velocity into a square region with a constant B field. By F=q(vxB) we know that the force is upward for negative charge or force is downward for positive charge. Also we know there will be a rotation due to the B field lines. When it leaves such a field, does it trajectory continue to "loop"? How does the velocity change? First i thought that maybe the force that acts on the particle outside comes from the lorentz force, but then i was not sure. F = qE + qvB, but outside B = 0 and there is no E field so E = 0. It doesnt seem reasonable that after the particle has experienced a B field it travels on a straight path with the same velocity as it was doing so initially (before entering the B field).

ANSWER:
First, you are using velocity wrong because velocity is a vector, it has both a magnitude (called the speed) and a direction. So, if you have two cars, one going north and one going south, both with a speed of 60 mph, they do not have the same velocity. Acceleration is the rate at which velocity changes and there are two ways that can happen: the magnitude can change (speed up or slow down), or the direction can change (move in a curved path) at a constant speed (or, of course, a combination of both). So, when a particle enters a magnetic field which is perpendicular to its direction, it experiences a force perpendicular to its direction and the direction of the field. A force perpendicular to a velocity changes its direction but keeps its speed constant. So, the particle will move in a circular path. So, the charged particle will continue until it leaves the "box" at which time it will move in a straight line with the speed it came in with but not in the same direction.

QUESTION: 
I noticed in a textbook that 'g' varies directly with distance inside the Earth's radius but is inversely proportional to it outside. I understand the reason for the latter but would like to know more about the former - thanks! Please keep it simple.

QUESTION: 
I have seen videos where astronauts release a small handful of objects like M&Ms, and they immediately drift away from one another, why do they do this rather than staying clumped together?

ANSWER:
When they are released, it is inevitable that each will have some random small initial velocity and, in the apparent absense of gravity, they will all just keep moving exactly like they were when released. These speeds will be rather slow but very apparent over several seconds. When on earth, the same experiment is over in less than half a second as they fall to the floor.

QUESTION: 
If a block placed on an inclined plane having base length say,L,moves towards right by distance x,what will be the horizontal displacement of the inclined plane?Why would it be towards left?

ANSWER:
I assume the plane is free to move, there is no friction. Before the block starts to move the system has zero momentum. If there is no friction, the only way the system can have zero momentum after the block is released is for the two to move in opposite directions. To know the distance, you need to know the relative masses. For example, if the block mass is m and the incline mass is M, the plane will move mx/M when the block has moved x.

QUESTION: 
Since kinetic energy = 1/2 mv^2, it follows that, for a given mass, a doubling of velocity requires a quadrupling of energy. It seems to be certain then, that 2 joules are required to accelerate a 1 kg mass from zero to one meter per second in one second (ie accel=1m/s2)

Now, according to the KE formula, to further accelerate the mass to 2 meters per second requires an additional 6 joules... because at that velocity, the energy now possessed by the mass is 8 joules - 4 times higher than when it was moving at 1m/s.

W₂=½x1x2²-½x1x1²=3/2 J, not 6 J.

Here is my question: (in the form of a thought experiment) Scenario: I am in space Let's suppose I use 2 joules of energy to accelerate the mass to 1m/sec, relative to me. A spaceship, passing by with velocity 1m/sec and going in the same direction as the mass, has a second person onboard who intercepts that mass. (Because it's and the ship's speed are equal, the mass is effectively at rest with respect to the ship. Now let that person - on the moving train - apply 2 joules of energy to again accelerate the mass to 1 m/sec relative to HIM. So the actual velocity of the mass is now 2m/sec, relative to the first observer (me) (Assume same direction...) So, between us, only 4 joules of energy have been required to get the mass to 2 m/sec instead of the 8 implied by the KE equation. I have no difficulty in grasping that energy goes up as the square of velocity, but for the life of me, I cannot understand why both observers cannot use 2 joules of energy to effect an increase of 1 m/sec each Surely, what is true for one must be true for the other... ? (ie. that it only requires 2 joules to effect an increase in velocity of 1m/sec, relative to that frame) I have posted this on a Physics Forum and the only responses I have gotten allude to the fact I am getting it all messed up by changing the frame of reference and hence, the numbers won't add up. They say KE is not frame invariant and I can understand that - but no-one can explain where my logic is wrong... Can you possibly enlighten me ?

QUESTION: 
I need you to settle an argument between my wife and I. We are traveling down the road at 60 mph. My wife throws a piece of balled up gum out the window. When it hits the ground, my wife thinks it will roll backwards. This is not possible and I know it will roll forward at a speed less than 60 mph due to friction. Who is right?

QUESTION: 
If a 180 pound person "plops" onto an object (chair, couch, etc) forcefully by just letting their feet go out from under them, instead of sitting down properly, how much force is this creating on the object (equivalent to pounds)?

QUESTION: 
if black holes keep sucking in matter, wouldnt the entire universe eventually be sucked up in one big black hole?

ANSWER:
Not necessarily. If there were two black holes orbiting each other outside their Schwartzfield radii, for example. Or, if there were two black holes moving away from each other at speeds bigger than the escape velocity.

QUESTION: 
what happens if the constant "c" in e=mc squared is faster than light

ANSWER:
The entire universe collapses instantaneously to a purple singularity. JK. What happens is that you have a meaningless and incorrect equation.

QUESTION: 
As a gas bubble rises in a column of liquid, it expands due to the decrease in pressure from the liquid above it. Intuition tells me that the diameter of the bubble will increase at a decreasing rate; but is that true? Does the bubble's diameter increase at a decreasing rate over time, does it increase linearly with time, or does it expand at an increasing rate?

QUESTION: 
The electromagnetic spectrum consists of so many waves which are made up of photons. We know that photons do not have mass. But they say that all the electromagnetic waves possess different energies. How does a massless photon possess energy?

QUESTION: 
If a marble and bowling ball free-fall at a constant rate, why does a car falling tilt toward the side of the engine (landing vertically)? In other words, if the marble and bowling ball were connected by a stick, would it still land flat (horizontally) or would it land with the bowling ball down and the marble up  (vertically)?

QUESTION: 
Could you please explain Einstein's theory of time travel to me? I recently watched a program that explained he postulated the theory based on the idea that if he moved away from a clock tower at the speed of light the hands of the clock would apear to slow and even reverse. Though I am willing to accept that this may be true I would argue that he is only catching up to the light that was reflected when the hands were in that position and that he would not actualy be traveling backwards in time. Can you help me to understand this idea better?

ANSWER:
There is no such thing as "Einstein's theory of time travel". However, Einstein's theory of special relativity shows us that the rate at which time moves forward is determined by the motion of the clocks, it is not a universal rate for everybody. The result of this is that it is possible to travel forward in time. For example, you can go traveling out to space and back for 10 years, arrive back home, and find that 100 years have elapsed here. To get the gist of this, look at the earlier answer on the twin paradox. As far as physics knows, though, there is no way to travel backward in time.

QUESTION: 
I have just started a new job and have been asked to do a presentation on the ABS (Avalanche Bag system). Upon reading about the product they kept quoting that the system works on the "Law of Inverse Particles." I figure ok, it's a Law, it should be fairly simple to research, but when I started typing it into search engines I am getting no feedback on this Law at all. So my question is does this Law even exits? If so is there maybe another name that it goes by? If it does exist who created it? Any help that you can off me on this topic will be very much appreciated, in the mean time I'll keep on checking books, sites and asking old teachers!

ANSWER:
I have never heard of the law of inverse particles. However, just looking over how this thing is supposed to work, it just looks like Archimedes' Principle to me. Archimedes' principle says that when an object is placed in a fluid it experiences an upward force, called the buoyant force, which is equal to the magnitude of the weight of the displaced fluid. Hence, if the object has a density less than the density of the fluid, it will float, if it is greater it will sink. I am guessing that the snow in an avalanche acts much like a fluid and, if you can significantly decrease your overall density, you will "float" on the surface, much like a cork floats on water. The way you decrease your density is to increase your volume with the inflated airbags. Why they invent this screwy name for this escapes me, must be an effective marketing technique.

QUESTION: 
I am not getting the idea of length contraction.Does length of an object actually shrink when in motion or is it just apparent to the observer which is at rest.Is moving rods shrinking a measurement problem?

ANSWER:
First you have to define how you are going to measure the length of something. In physics, length is the difference between the positions of two points in space which are measured simultaneously. So, if a stick is moving by you, you have to be careful to measure the positions of the two ends of the stick at the same time. If you agree that this is a reasonable definition of what length is, then the answer to your question is yes, the object actually is shorter if it is moving. What is apparent to the observer is an entirely different thing. How long a stick looks is not the same thing as how long the stick is. For much more detail, see an earlier answer.

QUESTION: 
If two identical golf balls fall from the same height what differences (if any) would it make if one of the golf balls was spinning?

ANSWER:
Normally a projectile moves in a vertical plane because the only forces are gravity down and air drag opposite the velocity direction. But, if the ball is spinning, the air drag can be asymmetrical so there is a force perpendicular to the velocity. This is how a curve ball in baseball works or why some golf drives "slice" or "hook". So a spinning ball dropped might not fall straight down.

QUESTION: 
At the S.F. Airshow when a jet airplane reached a very fast speed over the bay an aura of a white circle, with black around the edge appeared momentarily around the speeding jet. What caused this display? Was it from the speed of sound? I have also noticed that when the planes fly by they seem to have a steam like trail coming off their wings momentarily. Is that the same as the white aura? What causes it?

ANSWER:
Did it look like the picture to the right? If so, it was a supersonic shock wave which causes air vapor to rapidly condense. The trail off a subsonic jet is not the same thing. Here the turbulence at extremeties causes the water vapor to condense. This is what you usually see with passenger jets going by; it is condensation, not exhaust fumes.

QUESTION: 
I know that as something travels closer to the speed of light, time dialates for it, like at 90% time passes at a 7:1 ratio (7 days outside:1 day inside) and at 50% it's a 2:1 ratio (I think). My question is if the two ships, 90% and 50%, were travelling 1 lightyear to the same place, what would be the difference in their arrival time? Would one actually be faster than the other? Or would they just age differently on their respective ships but arrive at about the same time?

QUESTION: 
I'm trying to understand the permittivity and permeability of free space and I've simplified the concepts to mean the resistance of a vacuum to electrical and magnetic fields (respectively). Assuming that's more or less correct my question is, why are these two constants non-zero values?

QUESTION: 
In a theoretical situation where an object is free falling in a vacuum: without any wind resistance and consequently without a terminal velocity, what would stop the object from reaching the speed of light?

QUESTION: 
Would an object in an infinite free-fall in a vacuum reach the speed of light or would its mass bring it to a maximum velocity?

ANSWER:
The question is what happens if a particle experiences a constant force "forever". The reason it does not accelerate forever is that Newton's laws (like F=ma) are only approximately true and are not correct for velocities comparable to the speed of light. Linear momentum has to be redefined. I have answered this question before, so for much more detail see an earlier answer.

QUESTION: 
I shoot clay targets with my daughter. I used to be pretty good at physics at high school but I've been puzzling over this for months without success so here goes. I was trying to create a "lead" calculator so she could roughly calculate the amount to lead the target by to take a more accurate shot. The more you look at it, the more complicated it gets. Straightforward to start. Take the middle station on a skeet field where the shooter is trying to break the target directly in front of them. Then we can assume that the target is traveling perpendicular to the direction of the shotgun. If the target speed is 20m/s & the shot velocity is 396 m/s (1300fps) & the target is 15m from the shooter how much lead do you need? But then I started to think, the target is decelerating & ascending/descending, the shot is decelerating and the shot is not all in the same plane. It is spread out an the shot "string" is quite long. It gets more complicated then as you move around the arc of the skeet field and look at the target, and shoot it from different angles. The target speed & distance from the shooter is fairly constant in skeet from each station. ie. target 1 at station 1 is the same each time, 2 from station 1 is the same each time etc. In simulated field target speed varies from station to station. Target size may change. Trajectory can be varied. Angle to the target also changes. I'm not even going to think about wind velocity. My head hurts enough already. Got any suggestions? Please?

ANSWER:
Maybe there is an app for it! Seriously, though, there is science, art, and skill. Science is wonderful, and, given modern computing capabilities, one could do the calculations you want but it would have to be done numerically, not analytically, because it is a very complicated, real world problem. But, what is the point of what you are doing? Isn't it to have fun and to acquire a skill which comes from practice and lots of trial and error? Why try to put that into an equation? You should just encourage her to have fun and get better over time. Of course, explain to her what the considerations are, but qualitatively, not quantitatively. You can't compute a home run or a hole in one or a field goal or a three-point basket, you gotta do it! Go blast those clay pigeons!

QUESTION: 
I am considering an electrical motor, which can also be used to generate electricity. From what I understand, an electrical motor consists of coils of copper wire rotating inside a magnetic field surrounding the rotating coils. My question is: take a cylindrical object - a metal rod base with copper coils, like in the center of a motor - in orbit around the earth, free from friction and gravity, could we start this object rotating, with the center of rotation being the metal rod going through the center, with the rod and copper coils being placed within a hollow magnetic cylinder and both objects orbiting at the same speed, so that the copper coils are spinning within the magnetic field to produce a constant electric current? This seems to be a source of near endless energy, but I am not a physicist and surely I am missing something, I just don't know what it is.

ANSWER:
Ah, the perennial question "why can't I get energy for free"! Forget all the orbiting stuff, and just cut to the chase. Imagine a coil which we get rotating in a magnetic field and there is no friction. Imagine that the coil is not closed but has two ends which I will call the terminals. It is happily rotating and will do so forever, provided there is no friction. And, if I connect an AC voltmeter across the terminals, I will find that there is a voltage there. But, voltage is not a measure of energy. However, I can use a voltage to drive an electric current and I can use that electric current to deliver energy to me, for example to make my breakfast toast. So, can I make toast forever for free? No, because the instant I start taking energy away from my motor physics is going to demand that the motor give it to my toaster, the coil will therefore quickly stop after all the kinetic energy it had has been given to my toaster. And, I haven't even gotten that little amount of energy for free because I had to put that much in to start it spinning in the first place.

QUESTION: 
Does hot water freeze faster than cold water and if so are the ice cubes clearer?

ANSWER:
See an earlier answer. I have no idea about the clarity of the ice cubes.

QUESTION: 
In my textbook time dilation formula was calculated using a special clock with a light beam,mirror,detector arrangement.So it was easy to calulate mathematically.But can you calculate the same using a simple clock,biological clock or any other clock mathematicaly or should i accept it only based on experimental evidences?

ANSWER:
What you suggest is a pointless exercise. What is required is that you accept the light clock to be a perfectly good clock which will run at the same rate as any clock at rest with respect to it. Your watch would tick out 100 seconds exactly like the light clock would. An atomic clock would tick out 100 seconds exactly like the light clock would. A bunch of bacteria would double its population in a way that could be perfectly measured by the light clock. A man would become 100 years old when the light clock ticked out 100 years. The beauty of the light clock is that all clocks run with it and it can be easily shown to depend on motion.

QUESTION: 
While in a 0 gravity, vacuum environment you hold a 600kg weight. You weigh approximately 80kg If then you push against the weight, does the significant difference in mass between you and the weight affect the reaction? A) you and the weight move away from one another equally B) you move away from the relatively stationary weight. Thank you in advance for ceasing this argument :)

ANSWER:
Nothing has weight in zero gravity. But you and the object have mass, 80 kg and 600 kg. You simply use momentum conservation to solve this problem. If you push such that you leave with a speed v, then the speed u the object has is computed as: 0=80v+600u, u=-(80/600)v=-0.13v, that is, the object has a velocity about 13% of yours in the opposite direction (that's what the minus sign signifies). Neither A) nor B) is correct because I would not call 13% of your speed relatively stationary.

QUESTION: 
If an object were ascend 10 feet, perpendicular to the surface it was on, and then return to earth through the force of gravity, would it land in the exact physical location from whence it left?

ANSWER:
Look, let's just say we drop it and ask if it falls straight down, straight down being in a straight line toward the center of the earth. The answer is only if you drop it at the north or south pole. The reasons are complicated, but result from the fact that the earth is not an inertial frame of reference, that is, it is rotating. I have ignored any effects from the moon or the sun in this answer.

QUESTION: 
My wife and I had an argument this morning over E=MC^2. She proposed the following thought experiment: If you push down on a spring, you have put energy into it. Correct? I said, yes. Correct. And, when you let go of that spring, it bounces, correct? I said: Correct. Then, she said this: Have you increased the mass of the spring? I said no. She said – well there, Einstein was wrong. You have increased the energy, but not the mass of the object, therefore, mass and energy cannot be equivalents. SO… I said, well, you have increased the constant. And, she told me that I was a moron, because the constant is… well, the constant and does not change.

QUESTION: 
How might buckshot propagate in a vacuum? I've found scores of websites and posts where questions about firing a gun in a vacuum (the vacuum of space) have been addressed, but not a single one about shotguns. What I am trying to figure out is if the pellets will maintain a tight grouping in a vacuum or if they will spread in the same fashion as they would at one atmosphere. Will the shot cup expand without a substantial atmosphere? Will the shock of the gunpowder blast transfer enough transverse energy into the pellets thus causing the shot cup open. Will the shot cup travel at the same speed as the much heavier pellets? Will the gasses vented out of the muzzle continue to move forward or will they instantly dissipate into the vacuum? I've been trying to figure this out for almost two weeks now and every time I think I've got it, some other variable or contradiction comes up to complicate matters. Help me Obi Wan Kenobi.

QUESTION: 
Sound is what we here when a disturbance is created in air by a vibrating body. At microscopic level, we see that air molecules have very high speeds and they collide with each other. Our ear drum feels sound if a disturbance is created in air . But what about already moving molecules?? Even they are disturbed every second as they collide with each other that doesnt create sound but a small vibration does. Why so??

ANSWER:
When there is no sound, our ear experiences many tiny percussions very closely spaced and the net effect is that there is a nice, smooth pressure on the outside of your eardrum. But, inside your eardrum there is also air at the same pressure. What your ear detects is variations in pressure. Sound is a wave of pressure variation, so you ear is subjected to not a constant pressure but varying pressure and, if the variations have frequencies within our hearing range, our brain interprets the pressure variations as sound.

QUESTION: 
If a person were thrown from a vehicle traveling a given speed, with no resistance (for arguments sake let's say they are riding on the luggage rack) what would be a good formula to determine how far they would travel before hitting the ground, assuming the ground is level?

QUESTION: 
Please can you explain me or tell me something energy-time uncertainty principle and what is del E and del t in it?

QUESTION: 
I was asked the question "How can anyone think Einstein could have been wrong about E=mc2 and the speed of light when we know that atomic bombs work based on the equation?" I did not think the atomic bomb was inherently based on the speed of light's importance in the equation, but I did not know for certain. Can you help? Does the fact that nuclear bombs exist and work serve as proof of the validity of E-mc2 and the speed of light? Or are the two in fact not related?

QUESTION: 
I'm trying to get a grip on quantum mechanics. What was the original question theoretical physicists such as Planck, Heisenberg, Bohr, Pauli & Schrodinger had that lead them to the uncertainty principle. (If Einstein never could accept it.... do I stand a chance?) Do the particles in question have to be traveling at high speeds for this question to meaningful? Why would their be uncertainty of measurements of a car hitting a solid wall at 60 mph.

ADDED QUESTION: 
why were these scientist so smart and at such a young age? It seems like the best of them were Jewish/Germans, e.g. Einstein and Oppenheimer .

QUESTION: 
Imagine I have a 10000 ft long water filled vertical tube (1" ID though I dont beleive it matters) and I have a cap on both ends. If I now remove the cap on the bottom of the tube how much fluid will come out due to density chages in the water column (assuming no air comes into the tube thru the bottom)

ANSWER:
Your question is not about compressibility. Water is extremely incompressible, only a tiny change in density occurs for a huge change in pressure. The simple fact is that atmospheric pressure, about 15 psi, is only able to hold up a column of water of about 34 ft. So, as soon as you unstoppered the bottom of the column, water would flow out until the column was 34 ft high (with the remaining 9,966 ft a vacuum). This has nothing to do with compressibility of water. You have made a barometer.

QUESTION: 
I have been conducting some simple experiments in my local park (i have been launching carbon-fiber rockets of between 3-6kg using compressed air out of a tube) and have come across a confusing contradiction relating to the projectile in motion: with a near constant starting velocity (speed and angle of projection) and near constant shaped objects, the distance they are thrown varies with the mass of the objects in a positive manner (that is, make them heavier, they go further). I am guessing this is because Ke=Mass x Velocity squared, therefore the heavier ones, having more energy, travel further, even though the angle and speed are more or less constant. However, I have noticed that after increasing the speed above a certain point, the relationship between increased mass and distance travelled has a negative relationship ie. the distance starts decresing if I increase the mass too much. Why is this?

ANSWER:
I guess my first question is how do you get constant velocity using compressed air? Surely you need either bigger pressure or longer tubes for the more massive rockets. Anyhow, assuming they are all launched with the same velocity, the reason the heavier ones go farther is explained in the answer just two answers down from yours: they all experience about the same air drag force but this force is more effective in slowing the lighter ones because of inertia. Regarding your results if you continue to increase mass, I would guess that you simply do not give the big mass nearly as much initial velocity as the lighter mass.

QUESTION: 
Suppose there is a completely white room with a source of light. The room is white, therefore the light from the source will, at least mostly, reflect off all it's surfaces. Now let's say the light was suddenly turned off and the room gets dark; well if all that light from when the light source was on was reflected and not absorbed, shouldn't the room still stay lit? But it doesn't, so where does the light "dissapear" to?

ANSWER:
There is no such thing as a "completely white room" which reflects all the light; even really good mirrors reflect only on the order of maybe 90-95% of what hits them. But, let's say we have an incredibly white room which reflects 99.9% of the light which hits it. Now, let's say the walls of the room are 3 m apart. Then, since the speed of light is 3x10⁸ m/s, the time light takes to cross once is 10^-8 s. So, in 1 ms=10^-3 s, the light bounces back and forth 10^-3/10^-8=10⁵ times, each time losing only 0.1% of its intensity. So, how much is left after 1 ms? 0.999^100,000=3.5x10^-42% of the original intensity. That is really gone!

QUESTION: 
Why do I go faster than my lighter weight friends when on a bike? I have noticed this downhill, but also on flats, I'm pedaling less. Assuming we both have top of the line bikes (we do). I thought things fell at the same rate regardless of mass. I think momentum and friction have something to do with this, but can you remind me of the principles involved here?

QUESTION: 
I recently heard about the clever creation of Hydrogen 4.1 and its use to study reaction kinetics. However, I cannot find an explanation in any of the reporting, or original Science paper on why the muon orbits so close to the nucleus beyond "it's 200x heavier than electron." Surely they did not apply gravity to a Bohr model of a muonized atom, so why does the muon orbit closer?

ANSWER:
I actually never heard of hydrogen 4.1. Apparently what it is is a helium atom with one of the two electrons replaced by a negative muon. The radius of an orbit is inversely proportional to the mass in the Bohr model of the atom. It has nothing to do with gravity, it essentially enters from the mass in Newton's second law. So the muon, having about 200 times larger mass has a ground state orbit 200 times smaller. So, you may think of the "nucleus" as being of atomic mass of around 4 amu and charge of +1. Therefore this will look like a hydrogen atom with a much bigger mass.

QUESTION: 
In a simple series resistance circuit current remains constant through out the circuit.A resistor simply reduces the current drawn by the circuit.Just at the instant when the switch is pressed current is already determined by the value of the resistors,even before it encounters a resistor.I think the resistor should alter the value of velocity of charge & therefore current but current remains same even after encountering resistor ?

ANSWER:
If the current were different in the resistor than it is in the wires connected to it, charge would be building up in the resistor. But, the value of the current is not solely determined by the drift velocity. Also important are the number of charge carriers and the geometry. So, almost certainly, the velocity in the resistor will be different, but the current will be the same.

QUESTION: 
Why is it that if you jump from a airplane you will stop acceleration at some point of going down?

QUESTION: 
My wife bought a medical device that claims to heal joint pain by emitting photons into the affected area is it possible that such a device purchased at the holistic pharmacy could actually shoot photons into your body?

ANSWER:
And, I bet she paid a bundle for it! A flashlight will shoot photons into your body. Lying in the sun will let you get a bunch of photons. For joint pain, heat is probably good and the purchase of a simple infrared heat lamp would probably be just as good as this device and a lot cheaper. On the other hand, if this thing does not plug in and create heat or light you can see, like the copper bracelets you can buy, it is likely a fraud. Also, see a recent answer.

QUESTION: 
If antimatter and matter collide, they would annihilate each other. The definition of annihilate is to destroy completely. If these two meet and annihilate each other, wouldnt that make the law of conservation of matter - which states that matter cannot be created or destroyed, only changed - obsolete or false?

QUESTION: 
Can I have a non - mathematical, and simple intuition on why the speed of light is constant? Thank you. (I am 13 years old.)

ANSWER:
First, do not expect it to be intuitive. Our intuition tells us that if we run with a flashlight that the speed of light is what it was when we stood still plus however fast we are running. Nevertheless, the speed of light is always the same, regardless of the motion of the source or the observer. I have answered this question many times, nonmathematically, which you could have found on the FAQ page.

QUESTION: 
I have been reading up on general relativity and i have a question regarding it. If as matter approaches the speed of light it compresses along the axis it is travelling in (please correct me if i got this wrong). why wouldnt the same be true for time? let me clarify.. if travelling at near the speed of light through time, why doesnt your displacement in time compress, or conversely if you are travelling slowly through time but quickly through space, why wouldnt you have a larger displacement in the time dimension? Basically, i am asking, since matter displaces space, and travels through time, why doesnt photons, or electrons displace time and travel through space? from what i have read (so far) i dont see anything that touches on displacement, and i dont think this violates anything (yet).

ANSWER:
This is not general relativity, it is special relativity. Rather than go through your rambling, let us just say that moving clocks run slow. To understand why, see the FAQ page.

QUESTION: 
When we say we consumed electricity at home ? what actually we consume Voltage,current or energy of the charge flowing ?

ANSWER:
You pay for energy and that is what you should think about consuming. The electric bill is billed at some rate per kilowatt-hour. A watt is a joule per second, so 1 kW-hr=1000 J hr/s (3600 s/hr)=3,600,000 J.

QUESTION: 
Here is the scenario: a pool ball is traveling at certain pace with an enormous amount of spin on it. Once it hits the rail at about a 45 degree angle is it possible for it to shoot off of the rail and travel faster than it was going before when it was on it's way to the rail?

ANSWER:
It does not even need to have "an enormous amount of spin", a modest amount will probably do it. The ball, coming into the rail, has translational kinetic energy and rotational kinetic energy. The friction between the ball and the rail can transfer some of the rotational energy to kinetic energy, so after the collision it will have a higher speed and a smaller spin.

QUESTION: 
If you take a rock and weigh it, then take the exact same rock and crush it into fine powder and weigh all the powder (assuming no powder escaped), will the powder weigh the same as the solid rock?

ANSWER:
Two possible answers:

QUESTION: 
Why is there angular momentum in empty space? I.e., if there are no frames of reference in empty space, how does a spinning top "know" it's spinning?

ANSWER:
A constant velocity in an inertial frame of reference can always be viewed from another inertial frame in which the velocity is zero. So, we say that velocity is relative, absolute rest has no meaning. Why can't we extend this to rotating systems? Quite simply, because if we view the rotating object from the frame where it is at rest (which would be rotating with it), it is not an inertial frame, that is, Newton's laws are no longer valid. There is such a thing as "absolutely at rest" in a rotational sense. If an object has an angular velocity in one inertial frame, it has the same angular velocity in all inertial frames.

QUESTION: 
How do I calculate the impact force of an 800 gram object dropped 2m?

ANSWER:
Ah, the perennial question which demonstrates how folks do not understand force! See below.

QUESTION: 
Would a "fuzzy" tennis ball make any difference in the distance the ball travels?

ANSWER:
The fuzz on a tennis ball plays the same role as dimples on a golf ball, to reduce air drag. So, indeed, a fuzzy ball will travel farther than a smooth one with the same initial speeds.

QUESTION: 
A thermos bottle consists of two glass vessels, one inside the other, with air removed from the space between them. The vessels are both coated with thin metal films. Why is this device so effective in keeping its contents at a constant temperture?

ANSWER:
Removing the air prevents heat loss or gain by conduction or convection, although the vacuum is never perfect. There is still the possibility of heat loss or gain through radiation, and the silvering reduces this.

QUESTION: 
if earth's mass was , say 10 times lighter, would the gravity be 10 times weaker? if so , why as it would still occupy the same amount of space.

ANSWER:
Yes. The gravitational force on a mass m a distance R from the center of a spherically symmetric mass distribution M and at or outside the surface is given by GmM/R² where G is Newton's universal gravitational constant. It has nothing to do with the size of the sphere. So, if G, m, and R stay the same, making M 10 times smaller makes the force 10 times smaller.

QUESTION: 
To what degree does buoyancy affect actual weight as opposed to apparent weight? The support for a 2nd story soaking tub/shower takes into account the weight of a person standing to take a shower and the weight of however many gallons of water it holds, but does the weight of the person plus the weight of the water need to be calculated for the amount of building support necessary or does buoyancy mitigate the person's actual weight?

ANSWER:
The weight of tub, water, and bather must all be supported by the floor. True, the weight of the floating bather is supported by the water, that is the water exerts an upward force on the bather equal to her weight. But, Newton's third law says that if the water exerts an upward force on the bather, the bather exerts an equal and opposite force down on the water. So, your question would be like asking: if we have a 5 lb box on top of a 10 lb box, does the floor only have to hold up 10 lb because the 10 lb box holds up the 5 lb box?

QUESTION: 
when electric field is applied in a circuit ,charges experiences a force in a direction opposite to it.So why doesnt current accelerate in a circuit?why do they move with constant drift velocity?

ANSWER:
Because the electric field from the potential difference is not the only force on the electrons. The wire is full of atoms and the electrons keep bumping into these, getting stopped or scattered, then accelerated again, etc. So it is a constant stop-start process which averages out to some average drift velocity.

QUESTION: 
Please can you tell me how fast force is transmitted in a solid object? I am asking as I am wondering if it is possible to transfer information faster than the speed of light using a piece of string?! That is, for a (very) tight piece of string between two points A and B, when you pull the string at point A, does it not move instantaineously at point B? In answers by others I have been told it relies upon the speed of sound in the material - if you agree with this then please could you elaborate upon this for me?

ANSWER:
When you pull or push on the end of a long rod or string you are actually only pulling or pushing on those atoms you are holding on to. Then (let's say we are pulling, as in your example), the bonds between those atoms and the next one down the string get stretched and that stretch (think of all the atoms as being attached to their neighbors by tiny springs) pulls that group of atoms toward you, but that stretches the next layer of atomic bonds and so a pulse travels down the string. That is what sound in a string or a rod is, compression/stretch between bonds traveling through the solid. So, the information traveling down the string travels much slower than the speed of light, the speed of sound. You could have read an earlier explanation if you had gone to the FAQ page.

QUESTION: 
Is there a measure of time that does not make use of, or reference any planetary body? As an example, a day on earth is based upon its period of rotation as a year depends upon how long it takes to make a circuit about the sun. I am curious if there is a system of time measurement that does not use of any planetary reference.

ANSWER:
The unit of time, which is the basis for all time measurements, the second, is not based on an astronomical measurement. Widepedia says "Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." Atomic clocks are used for precision time measurements nowadays.

QUESTION: 
My question relates to atomic theory in regards to how I was taught that a nucleus is surrounded by electrons depicted by concentric circles. However I was listening to a Pink Floyd song on Youtube and in the video it depicted the Flower of Life symbol in a unique way. It struck me the Flower of Life could be another way of explaining atomic theory. Notice in the video how the electrons zip through the nucleus and reaching various atomic levels in a more believable manner. This depiction seems more real than what I was taught in grade 12. It shows both wave and particle in its depiction. Your thoughts would be appreciated.

ANSWER:
This is less real than what you were taught in school. Admittedly, well defined electron orbits which go around the nucleus are a simplistic picture of reality, but such a model (the Bohr-Sommerfeld model) was quite successful in many respects and led to better descriptions of atoms, particularly those using quantum mechanics. This picture, as you note, has the electrons moving in well-defined orbits which pass throught the nucleus; although there is a small probability that an electron could be found inside the nucleus, your picture would have us believe that all the orbits passed right through, simply not true. The best way, as determined from solving Schrodinger's equation, to think about the electrons is as clouds surrounding the nucleus. Where the cloud is dense, the likelihood of finding an electron there is high and where it is thin the probability is low. See an earlier answer.

QUESTION: 
While the vehicle is sitting on the launch pad at zero velocity place a detector that will light when the photons hit it, at the position the photons hit the sidewall. Launch the vehicle and accelerate to 0.5 c. After reaching a constant speed of 0.5 c cause the laser pen to fire the photons. Will the photons hit the detector and cause it to light or will the photons travel in a straight line from the point emitted and miss the detector?

ANSWER:
Hit it.

FOLLOWUP QUESTION: 
Now could you please explain what causes the photon to travel 2 feet in the direction of the vehicle while traveling 4 feet independent of the emitter across the vehicle? If the laser pen is pointed in the direction of the travel of the vehicle the speed of the vehicle can not be added to the speed of the photon. So why can the speed of the vehicle be added to the photon when the photon is traveling at a 90° angle relative to the travel of the vehicle?

ANSWER:
Your example is a variation of the old light clock problem. In the spaceship, you have no way to know you are moving, so it is a no-brainer that the light goes straight across the ship and hits the detector, just like you aimed it. In the earth-bound frame, the ship moves with speed c/2 but the light must still hit the detector (it cannot both hit and not hit). So the path it travels in the earth-bound frame is, as you note correctly, 4 ft across and 2 ft along, a distance of √20 ft>4 ft. But, as relativity postulates, the speed as seen by the earth-bound observer is still c and so it takes longer than the time the guy on the ship measures. This is time dilation, moving clocks run slow.

QUESTION: 
I have a question regarding special relativity. I was pondering over a thought experiment that I came up with using a modified version of a light clock. This modified light clock, with every tick, sends a signal to a space ship which is moving towards it at a uniform speed. This is where I get a little confused. It would seem that since the spaceship is moving towards the light clock that the frequency of the signals being received would increase. Intuition would tell me that the rate of the ticks has therefore increased. And yet I find it hard to reconcile with the fact that, since the light clock is in motion relative to the spaceship, that the ticks ought to be slowing down. I realize that my argument is flawed somewhere as special relativity has already been proven through countless experiments. I ,however, need help understanding where exactly that flaw is. Do you have any thoughts on this?

ANSWER:
You have stumbled upon, as many of my loyal readers will tell you, one of my favorite points of emphasis in special relativity. How things are is not necessarily how things appear to be. To answer your question, read my explanation of the twin paradox where I emphasize that the rates of clocks as you see them is not the same as the rate at which clocks are ticking. See also another answer. Time is defined as the time interval between two events at the same point in space and the clock coming toward you is not at the same places when it sends you sequential ticks, so the ticking you observe is not the time interval between ticks on that moving ship. If the space ship is going away from you the observed frequency of ticks would be less than the real tick rate. The same kind of phenomena are observed for length contraction. Length is defined as the distance between two points in space measured at the same time. But if a stick is moving toward you, it appears longer than its rest length even though it is actually shorter and the stick moving away from you looks shorter than its actual contracted length. Again, see an earlier answer and links there.

QUESTION: 
I was just thinking ... I'm about to fix up a very old bicycle that has one of those generators on the rear tire which supplies power to the headlight. Does anyone out there wonder if something similar could be applied towards a hybrid automobile? Perhaps it could be installed in a switching primary/secondary battery powered vehicle to give a charge to one while the other provides power to the vehicle.

ANSWER:
Well, this is exactly what cars have been doing for generations, that is what the alternator or, on older cars, the generator does, generate energy and store it in a battery. However, your idea of having two and have them swap off isn't realistic because somewhere something has to put energy into the system (to make your car move in the first place). On the bike it is you putting energy in, in a conventional car, it is the engine putting the energy in. A hybrid goes one step beyond and that is to get energy from the braking of the car. In conventional cars, when you brake you use friction and all the kinetic energy of the car (energy it has because it is moving) gets turned into heat (brakes get very hot) and is just lost. In hybrids, the same ideas as generators allow us to capture that kinetic energy and put it into the car's batteries.

QUESTION: 
What would you consider to be a good representation of the distance between the earth and the moon, and the relative scales involved, in terms of household objects, with regards to a total arm span of 6 feet, so as to be easily replicated without the need of a measuring tape?

QUESTION: 
In a step up transformer,the number of windings in the secondary side is greater than in the primary side.And this increases the voltage by decreasing the current(satisfies law of conservation of energy).how is this even possible if we consider this in terms of work done?How can a huge potential create only a small current?Rate of flow of electrons should be proportional to the work done?Or where is this huge voltage hiding without producing current?

ANSWER:
Why do you associate huge potential with huge current? If you connect nothing across the terminals, you have a huge potential and zero current. If you connect a huge resistance across them, you will have a small current. If you connect a small resistor across them, you will have a big current. But, in every case you have to ask whether you have the energy which allows that to happen. In the transformer, if you ask for more energy out (by forcing a large current) than you are putting in, the machine will refuse to oblige you. That is, the potential will simply drop. Here is a simple DC example. Suppose you have a capacitor which has been charged up to some high voltage. Can you simply attach a resistor across it and use it as a battery to give you a big DC current? Of course not because when you put energy into the capacitor by charging it, you only put in so much and you can only get that much out.

QUESTION: 
I have a question I can't seem to find. I was wandering what the orbit would look like if there were three objects of equal mass all orbiting each other. I can only find simulations of two objects orbiting each other. So my question would be first, is this possible? and if so, what would the orbit look like?

ANSWER:
Your question cannot be answered because the three-body problem cannot be solved in closed form. There are special cases which can be solved, for example if one of the three is held in place and the others move around. I believe that the orbits are chaotic, that the objects do not have single closed orbits. And the behavior which could be simulated depends sensitively on the initial conditions. That is not to say that there cannot be such systems in nature. A recently discovered planet has been found to orbit a binary star system. But if you looked at the planets orbit this year, it would not be the same shape as next year. For more information, google "three body problem".

QUESTION: 
How do moving electrons or charge particles produce their own magnetic field? For instance when the current in the wire produces its own magnetic field. How is it happening?

ANSWER:
Well, in classical electromagnetism, where the electric and magnetic fields are treated as different things, it is just an empirical fact that magnetic fields are produced by moving charges. But, in relativistic electromagnetism, electric and magnetic fields are just components of the electromagnetic field tensor, all just a single field. Then, if you start with a tensor which has only electric fields in it (that would be for a charge at rest, for example) and then transform the tensor to a frame which is moving, the transformation causes magnetic field components to appear.

QUESTION: 
I'm designing an experiment for my physics teacher, involving gas density and how it effects a ball's acceleration. The problem is finding a ball that is light enough to be effected by the change in density, but heavy enough to fall no matter what. Would a tennis ball be appropriote?

ANSWER:
In order for the ball to not fall, the weight of an equal volume of your gas would have to be more than the weight of the ball; there is always a buoyant force when you put something in a fluid (Archimedes' principle). I do not know how big pressures you propose using, but I would think something less massive than a tennis ball might give you results easier to measure. When you are analyzing your results, be sure to include the buoyant force in the forces acting on the ball.

QUESTION: 
If neutrinos are traveling through the earth what distance from each other would be typical? Can they be "seen" say at a light spectrum not typically seen by the human eye.

ANSWER:
The solar flux at the earth is about 10¹¹ neutrinos/cm²/s, 100 billion pass through a postage stamp in one second! If you really care, you can compute the average distance between them but it is not that interesting a number, flux is what physicists care about. They are not part of the electromagnetic spectrum (like x-rays, gamma-rays, radio waves, etc. are). That is, they are not photons, they are neutrinos; a neutrino interacts extremely weakly with matter whereas photons react very strongly. As a result, neutrinos are notoriously hard to detect. It was on the order of 50 years between the time we understood that neutrinos exist until the time when we actually directly observed them.

QUESTION: 
is it possible to see an image of the past (ancient earth) reflected from a surface in space millions of light years away. if no then how far away can an image be reflected and still be seen with some degree of clarity?

ANSWER:
The issue would be intensity. Since the light from earth spreads out, by the time you get "millions of light years away" there will not be enough photons per square meter per second to form an image; like, with a single photon you could not tell if it came from a dinosaur or a bowl of jello. I have done some rough estimates in an earlier answer for tv signals being received at a much closer (4 light years) distance and found the size of the detector to be impractically large.

QUESTION: 
Is it possible for a projectile to travel in more than one direction, while in a single unobstructed flight ? I.E. Vehicle traveling at 30mph, an antelope running parallel to vehicle at say 50yds. distant and also moving in same direction at 30mph. A firearm is aimed at the antelope from in or outside the car. A firearm is discharged toward the antelope with the projectile leaving the weapon at say 2,000fps. The firearm is exactly 90o from front of vehicle and nose of antelope. Will the projectile move at the speed of the car and equal speed of antelope once it leaves the muzzle of firearm, therefore having it move forward with car and animal at 30mph, while still moving at 2000fps in direction of the antelope?

ANSWER:
I will assume there is no appreciable air drag on the bullet; that is probably not true but is not the issue I believe you are trying to address. When you shoot the gun it gets the speed relative to the barrel of the gun but it also gets the speed of the car, 30 mph=44 fps. So the velocity is a tiny bit bigger 2000 fps and has a component which carries it forward with the speed of 30 mph. Therefore, the bullet, if you have corrected for gravity, should hit the prey. The path still will be a parabola in a vertical plane.

QUESTION: 
If 2 people the same size and weight are racing the exact same car on a flat service, but one car weighs 10 pounds lighter, which car would win? We think it's the lighter car, but my husband says the heavier car because it would get more traction.

QUESTION: 
I have heard that Newton's laws fail at some points like at the speed of light or at the quantum level. Please explain how they fail in the above cases.

QUESTION: 
suppose there is a toroidal device and u take cylindrical coocrdinate (r,theta,z) system in to solve it. now the magnetic field in this toroid is non uniform and its along theta direction. iwant to find the curl of magnetic field i.e. theta component.so hw could i find it?? will u b able to give a soltion to it?

QUESTION: 
A particular type of string of say 1 metre length (and mass 10 grams per metre) can handle 100N of force being applied before breaking. But if a string of the same substance was the length of the solar system, just floating in space and tethered to nothing, I'd assume that it couldn't be pulled because the total mass of the long string exceeds the mass that could be pulled by the string. Perhaps my assumption is wrong? Perhaps that string could be pulled extremely slowly with less than 100N of force? Please consider it if there was no gravity acting on the string from planets or the sun, so there's (hypothetically) no minimum force required to overcome gravity.

ANSWER:
As long as you do not pull with a force bigger than 100 N, the string will not break and will move. Newton's second law tells you that the acceleration a=F/m, so unless m is infinite, which it will never be, there will be some acceleration.

QUESTION: 
My friend is writing a childrens book and somethings have come up in his book and he would like to make it as accuratly as possible even though this make believe he would like the children to learn some physics. First question is, in theory how long would someone have to travel faster than the speed of light to go back in time a month? And would this vary depending on how much faster than light you can travel? Second question, in theory what would someone experiance while traveling faster than light?

ANSWER:
Well, if by "in theory" you mean according to the laws of physics as we know them, then none of your questions are possible. Nothing can travel faster (or even as fast as) the speed of light. It is possible to travel forward in time but not backwards in time. So, if he wants children to learn some physics, this is fantasy or science fiction, but certainly not physics!

QUESTION: 
I am curious to know. A brick with a crack in it was left outdoors in a cold country. Water that was collected in the crack froze into ice. When the temperature rose a little, the ice melted into water, only to freeze again when the temperature fell. This continued for a long time until the brick finay broke into two. Why is this so?

ANSWER:
When water freezes it expands. So freezing water in the crack tries to pry the brick apart. As it expands, the crack enlarges a little bit and when a thaw comes, water goes deeper into the crack. Eventaually, as this process recurs, the crack expands far enough that the brick breaks. It is sort of like slowly driving a wedge into the crack.

QUESTION: 
Could a simple aluminum spiral planar array be expected to equilibrate or harmonize our bodies electromagnetic field? I am a family physician in Columbia, SC and am looking at the C prime bracelet which touts said planar array to create a balanced EMF field around our bodies. I am most interested in the effects on the cellular level for release of stress hormones that lead to anxiety and illness and a heightened pain response. Studies in EMF seem to suggest increased release of stress hormones and somthing simple like a basic planar array worn in the clothes or on the wrist, if it worked, would be a very interesting idea to me. Thanks for any help.

ANSWER:
I have done some research into this kind of thing (my wife has some serious medical conditions and somebody was trying to sell her a magnetic matress pad) and I have to tell you that what you are talking about is pseudoscience. "Harmonize our body's electromagnetic field", "spiral planar array", "create a balanced EMF field around our bodies", what on earth do these things mean? Nothing in physics I can assure you and nothing in biology that I have ever heard of. The companies who market these things are, in my opinion, composed of nothing but charlatans and the trash they peddle is worthless. I have heard of cases where such devices may help a patient, but you may be assured that nothing more than a placebo effect is in play.

QUESTION: 
Ok me and my brother had a very long argument over this hopefully you can set this right. A helicopter that can lift 10,000 lbs is attempting to lift off. A man weighing 200 lbs but can lift 20,000 pounds grabs a hold of the helicopter. The man begins pulling down, what happens? I said that the helicopter would lift the man off the ground and the man would basically be pulling himself up. My brother believes that the man would be able to hold the helicopter on the ground. And this is all without an anchor on the ground for the man to grab a hold of or somehow bolt himself to the ground.

QUESTION: 
If a super-hero was standing on the ground and a helicopter was taking off next to him, he reaches out to grabs the helicopter to the ground. Would it take him up in the air or would he be able to hold it down? He weighs about 200lbs and can exert more force in his muscles than the 10,000lbs helicopter can lift.

ANSWER:
Looks like both you and your brother decided to "Ask the Physicist". You are right, the strong guy could not hold down the helicopter unless he was tied to the ground, and even then, even if he can lift 20,000 lb he would not necessarily be able to pull down with a force of 20,000 lb.

QUESTION: 
There are numerous problems connected with air drag acting on the falling object. What if it acts on the object thrown up with a given initial speed v(0) and has a given terminal velocity of v(term)? What height would an object rise to?

ANSWER:
This is a standard problem, and if the object is such that we can approximate a drag force of the form F=-cv², the solution is h=(v_t²/(2g))ln(1+(v₀/v_t)²). The graph to the right shows heights achieved for various initial velocities if the terminal velocity is 10 m/s compared to the case of no air drag.

QUESTION: 
I hold a tennis racquet by the handle out in front of me, with the flat head parallel to the ground. I flip it upwards, the head rotating toward me, to do a complete flip. I catch it by the handle again. The odd thing is that it rotates, or twists, parallel to its long axis halfway through. My friend says I'm spinning the racquet. I think it's some quirk of physics. Maybe the Coriolis effect, though I have never understood it, with all its "conservation or angular momentum" stuff.

ANSWER:
This is a complicated problem only dealt with in intermediate or advanced classical mechanics, not really feasible for me to try to reproduce it here. The reason it works is that the object you are tossing has three (x,y,z) different moments of inertia along its principal axes. If it is rotating freely, it rotates about its center of mass and if you start it with it rotating about the axis with neither the biggest nor smallest moment of inertia, it will do what you observed. The picture at the right shows this problem illustrated in an intermediate-level classical mechanics textbook. If you rotate it about the axis with the smallest moment of inertia, the axis running through the length of the handle, it will be stable, i.e. not do the flip you observe. If you rotate it about the axis with the largest moment of inertia, the axis running through the center of mass and perpendicular to the paddle, it will be stable, i.e. not do the flip you observe. But, if you rotate it about the axis with the intermediate moment of inertia running through the center of mass and in the plane of the paddle and perpendicular to the handle, it will be unstable and the axis of rotation will precess so that it does exactly a 180⁰ flip in a 360⁰ rotation as you observe. You are not spinning it, just get your friend to try it. It works nicely with a hammer also which is a little more compact and easier to toss.

QUESTION: 
why do you keep your legs far apart when you have to stand in a fast moving bus ?

ANSWER:
If you are standing in a bus, when the bus accelerates the friction between your feet and the floor is the force which accelerates you forward. But this force causes there to be a torque which tries to tip you over. To tip over, your center of mass must go beyond either of your feet. If your feet are close together, you do not have to tip very far to fall over, but if they are far apart you have to tip much farther before you fall over.

QUESTION: 
Some people been debating about the impact force on a bike of a bike rider riding off a 6 foot drop to a flat landing. Mainly the debate is about if speed would lessen the impact force on the bike because of the impact angle. So, if a bike rider was to ride off a 6 foot drop, would it lessen the impact force on the bike jumping off the 6 foot drop with faster speed rather than slower speed and just drop off? In both examples the bike rider is landing on both tires exactly at the same time.

ANSWER:
Both bikers hit the ground with the same vertical component of their velocities. The one dropping straight will experience only a force up when he hits. The other will experience a force up and a force backward due to the friction which would eventually stop him if he just sat there; the horizontal force is very small compared to the vertical force and I will ignore it. The vertical forces are determined by two things, how much the velocity changes (the same for both, from the speed just before they hit to zero) and how quickly the velocity changes. It seems to me that the only thing which will determine the time the collision with the ground lasts is the pressure in the tires which I will take to be identical. Therefore, I believe there will be no significant difference.

QUESTION: 
I want to move 20000 lt of water up hill every hour. the drop is 30 mt. the didtance uphill is 60 mt an the ange is about 30 degrees. can this be done using solar or wind. How much power would it take in watts or kW to move this much water .

QUESTION: 
If I spin an object, and a piece of that object comes off - will it continue to spin in the same direction as the original object was spinning? Do you have any ideas of how I could demonstrate this to a class?

ANSWER:
As long as the piece does not come off in a direction opposite the direction it was going before coming off (which it would only do if something impelled it rather than its just coming off), the spin of the object has to be in the same direction as the object+piece were rotating beforehand. This is simple conservation of angular momentum. To make a demonstration, you just have to have a spinning object where you can release a piece of it remotely so that it just leaves. How about a big disk rotating and a mass on the surface of the disk rotating with it; but the mass is also attached to a string which passes through a hole in the center and which you are holding on to. Let go of the string and the mass will slide off the disk? You can probably come up with something more clever than that! Be careful that the piece coming off does not fly into your students!

QUESTION: 
Hi, Hope you can help! I am driving a 6000 truck at 60 Mph. I am going to T-bone another vehicle (3500 pounds). Would I be better off putting my foot to the floor and hitting him at 80mph OR hitting the brakes and impacting at 40 mph??? My thought is that I should speed up and try to drive right through the impact.

ANSWER:
Sorry, I cannot say for sure! However, I can say this: the force you feel will depend on two things, how much your speed changes and how long it takes to change (i.e. how long the collision lasts). The force is proportional to the velocity change (bigger change means bigger force) and inversely proportional to the time (smaller time means bigger force). With certain assumptions you could maybe get some idea. Suppose the collisions are perfectly inelastic, that is, the two stick together. Then using momentum conservation you can show that the final speeds are 50 and 25 mph for the initial speeds of 80 and 40 mph, respectively; so your speed will change by 30 mph if you are going 80 and 15 mph if you are going 40. The bigger speed change would result in the larger force if the times of collision were equal. The times of the collisions will be determined by how long is taken to crush the colliding surfaces (the front of your truck and the side of the other vehicle). But that probably depends on the average speed during the collision, 65 mph if you are initially going 80, 32.5 mph if you were going 40; again, 80 in the bigger force since the time of collision would probably be shorter. So, if the collision is inelastic, I estimate that the average force during the 80 mph collision will be about 4 times bigger (30x65/(15x32.5)).

QUESTION: 
Why is it that I can sit on my front porch and get no wifi signal on my iPhone, but if I step inside and pick up the signal, then walk back out to my front porch I will continue to have the signal when I then sit inthe same place on my front porch?

ANSWER:
I guess because these devices scan for signals with a particular threshhold. Your wifi is there when on the porch, but the strength is below threshhold. When you move to where there is a stronger signal, it locks in and connects. Once you are connected, there is probably no threshhold level to stay connected, so you can reduce the signal but still stay connected when you go back to the porch. If this is the explanation (I'm really just guessing), a good analogy would be scanning your radio for stations. A weak station will not stop the scan, but if you manually tune to that station, you can listen to it.

QUESTION: 
Hello, my science class is currently studying air, and we have gotten to a section on air pressure. An interesting question was brought up, and the debate became so wild that my teacher asked me to google the question and come back with the answer. But, it seems that google has failed me, so I turn to you for advice. The question is: What would happen if all the air was sucked out of a room in the middle of no where? Would the room collapse? Has this been tried by scientists? If so, what happened? If not, what do you think would happen?

ANSWER:
The issue is more what would happen to the room before you pumped out the air. If there is air inside and none outside, the pressure in the room is trying to blow the room apart; so what happens all depends on how well the room is designed, how strong the walls are. When you pump the air out, it is no longer in danger of exploding. Actually, you do not need to "suck" it out, just punch a hole in one of the walls! Has it ever been "tried by scientists"? Every time we put up a spacecraft without a pressurized compartment (essentially all but those carrying people) we are trying it.

QUESTION: 
If Newton's first law of motion is true, why do you have to keep pedaling your bicycle to maintain motion?

ANSWER:
Because there are other forces other than the ones you bring about by pedaling. Always there is air drag and friction in the wheels. If you are going up a hill there is gravity trying to pull you down the hill. These forces are opposite the direction of motion and, if you want to proceed with constant speed, you are obliged to provide an equal forward force (as Newton's first law stipulates).

QUESTION: 
Yesterday, i studied about the spin quantum number and i came to know that electrons spin around themselves under magnetic field in clockwise or anti-clock wise. But my question is that why will the electrons not loose any energy when they rotate around themselves? what actually happens?

ANSWER:
First of all there is no energy associated with the electron's intrinsic angular momentum, nobody had to do any work to get it spinning, its presence is simply a property of electrons, that is what intrinsic means. Second, you should not really think of them to "rotate around themselves" since this is not your classical angular momentum and if you try to imagine a little sphere rotating you will get into all sorts of problems. Finally, there is no reason to expect an angular momentum to change unless there is a torque acting on it; a magnetic field will exert a torque on the angular momentum (because the electron has a magnetic moment), but the result is to change the direction of the angular momentum, not its magnitude.

QUESTION: 
My question starts with fundamental laws: gravity, electromagnetism, etc. As I understand it, these are the forces with the capacity to exert work on the physical world. I'm curious whether the work that biological systems produce are the product of fundamental forces, and if not, whether physics defines the source of the force that produces work in biology.

ANSWER:
Biology is essentially applied chemistry and chemistry is essentially applied atomic and molecular physics. The four fundamental forces (gravity, electromagnetic, nuclear, and weak) are the only ones we know in nature. Biology, at root, is mainly electromagnetism because that is the force which governs interactions among atoms and molecules. The other three forces play little role in biology.

QUESTION: 
A metal weighing 10kg and wood also weighing 10kg are both dropped from a building which would hit the ground first? If they are of the same shape,which would hit the ground first?

QUESTION: 
i want to ask that how are neutrons obtained to bombard on U235 in fission reaction

ANSWER:
It depends. Usually in reactors there is no neutron source. There is a small probability that a fission will occur spontaneously. When that happens neutrons are released and these can be used to cause more fissions. See an earlier answer. However most modern reactors have a starter source consisting of an alpha particle source inside a coating of Be or Li; the alpha particles react with the nuclei and neutrons are released.

QUESTION: 
How do atoms, which are 99% empty space, create the solid world around us? Is it simply that they get packed very close together?

ANSWER:
Most of the mass is in the nucleus, a tiny fraction of the size of the whole atom, but most of the size is outside where the electrons zoom around. The electrons form a cloud of negative electric charge around the nucleus. For more detail, see the FAQ page.

QUESTION: 
I have been following football since 88, and I have often heard that its easier to kick a long Field Goal at Denver, because of the thin air... I believe the regulatives says that a football has 13 psi, so if they check the ball-pressure at a higher altitude, and corrects it, it will still have 13 psi.. or at least the same relative difference to its outer surface .. Then there is the fact about lesser resistance in thin air, but since the pressure has been evened out compared to sea level/high altitude, the object should have the same relative weight..

ANSWER:
In an elementary physics class, we almost always do examples where one stipulation is "ignore air friction", the drag force on the object passing through the air. But, in the real world, air drag is seldom neglegible. This is particularly true in sports when balls often have relatively high speeds because, approximately, the drag force is proportional to v². When you say that the air is "thin" what you are saying is that it is less dense, that is there are fewer air molecules per cubic meter than there are at sea level. The air drag force also depends on the density of the fluid (how far do you think you would be able to kick the ball if the air were as dense as molasses?). Less retarding force on the football guarantees that it will go farther than a ball with the same initial velocity at sea level. This same effect is seen in baseball where more home runs are hit and curve balls do not break as much at higher altitudes.

QUESTION: 
If there is a magnetic bar with a strong magnetic field pointing from the floor to the ceiling. am I correct that if protons, neutrons and electrons are brought near it the protons will be least effected?

ANSWER:
You have to be careful to specify how the particles are moving. If they are at rest or moving in the direction of (or opposite) the field, then the neutron will experience zero Lorentz force because it has zero charge. Let's simply ask about putting each at rest in a uniform magnetic field. None will experience a force but all will experience a torque because each has a magnetic moment, that is, each looks like a tiny bar magnet. The electron has, by far, the biggest magnetic moment and will therefore experience the biggest torque; the proton has a magnetic moment larger than the neutron's and so it will experience a larger torque than the neutron. If you put them in a nonuniform field (which would be the case for the bar magnet you specify), each will feel a force in proportion to its magnetic moment, so again, the electron would feel the strongest force and the neutron the weakest.

QUESTION: 
what the direction of motion of electron around nucleus clock or anticlockwise?

ANSWER:
I guess that depends on whether you look from above or below! Once you choose a coordinate system relative to which you can make measurements, you might find it going in many possible directions. The direction of the angular momentum vector is a topic which is too involved to go into here.

QUESTION: 
I fly alots of transatlantic flights and a friend of mine in discussion have a disagreement. Here's the problem- If I am flying in a plane at 60,000 feet and he is flying in a plane at 20,000 feet and our airspeeds are 400 mph. Who covers more ground faster the higher aircraft or the lower aircraft? I contend the higher aircraft will have a higher ground speed than the lower aircraft even though they are both travelling at the same speed of 400 mph. I believe the further you are away from the earth the distance you travel in a straight line is lengthened by the curvature of a sphere. Whose right?

ANSWER:
I think it is a good idea to define "ground speed". Let's say it is the speed of the plane's shadow as measured by somebody on the ground. Then if you have a speed v and are a distance h above the earth's surface, the ground speed v_g can be shown to be v_g=v(R/(R+h)) where R is the radius of the earth. Alas, you are wrong! (By the way, I do not know what you mean "travel in a straight" line because if you fly at some altitude you are flying in a circular path.) To make this plausible, consider the following: two planes, each with speed 400 mph, one at altitude 1000 ft, the other at altitude 100,000 miles and both are keeping their altitudes constant; which will go all the way around the world first? Because the altitudes of your example are so small compared to the radius of the earth, there is practically no difference. I calculate the ground speeds to be 398.9 for the higher plane and 399.6 for the lower. [If you care, here is the proof. The ground speed and the plane both have the same angular velocity which is essentially the rate you are going around the circle, for example 1 revolution per month is the angular velocity of the moon around the earth, 1 revolution per day is the angular velocity of the earth on its axis, 1 revolution per year is the angular velocity of the earth around the sun. If angular velocity is measured in radians per second, then the angular velocity is v/r where r is the radius of the circle v is going. Then, v_g/R=v/(R+h).]

QUESTION: 
Please settle a bar room argument. 1. An ice cube is floating in a bowl of water: how does the water level change as the ice melts? 2. An ice cube containing the same amount of water as before but this time also containing an air bubble is floated: what happens to the water level as the ice melts and how does this level differ from the change before? 3. The same ice cube as in 1 but this time with a nail (or some heavy object) embedded in it is sunk; what happens to the level of water as the ice melts and how does this level differ from the previous 2 cases?

ANSWER:
Case #1 is a classic problem. Archimedes' principle states that the buoyant force equals the weight of the displaced water. Since the ice just turns into water, it exactly fills the void it originally created and the water level stays the same. Note that it should be clear that the density of ice is less than water since it floats. Case #3 is essentially to hold the ice totally submerged until it melts. But since the melted ice has a smaller volume than the unmelted ice, the water level will go down. Case #2 is essentially the same as case #1 because the buoyant force must still hold up the weight of the ice so the weight of the displaced water is still equal to the weight of the ice; so, when the ice melts it is water of that same volume. I have assumed that the weight of the air in the bubble is negligible.

QUESTION: 
although you can't see an atoms through a microscope, at some point a clump of atoms is large enough to see as a dot through the microscope. What determines when the clump of atoms is big enough to be seen?

QUESTION: 
How fast do you have to travel before you experience significant relativistic effects. For the purposes of this question, let's say 1 hour of travel at "X" velocity equals 1 hour plus 10 seconds.

QUESTION: 
Suppose that two water droplets detach from a liquid, is it possible that bigger droplet reaches to its spherical shape sooner than smaller droplet. Because as you know, after detaching, the droplets have elliptical shape because of drag force.

ANSWER:
Well, actually, water drops do not reach a spherical shape as they fall. Nor are they ever shaped like the traditional raindrop/teardrop shape. If the drop is very small, it will stay approximately spherical but if you now look at bigger drops, first it becomes oblate (doorknob shape) then oblate with a dent in the bottom and finally like a parachute. See an earlier answer for more detail.

QUESTION: 
I was doing some physics revision and completely off the topic I wondered if theoretically a pendulum could travel faster than light. although our given formula: T = √(l/g) does not mention anything about velocity, there seems to be no reason that it couldn't (theoretically), so i searched around and round a formula to find the velocity of the pendulum at the bottom of the swing: v = √(2gl(1-cos(a))) v is the velocity of the weight at the bottom of the swing g is the acceleration due to gravity L is the length of the wire a is the angle from the vertical were the pendulum is released cos(a) is the cosine of angle a assuming that there is no change in gravity as the pendulum comes down (its theoretical! im keeping it simple), that the length is not able to bend, friction is ignored... etc... a few simple calculations found that: √(2 × 9.8 × 4.6×10^15 (1 - cos(90))) = 300266548.3 m/s AHHHH!!! (even though this assumes we could have a pendulum with a length of 4.6 quadrillion meters... its theoretical alright!)

ANSWER:
Well, the formula for the period of a pendulum or for its velocity at the bottom of the swing are classical and simply incorrect for very large velocities. In addition, the equation for the period itself is not even exact, it is based on a small angle approximation. It is possible to get the speed at the bottom exactly (classically, that is) using energy conservation; but, again, this is not applicable to high speeds because as the speed of the pendulum approaches the speed of light, the mass increases. Finally, where do you propose getting a uniform gravitational field whose extent is quadrillions of meters?

QUESTION: 
I am trying to prove that the energy of a ball in its trajectory is conserved. I believe that a correct approach is to prove that when the derivative of E = K + V (potential energy) is taken, it will equal zero because E = constant. I know that I need to find h(t) and v(t) but I am having difficulty doing this. I seem to be coming for circle. Am I missing something? I feel that I can't use typical mechanics equations here because then I have to deal with components and angles. It's been a while since I've done physics so I feel a bit stumped.

QUESTION: 
A friend of mine and I were discussing crowds and applause. We could not quite figure out why a crowd of people clapping is louder than one and if indeed it even is louder. Assuming that everybody clapped at the exact same volume, would one thousand people clapping be louder than one person, even though they are producing the exact same sound at the exact same volume just from a thousand different sources?

ANSWER:
Sound, like most waves with not too large an amplitude, obey the principle of superposition; this principle states that if there are two different sources both radiating waves, then the sum of those waves at any point of space is what the total wave is. Clapping is a sort of special case because it is a series of short pulses, so if there are three people clapping randomly, you just hear more claps, usually not louder sound. But if all three clapped in unison and were equidistant from you, you would hear sound with three times the amplitude of a single clap. So, if your thousand clappers are not happening simultaneously, you will hear a more or less uniform sound at about the same volume as each clap is.

QUESTION: 
This is my understanding: They (Einstein I think) say that we cannot travel at the speed of light due to the fact that we will be traveling faster than time. Couldn't they have said the same thing about traveling faster than sound? I understand that the light we see from space is old. Like when watching someone hit a baseball from 1/4 mile away. You will see the bat hit the ball then you will hear it. BEcause sound travels slower than light. If I were to travel at twice the speed of sound going towards the ball being hit and then back to my current position would that alter time in any way? Then why do scientists think If I were to travel towards old light, faster than the speed of that light and then back again anything would happen to time? Is there a simple answer for my pea brain?

ANSWER:
Light is a perfectly unique wave. Unlike all other waves, it can propogate without any medium. A sound wave needs air, for example, a water wave needs water, earthquake waves need the ground, a guitar needs the string. I cannot derive all of special relativity for you here, but the result of the fact that light can move through a vacuum ends up completely shaking up our concepts of space and time. I do not think that saying you are "traveling faster than time" is fruitful, in fact it does not make much sense because time does not travel. Why you cannot travel the speed of light is that it takes an infinite amount of energy to accelerate something there and, of course, there is not an infinite amount of energy in the entire universe. See earlier answers in my FAQ page.

QUESTION: 
does the earths speed around the sun stay the same? Or stay constant during the entire earth year orbit? It seems that it speeds up during fall and spring.

ANSWER:
Kepler's second law says that a line drawn from the sun to a planet sweeps out equal areas in equal times. That means that the closer the earth is to the sun, the faster it goes. The figure to the right shows that the earth's orbit is slightly elliptical and it is closest to the sun in the winter (January 3) and farthest from the sun in the summer (July 4). It is not much of a difference, though, just about 3%. What puzzles me is how "[i]t seems that it speeds up during fall and spring". How would you feel that?

QUESTION: 
Would adding a greater amount of force acting against the force of a spring slow the movement of the spring? So say i have a compressed compression spring with weights in front of it (the weights get pushed as the spring expands), would adding more weights slow the rate at which the spring expands? I know this is kind of a boring question, it's for a sculpture i'm working on, and the only other way i can find out is to build the whole sculpture and spend hours and hours fooling around with weights and springs.

ANSWER:
The force which a spring exerts is F=kx where k is the spring constant (how stiff the spring is) and x is the amount of stretch or compression. If an object with mass m is attached to a spring, Newton's second law says F=ma=kx and so the acceleration of the mass is a=kx/m. So, the bigger m is the smaller a is which means that it will slow down how fast the compressed spring pushes out the masses.

QUESTION: 
I have a question of which I cannot find the answer on the internet. Hope you can help me. I'm writing a book and I need to know if it's yes or no possible for an 11 year old boy to push a car over (back) that is on it's side. i have no idea and it's not like I can try this at home. My instinct says no, but I want to make sure.

ANSWER:
It is certainly possible. It depends on the geometry of the car and how it is sitting on its side. All he has to do is push it far enough so that the center of gravity is beyond the furthest point of support; once that was achieved, gravity would do the rest. Imagine a 10 ton object sitting, carefully balanced, on a base of the diameter of an apple. It would be incredibly easy to push it over.

QUESTION: 
My question is about space and objects. If you were to lay a sheet of steel on top of another sheet of steel, assuming that they are both perfectly flat, is there a measurable space betwen the sheets. My intuition tells me that there is space there, which however small keeps the objecst seperate, but I am not sure.

ANSWER:
To measure this space measure the thickness of the two sheets and show that it is larger than twice the thickness of one. However, this is likely to be a difficult measurement and could maybe be achieved by stacking many sheets. What is between the sheets is air, a thin film of it. I have been told that if you press two smooth sheets of metal together in a vacuum, they will weld together.

QUESTION: 
let us say we have a string passing through a circular hole on a frictionless table with on end of the string(that lying on the tabe) attached to a body which is rotating with certain constant angular velocity initially and the other end (that hanging through the hole) is being pulled with a constant velocity.now since torque on the body is zero its angular velocity will start increasing as it slids towards the hole on account of conservation of angular momentum.my question is what is the internal torque (analogous to internal forces in conservation of linear momentum) responsible for the increase in the angular velocity of the body?

QUESTION: 
What spring constant would be needed for a tension spring to give us a 18000 J of energy?

QUESTION: 
I was playing with my son's wooden train set and it got me to thinking. Each train car has a magnet on each end used to hook up the train cars to each other. Turn the cars one way and they hook up, turn them the other way and they repel. Now my question is, does it take the same amount of energy to push the train around the track whether the cars are repelling or attracting or does it take less with the cars in the repel position with the magnets doing some of the work, like winding up a spring and letting it go?

ANSWER:
You may be sure that if you (or the little spring motor) does a certain amount of work, the same amount of energy is used either way (pulling coupled or pushing decoupled train). The magnets do not do any work because if the magnet of car #1 exerts a force on car #2, the magnet of car #2 exerts an equal and opposite force on car #1; hence the net work done by that pair of forces is zero. It is also true, incidentally, that a magnetic field never does work, but maybe that is a bit too technical here.

QUESTION: 
why do people say to shut down electrical appliances during lightning?l?

ANSWER:
Due to lightning strikes, the voltage can sometimes surge, become much bigger than your appliance is designed to operate with. Turning it off is one level of protection for your stuff because the power switch opens the line between the outside and the appliance. However, it may not be enough and to be really safe you should unplug it. Another option is to use "surge protectors" which are designed to not let the surge pass. Also, remember that a surge could also come in your telephone line and whatever the line goes to (fax, telephones, computers) may be damaged.

QUESTION: 
You and a child half your height lean out over the edge of a pool at the same angle. If you both let go simultaneously, who will tip over faster and hit the water first? Or will they hit at the same time?

QUESTION: 
I recently performed the "Determining the Densities of Pennies" lab. In this lab, I know the answers to my percent error calculations should NOT be 0%, but what I don't know is WHY. Can you help? Your time is greatly appreciated.

QUESTION: 
A mass amount of an object is traveling down on to a conveyor belt and then in to a basket. Maintaining the same amount of mass on to the belt, and then you slowed the belt down, would the basket fill up slower or stay the same?Also vise versa if belt speeds up would the basket fill faster?

ANSWER:
It is not clear what "maintaining the same amount of mass on to the belt" means. If you mean you do not change the rate at which mass is added to the belt when you change the speed of the belt, then the answers to your questions are same and no. If you mean that you adjust the rate at which mass is added to keep the total mass on the belt constant, then the answers are slower and yes.

QUESTION: 
If the distance between the south poles of two long bar magnets is reduced to half its original value, will the force between these poles be doubled?

ANSWER:
If there were such a thing as magnetic monopoles, they would behave like electric charges, the force would be inversely proportional to the square of the distance between them. When you say "long bar magnets" I presume you are saying that the other poles are so far away to have negligible effect. In that case, halving the distance would increase the force by four times.

QUESTION: 
suppose an astraunaut landed on a planet where the acceleration due to gravity is 19.6 meters per second square,compared to earth,would it be easier,harder,or just as easy for her to walk around? would it be easier,harder,or just as easy for her to catch a ball that is moving horizontally at 12 meters per second square? assume that the astraunaut stay suit is a lightweight model that does'nt impair her movements in any way.

ANSWER:
Essentially, everything would weigh twice as much as you are used to. So it would be difficult to stand, let alone walk. If the ball were truly going horizontally, it would be just as easy to catch as on earth because it still has the same mass even though it has greater weight. If it were falling, though (high fly ball) it would be like catching something twice as heavy.

QUESTION: 
Regarding gravity and light outside of a vacuum: Assertion 1) Object A pulls on object B, as object B pulls on object A. Assertion 2) Gravity from object A can alter the direction of travel of a photon (object B). So the photon (object B) should then in turn alter the travel of object A. Given enough light, could you actually move object A? If we adjust the scales of A and B to be ideal/non-realistic, the question can be reduced to "Would the light from a (very bright) flashlight pull (or perhaps it would also be pulled back from the opposite side as the light passed it) a (very light/non-massive) balloon?"

ANSWER:
I am not an expert in the fine points of general relativity. However, it is my impression that, although it is usually mass which everyone thinks of as "warping space-time" and hence creating gravity, that it is more correct to say the presence of an energy density also does it. Since mass contains so much energy (E=mc²), that is what is, by far, the most important source of gravity. However, it is not incorrect, in this context, to say that a very intense beam of photons would also be a source of gravity, but it would probably be totally impossible to measure it because it would be so weak.

QUESTION: 
How many rpm do i need to spin a rope 30 feet long with a 6inch piece of chain on end weighing 1lb. to keep rope straight out?

ANSWER:
Infinite! It cannot be done. You could get it nearly "straight out" but never all the way.

QUESTION: 
When driving a vehicle that has a camera equipped inside it and you are going around a curve in the road will the camera follow you in the curve or go straight making the line in the road appear as if you crossed it when in fact you didn't?

ANSWER:
A camera and your eye work just the same way: the image of what is aimed at is focused onto the film (retina) for the camera (eye). The camera is just a mechanical eye. Whatever your eye sees will be the same as what the camera sees.

QUESTION: 
We have ten fingers and so we made a base 10 number system. We have based everything from maths to physics to computers on this assumption. It seems a bit convenient. If I ask a calculator to calculate 1 divide zero it cannot cope. How therefore can we be confident that results from theories such as 'big bang' or theoretical physics are anything other than arbitrary?

ANSWER:
First of all, computers usually use other base number systems, binary, octal, or hexadecimal. But the bottom line is that it makes no difference what number system you use, any given theory would be the same. The equations and such would look different but mean the same thing. What a theory means is independent of the base of the numbers. The fact that you cannot divide 1 by 0 has nothing to do with the fact that you are working in a 10 base system, 1/0 is the same in any number system, undefined (which is why, of course, your calculator cannot "cope"). You can put 2 into 6 just 3 times, but if you start putting nothing into 1, you will continue putting it in forever and never get there. Finally, lots of things like the big bang are not the result of a theory but the result of experimental measurements. There is lots of evidence that the universe originated about 14 billion years ago in a huge bang and the evidence has nothing to do with theories.

QUESTION: 
If 225 pounds falls 24 inches straight down to the ground, propelled only by the force of gravity, what is the force of impact?

ALMOST SIMULTANEOUS QUESTION: 
what is the striking force of an object that weighs 300lb falling from a height of 3 feet

ANSWER:
My God, will I never stop getting this question? It depends entirely on the details of the collision. In essence, what you need to know is how long did it take for the weight to stop? If you had checked the FAQ page you would have found an answer. There is no such thing as "striking force".

QUESTION: 
If you take coherent light of a fixed frequency and add more coherent light of the same frequency, but out of phase by 180 degrees then can you reduce the intensity due to canceling out the waves?

ANSWER:
Yes, you can change intensity at points in space by causing destructive interference. But you cannot simply reduce the intensity everywhere, only at places where the two waves interfere destructively. There will inevitably be other places where they interfere constructively depending on path differences. There is a very lengthy discussion of a similar question in an earlier answer.

QUESTION: 
I am creating an educational physics poster for a class assignment. I am representing terminal velocity of two ~10lb objects (let's call them spheres), one dropped from 100 feet, one from 50 feet. The sphere dropped from 100 feet hits terminal velocity and the one from 50 feet doesn't. I need to find out how long it takes the sphere to hit terminal velocity (i.e. the average human takes 12 secs), and where in the 100 feet it hits it (half way down, 3/4 way down, etc). There is one slightly confusing matter in that during this example the words "feet" and "stories" was interchanged, so if it turns out to be impossible for a 10lb sphere to hit terminal velocity in only 100ft, change it to 100 stories (1000 ft) instead!

QUESTION: 
I'm confused. If there's a ball at rest on the ground (floor), does that mean it has zero gravitational potential energy? When we say "zero", do we mean that it there is none or it just refers to the "position"? In relation to the center of the earth, does the ball have gravitational potential energy?

ANSWER:
Your last sentence indicates that you know what the answer. Potential energy (PE) has no meaning, it is the change in PE which matters; you have to specify where you choose PE to be zero. If you have chosen the floor as your zero of potential energy, the ball has zero potential energy. If you have chosen the basement floor for zero PE, and it is 10 m below the ball, then the ball has PE=mgy=10mg. If you have chosen the upstairs floor for zero PE, and it is 10 m above the ball, then the ball has PE=mgy=-10mg. (PE=mgy implies that up is the +y direction.)

QUESTION: 
If the faster you go the more mass you achieve, the why dosnt light achieve mass while traveling at..... light speed.

ANSWER:
See a recent answer.

QUESTION: 
an electron moving perpendicular to a magnetic field experiences lorentz force , here there is action but where's the "equal and opposite" reaction force . Is the newton's third law of motion being violated here.

ANSWER:
The field is being produced by a current somewhere and the "reaction force" will be on that wire. However, that force is not necessarily equal and opposite to the force on the electron. Newton's third law, as taught in elementary physics, is not really absolutely true and magnetic forces, since they are velocity dependent, are the best example to show this. The figure shows two positive charges moving with velocities shown by black vectors. The electrostatic forces are red and are equal and opposite. The magnetic force felt by the particle moving horizontally is zero because the field there (due to the other charge) is zero. The field due to the charge moving horizonatlly points up out of the screen, so the magnetic force (blue) on the other charge is to the right. The net force on each particle is shown in green and they are neither equal nor opposite. Newtonian mechanics takes on new forms in electromagnetism and everything still all hangs together, but you need to include the energy and momentum densities of the fields. This is way beyond the scope of this site to address.

QUESTION: 
Is it possible to hover above the ground and then "travel" to other places by letting the world revolve beneath you?

ANSWER:
See earlier answer.

QUESTION: 
If work is defined as force x distance then if you are pushing against an immovable object, no work is being done. Nevertheless, you are expending energy as you are pushing. It seems to me that there is some kind of paradox here--you are losing energy but you are not doing any work. Can you explain?

ANSWER:
Physics would say no work is being done because the force is not acting over a distance; you and I both know, however, that sugar is being burned to provide the energy necessary to push on this stationary object. What is going on here, as I understand it, is that the muscle fibers in your arm are continually slipping and retensioning thereby doing lots of little parcels of work to hold your arm steady.

QUESTION: 
I have several questions involving the law that energy cannot be lost or created, only converted. Say a person was cranking an electric generator. it made electricity and lit a lightbulb. Now, what if the generator wasn't a generator, but simply a container with an equally-hard-to-push crank. The person would crank it, outputting just as much energy, and creating movement but no electricity. This makes the assumption that the man is not making as much energy. ???

ANSWER:
Site groundrules stipulate "single, concise, well-focused questions" so I will only answer your first. If a person does a certain amount of work cranking the generator, that energy will show up somewhere. If your "container with an equally-hard-to-push crank" is the recipient of the work, that container will have some built-in friction to make it equally hard to crank. When work is done against friction, the result is mostly heat. Your container will get hot.

QUESTION: 
How can we say that the Earth rotates the Sun? Which one rotates in which the two or more other planets? Which point is defined as the static point in the space to say that what rotates which? Can there be the static point in the space without first thinking the Sun as a static point? To say that an object is rotating or moving, there must be the static point first beside that as a criteria. So, What is the axis or the criteria in the space to describe an object is moving without regarding the Sun as a static point first?

ANSWER:
The sun is not static. The earth does not revolve around the sun. Rather the sun and the earth each revolve around the center of mass of the earth-mass system. However, the sun is so much more massive than the earth that the center of mass is close to the center of the sun and therefore you get excellent numerical results if you just assume the sun does not move.

QUESTION: 
If we transfer compressed air from a one-hour bottle rated at 4500 psi into a 30-min empty bottle also rated at 4500 psi will the pressure of both bottles equalize at 2250 psi each? And will there theoretically be 30-min of breathing time left in both tanks?

ANSWER:
I presume that the volume of the 30 min bottle is half that of the 60 min bottle. Also, 4500 psi is, most likely guage pressure, the pressure above atmospheric, the "empty" bottle is not really empty but has atmospheric pressure in it. But, atmospheric pressure is only about 15 psi, so let's just ignore that. What is operative here is Boyle's law, PV=constant, so P₁V₁=P₂V₂ where P₁=4500, V₂=1.5V₁, P₂ is the final pressure. That is, you are increasing the volume of the original gas by a factor of 1.5. So, P₂=3000 psi. I do not understand what you mean that both will have the same breathing time left. Seems to me that, for each, 4500 psi gives the full time, so each would have 2/3 of their maximum times, 40 min and 20 min.

QUESTION: 
This question was mentioned to me a long time ago by a physicist who I think was just trying to mess with my head - a task at which he succeeded. One of the simplest ways of measuring the circumference of a circle that many children are taught is rolling out the circle; and this is basically the starting point of my question. To make my question clear, I feel I have to describe the process : Take a wheel or circle; draw a radius on it from the center to the edge; position the circle on a flat surface or number line so that the radius points straight down at a known point; roll the circle along the line until the radius is again pointing straight down at a second point and mark it; the distance between the two points is the circumference of the circle. Now draw another circle on the first that is half the size of the original. If you repeat the original procedure of measuring the circumference of the larger circle, the smaller circle also starts and ends at the same position despite travelling a larger distance than it's own circumference? Why can't I get my head around this? I hope this doesn't count as a maths question - it was originally put to me by a physicist...

ANSWER:
The small circle is not rolling on the dashed line, it is slipping on it. At any given time the whole circle is rotating around the point of contact of the big circle with the ground, that is, the point of contact is at rest (that is what rolling without slipping means). Therefore, a point a distance s above the point of contact is moving forward with a speed vs/R where v is the forward speed of the center, and R is the radius of the big circle. The bottom of the small circle is going forward with speed v/2 and the dashed line is at rest.

QUESTION: 
Does light, a photon, have mass, any mass? If so, and a photon by definition travels at the speed of light, why doesn't it have infinite mass? If not, how is it subject to gravitation?

QUESTION: 
I'm puzzled by an Einstein thought experiment. The idea seems to be that gravity is the same as acceleration. If you are in a closed box you can't tell if you are resting on Earth or on a rocket ship being accelerated at 1G. Obvious nonsense; gravity is curved and acceleration is not. I can think of at least three ways to tell the difference - in gravity:
1 - weights hung from different sides of the box would not hang parallel.
2- gravity would be slightly (ok, very slightly) stronger at the bottom of the box.
3- time would proceed slower at the bottom of the box. None of these effects are true for accleration. Is this 'thought experiment' just meant to be a crude approximation, and am I just picking nits? It seems... unsatisfactory. Oh, and an interesting side effect - if you apply a certain acceleration to a spaceship in interplanetary space and then applied the same accleration deep in a gravity field, say close to the sun, you'd get a greater increase in velocity from option two - ehh?

QUESTION: 
Why was work done defined as Force x Displacement? Is there a sound reason or was it simply derived from the work energy theorem?

QUESTION: 
Could you help me understand the concept that you can't measure a particle's speed and position at the same instant. It seems to me that this is a technical problem. We know that they have a speed and position at any instant so they are knowable, thought perhaps never to us, and therefore their future is determinable.

FOLLOWUP QUESTION: 
I would like to have another go at my previous question about the uncertainty principle. I'm just an old farmer, long in the tooth and have just gotten interested in this physics stuff in the last ten or so years so let me present a thought experiment. Say I shot a cue ball at a racked set of pool balls, once that ball is rolling there is a definite route each ball will take and the outcome will not be altered whether I know the forces involved or not. I can't predict the outcome since I can't possibly measure the exact forces but that doesn't interfere with the outcome. Now I understand that by trying to measure particles, no matter how close I measure them, over the course of the life of a universe there will be drastic changes in any outcome I would predict. However, at the time of measuring, isn't there already a prescribed path set in concrete based on Newtonian physics and if it were possible to know the forces involved and we had the technical ability could we not define that path. I realize there are probably other factors that would make that impossible such as particles popping into and out of existence but I don't understand why not knowing the data makes the outcome any different. Isn't that why Einstein commented that god doesn't play with dice?

QUESTION: 
if i drop a paper clip and a 500 pound weight out of a plane at the same time which one will hit the ground first?

QUESTION: 
I am a flight attendant on a private jet. Flying at 43,000 ft we passed over a lage cloud formation toward another. All of a sudden the plane dropped about 1000 ft before contacting 'pressure' again. I was standing in the cabin - there were no passengers on this flight. Pilots were in seatbelts and said there was absolutely no indication of change of pressure and no up or down drafts. I did not hit the ceiling but i hit the floor like a bag of rocks. What speed was i falling and what were the g's on my body (5'5" 170 lbs)? Several people have asked and i would really like to know.

ANSWER:
As is often the case, I do not have enough information to calculate what you want. If the plane all of a sudden dropped in freefall, it would take about 8 s to go 1000 ft down; sound about right? If so, when you were 1000 ft down you would be going about 250 ft/s=170 mph in the downward direction (as well as about 500 mph forward). Now, if you hit the ground going 170 mph, you would certainly die. However, the plane did not suddenly stop falling, it happened over some period of time. So, if it happened over 2 s, for example, and you, having hit the floor, will also stop in 2 s. The force you would experience is your mass 170/32=5.3 times your acceleration 250/2=125 ft/s/s; so the average force you would have felt over that 2 s would be about 664 lb which would be about 4g. But all these are numbers I basically made up, but I think they are the best you can do given the information available.

QUESTION: 
My son has a question that I can't seem to find an answer to researching on the web. WHAT FORCE IS BEHIND A GOLF BALL WHEN HIT ON THE MOON? I appreciate your time on answering this for my son. I think it has something to do with Velocity, but not sure!

ANSWER:
I think there is some confusion about what force is here. Here are all the forces a golf ball experiences here on earth:

1. The club, traveling with some speed, hits the ball and exerts a contact force on it for a very short time but it is a very big force and it results in the ball acquiring a very large velocity. As soon as the ball leaves the club, there is no force "keeping it moving". If there were no other forces, it would keep going forever with the speed with which it left the club.

2. Once it is started, gravity pulls down on it which is the force which eventually does bring the ball back to the ground.

3. As it flies through the air, it experiences air drag which can be very complicated. Essentially, it is a force trying to slow it down and the bigger the speed is the bigger this force is.

4. If it happens to be spinning, the air drag can act asymmetrically so that the ball curves. This is what is called a hook or a slice in golf (depending whether it curves left or right, respectively, for a right-handed golfer).

5. Of course, when it hits the ground, it experiences forces from the ground which ultimately bring it to rest.

What is different on the moon?

1. If the club is the same club with the same speed, there is no difference for this force. Therefore, the ball launches just the same as on earth.

2. The moon is much smaller than the earth and the result is that the gravity on the moon is much weaker. Therefore, this force (trying to pull the ball back down) is much smaller and the ball will go a lot farther.

3. Since there is no air on the moon, there is no drag and this also results in the ball going much farther.

4. The ball will not curve on the moon, regardless of how much spin it has.

5. When it hits the ground, things are about the same as on earth except that all the forces are smaller, again because of gravity being smaller, so the ball rolls farther before it stops (also because it is going much faster when it hits the ground than it would have been on earth).

QUESTION: 
In a projectile motion, which has a farther range (neglecting air resistance, or in a vacuum) fired from a catapult with constant force and same angle of flight, a paper(crumpled) or a rock? Knowing that the paper's mass is less than the rock, I thought that the paper would have a farther range since mass is inversely proportional to acceleration; but my teacher says otherwise, that they would have the same range. Who is correct?

QUESTION: 
Heat recovery ventilators are becoming very popular as homeowners weatherize their homes. They are simple units that intake air from outside, exhaust stale air from inside. Heat is transferred from the exhaust air to the intake air through a plenum of aluminum. All very simple so far. Except, they work too well! They can take 0 deg F outside air, 70 degree exhaust air and instead of warming the intake air to 35 deg, like most of us would expect, they warm it to 50 degrees! How do they do this?! Does the answer lie in the fact that 70 deg F contains more than twice the energy of 35 deg air? (NB: No heat pumps, freon, electrical resistance, etc. are involved)

QUESTION: 
Does UV-C light delivered through fiber optics diminish the UV-C sanitising properties?

ANSWER:
I do not know about the "sanitising properties"  of UV-C. What I know is that a photon (of a particular frequency) is a photon. It does everything popping out the end of a glass fiber that it could before it entered.

QUESTION: 
Can A small cluster of conducting metal atoms on the nano size be given a positive electric charge using a laser (Preferably of modest power).

ANSWER:
It depends on how much the nano particle mimics the bulk metal. You are basically are dealing with the photoelectric effect, so the laser photons have to have enough energy to knock out an electron. Usually, ultraviolet would be the best bet.

QUESTION: 
How much energy is required to transmue a metal by removing a proton from the nucleus of the atom. For example remove one proton from mercury do I end up with gold?

ANSWER:
The exact amount depends on the particular nucleus you want to remove a proton from. An average over many nuclei is about 8 MeV (million electron volts) which is about 10^-12 J. If this is a get-rich-quick scheme, forget it!

QUESTION: 
I remember learning that momentum equals mass times velocity but this must not be strictly true since light is massless and has momentum. Why does light have momentum?

QUESTION: 
I am a member of a high school debate team in Vancouver Washington, and I am currently writing a policy debate case on Nuclear Fusion. I was hoping you could give me some advice on the likelihood of scientist's ability to produce a nuclear fusion reactor in the near future. Any other opinions on the overall convenience in nuclear fusion and it's energy production potential would be greatly appreciated. Thank you!

ANSWER:
I would not want to be arguing in a debate that controlled nuclear fusion is imminent. When I was a college student nearly 50 years ago, word had it that fusion reactors would be operating within 10-15 years. It is a very hard problem because you need extremely high temperatures for fusion to occur because the electric repulsion between the hydrogen ions keeps them too far apart for them to interact if they are moving around at normal room temperatures. It is extremely difficult to design magnets which are capable of containing a hot plasmas (essentially a 'soup' of electrons and positive ions) for longer than milleseconds; nevertheless, efforts are still underway to build such machines (tokomaks) at places like the Princeton Plasma Lab. An alternative approach is to make tiny pellets containing hydrogen and implode them with powerful lasers coming from all directions which is sort of like a tiny H-bomb. I may be wrong, but I do not believe that anybody has yet achieved "break even" which means you get as much energy out of the fusion as you put in to cause or maintain it.

QUESTION: 
If I drop a steel ball in a gaint never ending vacuum will the speed increase forever or level off at some point?

ANSWER:
I guess you want this "never ending vacuum" to also be a never ending uniform gravitational field? In spite of the fact there are no such things, it would continue accelerating for a while and then, when its speed became comparable to the speed of light, it would start accellerating less and less as it approached the speed of light which is the "speed limit of the universe." For details, see my earlier answer on this problem.

QUESTION: 
If the matter from a collapsed star can create a black hole (tear in space or however you define it) then how is it that after the big bang, say when ALL THE MATTER in the universe had expanded to no bigger than a baseball and gravity took effect, an ultra massive black hole wasn't created stopping the expansion of the universe dead in it's tracks.

ANSWER:
First, read my recent answer discussing the ultimate fate of the universe. Then you will see that, depending on how mass there is in the universe, it will either expand forever or turn around and collapse. If it is the latter, then everything, as you suggest, will coalesce into one giant black hole, sometimes referred to as the big crunch.

QUESTION: 
Why do magnets loose their magnetic properties on heating ?

ANSWER:
Ferromagnetism results from electron spins (each electron is, itself, a tiny magnet) all aligning themselves in the same direction; this results from quantum mechanics and happens in relatively few materials. However, when the magnet is heated the atoms all start vibrating more violently until a temperature is reached where the alignment is shaken apart. This temperature is called the Curie temperature.

QUESTION: 
Given the right technology, could the vacuum of space, be used to lift material into space?

QUESTION: 
Is it the case that the isotopes of an element are found together in the same ratio (in natural deposits) because some process prevents their separation?It seems to me that some process must keep them together.

ANSWER:
You ask why are they together? I might ask why not? All the isotopes of an element have exactly the same chemical properties and it is usually chemistry which causes things to move around. Or, the masses could play a role in the diffusion in a gas but the masses are so close that there would be no mechanism to cause any significant separation. There is simply no natural process which will cause isotopes to separate. I should point out that one of the major problems which had to be overcome in making the atomic bomb during WWII was separating the fissile isotope of uranium, ²³⁵U, from the more abundant ²³⁸U. I can think of one process by which a local abundance ratio might be different than other places. When spontaneous fission happens we are left with fission fragments which have a distribution of masses but tend to be around half the mass of the fissioning nucleus, and that would favor particular isotopes which the more common creation (in stars and supernovae) would not. Oh, I can think of another example: cosmic rays can interact with the atmosphere or surface of the earth and create isotopes there whereas they would not create them deep in a mine, for example. Carbon dating relies on such a process; if you look at the abundance of ¹⁴C in a leaf of a tree you will find a certain percentage. If you look at carbon in wood several thousand years old, you will find significantly less ¹⁴C because it is radioactive and decays away (half life about 5700 years); it is created by cosmic rays. So, if you look at carbon which is old or from deep in the earth, its isotopic abundances will differ from "new" carbon.

QUESTION: 
Does friction have fundamental roots or is it purely empirical? What I mean is like how gravity and electrical forces can be calculated at the most basic level as forces between point masses or charges. Do you know if there have been experiments that say tested a single hydrogen atom or some other particle moving along surfaces. Also is friction then really a fundamental force or is it merely a different version of the electrical forces of atoms?

QUESTION: 
If Helium-II has a viscosity of 0, does that mean you could accelerate it without friction? If so could you use an accelerator like the LHC (but smaller) to accelerate Helium-II instead of a particle beam? If so could it reach near light speed? If so what would happen?

ANSWER:
Superfluidity is a collective phenomenon, it depends on there being many atoms interacting with their neighbors. In an accelerator it is not friction which limits maximum speed, it is simply the design of the machine; friction plays no role for individual particles. Anyway, to accelerate it as you suggest, you would need individual helium atoms which would be singly or doubly ionized and it would therefore not be superfluid helium.

QUESTION: 
I understand that matter gains in mass as you accelerate it towards the speed of light, and that as you approach the sped of light, mass tends toward infinity, meaning you would need an infinite force to "push" it beyond the speed of light, and that's one of the reasons why nothing can exceed the speed of light. However, I also understand that with increased mass comes increased gravity, so... When they accelerate those sub-atomic particles to velocities approaching the speed of light at CERN, why don't they suddenly start exerting lots of gravity and sucking the entire CERN campus toward them?

QUESTION: 
I know water boils in a vacuum, but the question I can't seem to find an answer to is, does the temperature of the water remain constant? If I'm in a vacuum with a bowl of water, will the boiling water scald my hand or will it just be room temperature water in a boiling state?

ANSWER:
If you do not let any heat flow in or out and keep the volume constant, as the pressure is decreased the temperature will remain constant. You can see from the water phase diagram that the pressure will eventually reach the boundary between liquid and vapor at that temperature and boiling will commence.

QUESTION: 
What pattern does monochromatic light form when the double-slit experiment is performed in a vacuum? If it forms the same interference pattern that is seen in double-slit experiments that are performed in the presence of air, does that mean that a vacuum can propagate a wave?

QUESTION: 
I'm a 16 year old and i would appreciate if you help me answer a question. I was taught that the colour black absorbs all the colours of light. However, i find it puzzling to see smooth, black surfaces reflecting light. Did i have a wrong concept or not understand something?

ANSWER:
Well, this should tell you that all the world is not just black and white unlike some physics texts would have you believe. There is no such thing as an object which actually absorbs all radiation which falls on it. So, even if an object is not smooth, it will not reflect all the light on it. When you look at a rough black object, you can still see features on its surface, right? So some light must be reflected. If it is real smooth, it can work like a mirror, even though it looks black "behind" the mirror, it still reflects from the surface.

QUESTION: 
Two bikes are traveling at the same speed of 10 mph towards each other. Starting at a distance of 20 miles away. A bee is flying from front tire to front tire of the two bikes at a speed of 25 mph. After several trips back and forth the bikes collide and the crush the bee. What was the total mileage of the bee in the many trips back and forth before the crush

ANSWER:
I have answered similar questions before. This is probably your homework problem, not really allowed. Look at my earlier answer and figure it out for yourself.

QUESTION: 
I was playing paintball one day, and was shooting in a "range" for target practice while i as waiting for the next game. At the time i was reading a book called "The Big Bang" (I don't remember who it's by...). I had just read a part about space-time. Anyways, i was aiming for a particularly small target, and i just couldnt hit it. I would shoot at the target and it looked like it was going to hit it, but at the last second it would veer off to the left or right. I would adjust accordingly and the same thing would happen, but it would go to the toher side. I don't know why it was doing this. So my question is: Does space-time have any affect on small projectiles? I apologize if this is a stupid question, but im not particularly exceptional at physics...

QUESTION: 
Could the force of gravity be the direct, measurable result of the universe expanding? I really hope you get back to me about this, i have this idea in my head I just cant shake.

ANSWER:
I do not see how!

FOLLOWUP QUESTION: 
everything has an equal and opposite reaction. What is the opposite of the universe expanding? because matter and atoms etc are the only objects in the universe, they collectively counter the expansion. Maybe not equally (as some of the energy is expended in other forms). Am I just stupid? i have not done physics since high school lol.

REPLY:
"everything has an equal and opposite reaction"? This has to be one of the most misused, misunderstood, incorrect extrapolations in all of physics! Where does it come from? Newton's third law states that if one object exerts a force on another, the other exerts an equal and opposite force on the first. Some jerk (sorry, I get a little "het up" about this!) thought that a good paraphrase of this would be "for every action there is an equal and opposite reaction". It just sounds so cool and profound, it must apply to everything you might want to apply it to. And my years of of teaching have taught me that it is a neverending source of confusion for students. I often get questions like "If I push on something, why does it move? Why does the reaction force not cancel out my force?" Well, duh, because the reaction force is not on the same thing I am pushing on, it is a force on me. "What is the opposite of the universe expanding?" Well, I guess it must be the universe contracting? But, if it is expanding, it certainly can't be contracting, can it? Then where the heck is that "opposite reaction"?

I usually don't answer astrophysics/astronomy questions, but let's talk about the expanding universe a bit. So, the big bang happens. This imparts energy to everything which has come into existance and it is all flying apart right from the start. So here is the question which has been a longstanding holy grail for astronomers, is what is the ultimate fate of our universe? Since we observe the universe expanding, is it must be doing one of two things: since there is only gravity acting on everything in the universe, the gravity is trying to slow things down, either

1. if there is enough mass in the universe, eventually it will turn around a collapse back into a "big crunch" or

2. if there is not enough mass in the universe, it will keep expanding forever and, since everything is getting farther apart, the gravitational force will just get smaller and smaller until everything is just about drifting with a constant speed.

But, wait!

3. Several years ago an amazing discovery was made. Measurements on the most distant objects in the universe have shown that these objects are actually speeding up. So, there is some force other than gravity, it seems, which must be repulsive instead of attractive like gravity. This is what is often refered to as dark energy.

Anyhow, this is a longwinded answer but here is the bottom line: keep action/reaction to Newton's third law, not just anything which comes to mind!

QUESTION: 
If a train is moving with the speed of 180 km/h and a mosquito is flying inside the train with speed 10km/h. Will it collide with the train if yes! why yes ,if no! why no?

QUESTION: 
If one has two vectors equal in magnitude and direction but with different units, then can the two vectors be considered equal to each other? For example, a displacement vector of 1m towards east and a velocity vector of 1m/s towards east.

ANSWER:
No. You cannot write x=v because it would not be dimensionally correct. Does one watermelon equal one potato? However, it is not really "different units" you want to ask about because, if A=1 foot north and B=12 inches north, you may write A=B; these vectors have the same dimensions (length) but different units.

QUESTION: 
is the force applied by a vertically faalling body equal to its weight?

ANSWER:
If it is falling, it is not exerting a force on anything (except a little force on the air it is falling through). If you mean how much force does it exert on something it hits which stops it, there is no way to calculate that without knowing its speed and how long the collision lasted. First, see a recent discussion of Newton's second law. What determines the force is the mass of the object, its speed when it hits, and how long it takes to stop. If it is 1 kg moving with a particular speed, if it is a brick it stops quickly and hurts a lot (big force), if it is a piece of foam rubber it takes a longer time to stop and does not hurt (small force). But both were 1 kg moving with the same speed. If you drop them from higher up, the force is bigger because the speed at impact is bigger.

QUESTION: 
If water was preserved in a very solid metal container where air was completely extracted from inside the container and u put it to freeze, how wud the water expand when there is absolutely no way??

ANSWER:
See an earlier answer.

QUESTION: 
Is it ethical for cellphone companies to sell cell phones without a written health warning to the user?

ANSWER:
I personally feel that there is zero good evidence that cell phones are hazardous to your health. See my recent answer on this topic; if you are serious about forming an opinion, be sure to read the linked-to New York Times article. Given the current status of the evidence, if cell phones must have a warning, then so must coffee and pickles, both of which have been listed as "possible carcinogens" by the World Health Organization.

QUESTION: 
what is the difference in properties between dark matter, dark energy, and antimatter? How does this relate to the universal law of "self preservation"?

ANSWER:
Dark matter and dark energy are related to puzzling observations in modern astrophysics and have been discussed in an earlier answer. Antimatter is a very different thing (and unlike dark matter and energy, has been actually observed directly). Symmetries in nature make necessary in nature particles which are the antiparticles of corresponding elementary particles. The one commonly observed is the antiparticle of the electron, called the positron, which occurs naturally when a nucleus with too many protons to be stable decays by changing a proton into a positron, a neutron, and a neutrino. However, positrons are soon gone since if they encounter their nemisis an electron, the two totally annihilate each other and result in electromagnetic energy corresponding to all the mass energy the two had before annihilation. I have no idea "what the universal law of 'self preservation'" is, but it certainly is not physics!

QUESTION: 
why do paratroopers roll on landing

QUESTION: 
I hear all the wizards say that our H20 arrived on earth as a result of Comets and Meteors. I have trouble accepting that as the answer, because of the abundance of H20 on our planet? How many Comets and meteors would be required? As a common-sense thought: Something else seems to me, more a more practical and possible solution. We are told that the early composition of Earth's atmosphere was mostly Hydrogen. Plus, it was violently hot, with an abundance of volcanic and lightning activity. Fire + Hydrogen = BOOM! The by-product of burning Hydrogen = H20. Why wouldn't this be a better explanation for our abundance of H2O? I listen and read constantly. I've never heard a theoretical physicist suggest this as a plausible explanation. So, I automatically assume I can't possibly be correct. Then; I wonder . . . why not? So . . . that's my question. Why not?

QUESTION: 
If all inertial frames of reference are equivalent, and there is no way to identify which of two objects which are becoming further apart is moving, how is it possible to determine which of two objects is accelerating? For example, how do we know that a train is accelerating away from a platform, and not vice-versa, if it is impossible to determine which is moving when the train is traveling at a constant velocity? If all inertial frames of reference are equivalent, and there is no way to identify which of two objects which are becoming further apart is moving, how is it possible to determine which of two objects is accelerating? For example, how do we know that a train is accelerating away from a platform, and not vice-versa, if it is impossible to determine which is moving when the train is traveling at a constant velocity?

ANSWER:
One way you can identify an inertial frame is if Newton's first law is true (an object at rest or moving with constant velocity has zero net force on it). For example, sitting at the train station there is a pendulum bob hanging straight down; the earth is pulling straight down with a force (called the weight) and the string is pulling straight up with a force which may be measured to have the same magnitude as the weight. Now, if somebody is accelerating in a train going by and also has a pendulum, it will not hang straight down but at an angle. But there are still only two forces on that pendulum bob, the weight straight down and the string not straight up. So, if you are in the train's frame of reference, you conclude that Newton's first law is not true.

QUESTION: 
What is the function of neutron in the nucleus of the atom ?

QUESTION: 
Why don't iron filings simply align themself due to the magnetic field , why is it necessary to tap the board on which thay lie?

ANSWER:
The filings have friction between them and the surface they sit on. The force from the field is not big enough to move them. If you tap on the surface, they tend to "jump up" either eliminating or reducing the frictional force for a short time, time enough for the filings to align with the field.

QUESTION: 
magnetic powered water pumps by cars over road ways, and eme produced from traffic also on road way , based soley upon magnets placed in locations throught the hy system and on vehicels. Is this feasible, Or just another pipe dream??

QUESTION: 
I'm having a hard time understanding virtual particles in empty space. E=mc^2 says that matter and energy are equivalent. Is there an energy field in empty space? If not, then the math doesn't seem to work out. If empty space is literally nothing, no energy field, but nothing, zilch, zip, nada, NO THING, then how is it possible for an electron and postitron pair to form? The problem is, suddenly we now have an electron and positron pair that formed out of nothing, zilch, zip, nada, NO THING and both have mass. Then the electron and positron pair annihilate leaving energy in the form of heat: E=(e-)c^2 + (e+)c^2 Please make the appropriate corrections to the above equation if I have expressed it incorrectly, but what I am trying to say is that the mass of the electron and the mass of the positron become energy when they annihilate. This left over energy should be causing the universe to be heating up since now there is energy where there was none before. Of course if empty space is an energy field and the formation of electron and positron pairs is simply existing energy transforming briefly into matter then annihilating back into the same existing energy, then there is no net gain in energy. The universe will not be heating up. Even if these are not real electron and positron pairs or they exist so briefly that somehow the law of conservation of energy isn't violated, there is still the problem of Hawking Radiation. A positron and electron pair form out of nothing, zilch, zip, nada, NO THING near the event horizon of a black hole and the positron falls into the black hole leaving the electron--NOW A REAL ELECTRON--to drift off into space, we now have the creation of new matter--this new electron--where none existed before! Doesn't this violate the law of conservation of energy?

FOLLOWUP QUESTION: 
I will try to restate the second part of my question more soberly (with less rant) regarding Hawking Radiation. I heard somewhere that in Hawking radiation, the "virtual particles" such as an electron and positron pair pop into existence of nothing--you know the drill--nothing, zilch, zip, nada, no thing near the event horizon of a black hole. As in the previous question we need to consider Einstien's mass energy equivalency, E=mc^2 since now we have an electron and positron pair that popped into existence from nothing, zilch, zip, nada, no thing: E=(electron)c^2 + (positron)c^2 As in the previous question the positron falls into the black hole leaving the electron to drift off into space. So now we have: E=(electron)c^2 Doesn't this violate the law of conservation of energy?

QUESTION: 
Is the speed of light constant?

ANSWER:
The speed of light in a vacuum is a universal constant, it is the same regardless of the motion of the observer or the source. To understand why, see the FAQ page. In a material medium (think of glass, water, diamond, etc.), light travels more slowly.

QUESTION: 
I had a question regarding relativity. If I'm not mistaken, the principle of relativity states that different reference frames are essentially equivalent in that you can't discern a "stationary" reference frame from a "moving" one (that basically there is no such thing as absolute position). So, i was considering a collision of two objects, one of which is much more massive than the other (say, a train and a person). From an observer relative to the person, the train is approaching at a certain speed (say, 5 mph). if the train were to crash into the person, the result would be very bloody and cause severe damage to the person. If, however, this were viewed by an observer moving alongside the train, it would appear as if the person simply walked into the train at 5 mph, and would emerge damaged, but not severely harmed. How is it that this doesn't contradict the principle of relativity? An event's result seems to depend on a "stationary" reference frame (that either the person or the train is moving), when according to relativity the result should be the same regardless of the observer's frame of reference.

QUESTION: 
Why does speed of light reduce when entering a denser medium e. g. glass, and why does it return to initial speed when reentering original medium. Why doe different colours(frequencies) of light refract at different angles through a prism.

ANSWER:
I am afraid that there is no simple explanation for this, it is probably best to take it as an experimental fact. The answer requires consideration of how the oscillating electric fields interact with the electrons in the medium through which it is traveling. The reason for the the prism effects is that, when a more detailed analysis is done, it turns out that light of different colors travel with different speeds in a material. This is called dispersion. So, when we say the index of refraction of some medium is, say n=1.33, this is an average number and there are slightly different values for colors, that is the index of refraction is actually a function of the wavelength.

QUESTION: 
Bearing in mind that the earth revolves every 24 hours, if I wanted to travel from, say, England to Australia, why cant I float up into the sky and 12 hours later float down into Australia? Thus saving about 12 hours flying time and huge amounts of fuel!

ANSWER:
Even if this scheme could work, you could travel only due west, starting in England and ending up in Mongolia, maybe. But, it does not work because when you float you are at rest with respect to the air you are floating in and the earth drags its atmosphere along with it.

QUESTION: 
In E= mc2, c2 cannot merely be velocity squared because a massless photon would then have no energy.

ANSWER:
Always check the FAQ page before asking a question.

QUESTION: 
When we are talking about the spacial dimensions it confuses me how we can talk about the first and second and third dimensions as separate from each other. My question is essentially how can there actually be flat planes in the physical universe; are they only conceptual or do physicists think an object can have length without width or depth? It seems like common sense to me that it is not possible, but I was watching a Carl Sagan clip in which he talks about flat-landers. If they are flat they would still have a finite three dimensional shape I would think...

QUESTION: 
Why are massless photons stopped by paper while a neutrino (with mass), easily passes through the earth?

ANSWER:
A photon interacts strongly with electric charges of which all matter is composed. Neutrinos do not.

QUESTION: 
In V=W/Q or W=VQ where V is the potential difference, W is the work done to move a unit negative charge Q, from one point to another, then what is here the force and displacement as we know that work done is force multiplied by displacement ?

ANSWER:
The only thing that the work in moving a charge between two points depends on is the potential difference between the points. If V=10 V between points A and B, it makes no difference whether they are 1 cm apart or 1 km apart, the work necessary to move a 1 C charge from A to B is 10 J. But, it is still true that work can be computed as force times distance, although the force might not be a constant so you cannot just write W=Fs. It follows then, that the average force experienced by the charge in the example above must be 10,000 times bigger when A and B are separated by 1 cm; the fields are stronger when A and B are closer together.

QUESTION: 
I have a question on Law of Decay.I never heard of it before and I am quite confused with the question asked.Its about a radioactive material that is known to decay at a rate proportional to the amount present. What does the mass of the material initially,got to do with the time taken for example,5 hours, and after this hours ,the material has lost some percentage of its mass.Can you try explain to me what's this whole thing about and how do I even get to start??

QUESTION: 
Force is a push or pull. Is it just the transfer of energy from one body to another? If yes, why does it mostly result in the acceleration of the body? If no, then how does force travel?  

ANSWER:
A force transfers no energy unless it does work. For a force to do work, it must act over some distance. So, if you push against a wall, you do no work and the wall acquires no energy. If you throw a baseball, the force from your hand on the ball acts over a couple of feet and gives the ball kinetic energy. So, the answer is either yes or no.

QUESTION: 
hey man, we had a discussion at the bar and we were not sure about this. here it goes. when you knock on a vacuum? like, u have a trunk with a vacuum inside. what will be the sound. is it A. none at all? B. like a solid piece of wood?( in the case of a trunk ) c. something else It may not be the greatest of questions but i'd apprieciate any answer.

ANSWER:
What sound does something make when you "excite" it? Something has to move to make a sound. The classic example would be a guitar string. If you just had a string stretched between two nails in a concrete floor, you would barely be able to hear it when plucked. Now, if you connect the string to a flexible piece of wood (like with the bridge of a guitar), the vibrations of the string will be transferred to the wood which will also start vibrating in some complex fashion (like the head of a drum makes sound). You may now be able to hear it a little better. Now put a volume of air behind the wood (that is, make a guitar). Now the whole volume of air will start vibrating which will amplify the sound made by the vibrating wood which is amplifying the vibrating string. There is a lot more to a guitar than a plucked string. If you now close up the hole and pump out that vibrating air, you will still be able to hear the guitar but it will not be nearly as loud (or as pretty) without that vibrating chamber of air.

QUESTION: 
Will a stationary wave formed when two wave trains with same speed, frequency but different amplitudes moving in opposite directions overlap? I know if the frequency and amplitude were the same, a stationary wave is formed. But I am not sure when the amplitudes are different. My guess is it will form a stationary wave as long as the frequency is the same. But when tried it in a simulation, it looks as if the resultant wave is not stationary.

ANSWER:
No, the amplitudes must be the same to result in a standing wave. One of the characteristics of a standing wave is that there are times when the displacement is everywhere zero and this cannot happen unless you have equal amplitudes. Imagine one wave, going south, with an amplitude 1000 times greater than the amplitude of another wave with equal frequency going north; surely all you will see is a big wave going south.

QUESTION: 
I was taught in a meteorology class that the Coriolis force is proportional to the speed of an object. The faster an object is moving, the greater the amount of deflection. The classic example of the Coriolis force is the deflection of a ball when tossing it on a merry-go-round. On a counterclockwise rotating merry-go-round, a person tosses a ball to someone directly across from them. The ball appears to deflect to the right (from a rotating reference point). However, if you toss the ball fast enough it will reach the person on the other side before it has time to deflect too far to the right. So, wouldn't an object with a lower speed have greater deflection?

ANSWER:
You confuse force with deflection. Big force does not mean big deflection. Consider a bullet fired horizontally from a gun and an identical bullet thrown horizontally from your hand. Both have the same force on them but the fired bullet has considerably less "deflection". I hope you keep in mind that the Coriolis force is a fictitious force.

QUESTION: 
Hey, so it's pretty basic physics that tells us why some planets revolve around our sun, it's because of gravity, but gravity tells me that the Earth should just spiral into the sun, so the question is what is defying gravity and keeping us from colliding and keeping orbit?

QUESTION: 
Why does liquid flow out of a container faster (or in greater quantity) from a spout after a separate nozzle for air flow is opened?

ANSWER:
As the liquid leaves, the volume of air gets bigger so the pressure gets smaller. So the atmospheric pressure on the outside is larger than the pressure of the air inside the container which tries to keep the liquid from leaving. If another opening is made, the pressure inside will be the same as outside and the liquid will flow more easily.

QUESTION: 
under momentum in physics why is it that when you shoot a gun the gun moves backward

ANSWER:
Momentum of an isolated system (no external forces on it) is conserved. The gun plus bullet system may be approximated as isolated because the only force on it is the explosion of the bullet which is internal. The system starts with zero momentum. After the bullet is fired, the bullet has momentum so the gun must also have momentum in the opposite direction so that the momentum is still zero.

QUESTION: 
It was explained to me that if I jump off my roof into my pool, in free fall I am at rest and the pool is accelerating up towards me. The exact quote: "if you were to go jump off of your house into your pool, you aren't falling into the earth--you stay perfectly still and the earth comes up to catch you." This doesn't make any sense to me.

ANSWER:
The reason it makes no sense to you is that it is utter nonsense. The earth exerts a force down on you, called your weight (Newton's universal law of gravitation). This force causes you to accelerate toward the earth with an acceleration inversely proportional to your mass (Newton's second law). You exert a force up on the earth which, in magnitude, is equal to your weight (Newton's third law). This force causes the earth to accelerate toward the you with an acceleration inversely proportional to the mass of the earth (Newton's second law). So, you accelerate toward each other. However, since the mass of the earth is huge compared to your mass, the earth has a tiny acceleration, far too small to ever hope to measure. For all intents and purposes, you fall to the earth, it does not fall to you.

QUESTION: 
I refer to one of your previous posting where a school teacher asked if a weight of 1kg is dropped from 1m on to the head of a teacher how much force is exerted on the teacher. Your response was that F=M*A so unless you know how long the weight was in contact with the teachers head you could not calculate the force. I understand what you are saying that the longer the force is is contact the less the accel, thus the less force is imparted. But I think your answer, respectively is not quite correct.

1. Instead of calculating the Force in terms of F=MA we should be actually calculating the momentum of the of object in N.m/s ?.

2. My question is, if we calculate your way (F=M.A) then if the object hits the head of the teacher and instantaneously decelerates to 0 M/s we should consider instantaneous deceleration as A=1?

3. Because if Acceleration = (Vfinial-Vinitial)/1 which means the total force is imparted, however if the object was in contact for only 0.1 seconds then F=M*((Vfinal ¨C Vintial) / .01s) - F=9.8N *(0 ¨C 9.8M/s) / .01 = 9604Nm

4. (Can you see my point that 1 sec = full force but 0 .1 is still an incredible amount of force that cant be right.?)

5. Could you please clarify? And show how to calculate the force in terms of momentum force of the object which must be complexly transferred to the teachers head if the velocity of the 1KG comes instantaneously to a complete rest, and in 0.1 and 1 second intervals?

6. And also if the object was a ball and spinning as it falls which means we should calculate the impact in terms of Inertia = Energy transfer to the teachers head.

ANSWER:
You have incredible misconceptions. I have converted your question to a numbered list so I can answer your multiple questions.

QUESTION: 
Does gravity have a speed or is it instantaneous? My thought is...if two bodies are orbiting each other (Earth and Moon for instance), is the gravitational attraction between them a vector force that points directly at the center of the other's mass, or does the vector point somewhere behind its mass due to the finite speed of gravity?

ANSWER:
Always check the FAQ page before asking a question!

QUESTION: 
When your eye sees light, are those photons "lost" in the absorption and subsequent conversion to signals to the brain? Is the original source of light diminished by the amount of photons absorbed by the eye (or a measuring device is used in lab)? I am trying to figure out if light that travels through the universe remains whole or are photons lost due to to the simple act of "observation".

ANSWER:
Yes, the photon is destroyed in your eye. The original source lost those photons when they left, not when they got absorbed. A photon traveling through the universe continues until it interacts with something.

QUESTION: 
If the gas discharge tube contains MOLECULAR hydrogen, why do we only see ATOMIC hydrogen line spectra, not molecular?

ANSWER:
Because the tube ionizes the gas which causes the H₂ to dissociate into atomic H.

QUESTION: 
Why can electomagnetic radiation travel through the vacuum of space?

ANSWER:
Because the waves are time varying electric and magnetic fields which do not require a medium to exist. See earlier answer.

QUESTION: 
What is the smallest instance of time?

ANSWER:
Nobody really knows whether time is continuous or discrete. See an earlier answer.

QUESTION: 
Last night I heard an interview on the BBC with a man from Sweden who had tried to build a nuclear reactor in his kitchen. What disturbed me most was that he had bought radium on eBay. Admittedly it was only a small amount, about half a gram. I know that the conversion of mass to energy is normally inefficient, but for a moment, assuming 100% efficiency, then given that the square of the speed of light is an enormous number, at first sight it looks as though half a gram of radium might potentially produce a rather big bang. I wondered quite how big it might be, compared to, say, a firework, a stick of dynamite, or a ton of TNT, whichever is the most appropriate. I am so glad the police stopped him before he got his project underway!

ANSWER:
Radium is not a material suitable for a chain reaction. And, a half gram of it is not enough, even if it were plutonium or uranium, to make a critical mass for a chain reaction to occur. And, 100% mass conversion is so far off what is possible that it does not merit discussion. A completely "efficient" fission reaction (by which I mean 100% of the atoms undergo fission) of uranium or plutonium yields only on the order of 0.1% mass conversion. The danger in this instance is, mainly, to the man himself from the radiation, very little other danger.

QUESTION: 
Baseball pitcher throws 90 mph in 60-dgree ambient air temp. If he threw with the same force in 90-degree ambient air temp, would the pitch be faster or slower. If I remember my physics, shouldn't humidity play a role? If so, make humidity constant. If colder air is denser (irrespective of humidity), then wouldn't the same force produce a faster pitch in warmer ambient air temperatures?

ANSWER:
A couple of problems here. Constant humidity does not mean the same amount of water in the air at different temperatures; humidity is the fraction of the maximum water that the air can hold at a given temperature, and cold air can hold less. Density of a gas is not solely determined by temperature, pressure is also a factor. So, let's just say the humidity is zero and the atmospheric pressure is constant. Then, you are right, the 90 mph pitch (leaving the pitcher's hand) in cold air will have lost more velocity when it reaches the plate because the density of the air is greater. However, the effect will, I believe, be rather small; the difference in densities is only about 5%. Since the speed lost by a 90 mph fastball is about 10 mph, the difference in final speeds would be be about 0.5 mph (one arriving with 80 mph, one arriving with 80.5 mph). There are lots of FAQ links to questions about air drag.

QUESTION: 
I read that atoms come together to form molecules, which combine to form solid matter. I'll site the uranium atom which has I believe 92 electrons spinning at high velocity in orbit around the nucleus. some may be at lower orbit than others. So, all these negative charged electrons occupying this "shell" of <---> electric charge. Would that not create some type of a barrier, which would prevent the atom from approaching another due to this perimeter ?? of negative electric charge ? I know I'm reaching out too far here as a "novice", but I imagine this outer electron perimeter as having an electron located at any given point, at any given moment. Or, no electron located at any given point at any given moment. Because of their velocity they would seem to be everywhere all the time, thus creating this field (barrier) ?? of negative electric force. " LIKES REPEL" Also may I add another point here, If a single proton reaches out to attract a single electron, why does the electron stop at a great distance from the proton. What causes it to stop instead of going "all the way", BUMP ????

ANSWER:
The simplest way for you to think about this is to think of the electrons as occupying concentric spherical shells around the nucleus. And, only a limited number of electrons can occupy each shell. So, for example, H has 1 electron in the first shell, He has 2 electrons in the first shell, Li has 2 electrons in the first shell and 1 electron in the second shell which has a larger radius. Hence, the third electron in Li is "shielded" from the nucleus, sees only approximately one positive charge instead of the three that are really there. Also, Li looks a lot like H (one net positive charge inside, one negative charge outside) and, indeed, H and Li are very similar chemically. Now, you wonder why atoms bond into molecules; you are right that the tendency is, if brought close together, that two atoms will repel each other because of the outermost shells of each being negatively charged. But, if the atoms get close enough together, the whole shell idea starts to break down because, the outermost electrons (which chemists call valence electrons) in atom A start feeling an attraction to the positive nucleus of atom B and vice versa, and so the net result is an attractive force which may bond the atoms into molecules.

QUESTION: 
I have a question which is part of a project I'm working on and thought you might help out :) It is not homework! The objective is to calculate the volume and pressure of the air coming out of the following experiment: Imagine a huge piston and cylinder. The surface of the piston is 10000sq ft and it is moving up in the cylinder at a rate of 1 ft per hour. As a result air is being pressured to exit through a 1sq ft opening at the top. 1- what will be the pressure and airflow volume exiting the opening. If any one has a basic equation for this it will be wonderful.

ANSWER:
I presume the end with the hole is just open to the atmosphere. Therefore, since the piston is moving so slowly, the pressure everywhere will be atmospheric pressure to an excellent approximation. The velocity of the air exiting the hole will be 10,000 times 1 ft/hr, 2.78 ft/s. The equation you need is that vA=constant, where A is the cross section and v is the velocity. Technically, this is true only for an incompressible fluid, but with such small speeds, air may be approximated as incompressible.

QUESTION: 
I know that gravity decreases as you move away from its center, so at the top of a high mountain, you would weigh a small percentage less. Now, what if you had a very tall mountain, say 100 miles tall, how much less would you weigh at that height, and what general formula would one use to calculate the relative weights at various heights from sea level to any given altitude? I am not a mathematician or scientist, but it would be very helpful to have this information for a novel I'm writing.

QUESTION: 
Why do they say the center of the earth is like a bar magnet when the center is more like a perfect gyro scope? Seriously almost like a mini sun... a uber dwarf sun. That's why the earth plates work in a fluid motion cause fluids are sitting on plasma?

ANSWER:
"They" do not say the center of the earth is like a bar magnet, "they" say that the earth's magnetic field resembles the field of a bar magnet, has a north and south pole, for example. But, the field of any simple current loop resembles a simple bar magnet field also. The earth is thought to have, at its center, currents in the molten core, which give rise to the field. It does not really resemble the sun at all, no hydrogen and no fusion going on. Radioactivity is believed to be a source of energy for heating the core of the earth.

QUESTION: 
If dark matter/dark energy exists, would it be distributed throughout space, interspersed or integrated, so to speak, with observable matter/energy? Or is dark matter/dark energy thought to exist separately, in regions of what appear to be vacuum, apart from ordinary matter/energy?

ANSWER:
First of all, my take on dark matter and dark energy is not mainstream, see the FAQ page. Also, note that dark matter and dark energy are quite different things, neither of which are well understood because they have not been directly observed. Dark energy is already a part of general relativity, represented by something called the cosmological constant, again, not well understood what its origin or value are. Regarding whether dark matter is distributed uniformly throughout space, the answer has to be no. If it were, there would no effects because you should interact only gravitationally and, no matter what direction you looked, you would see the same amount of mass and therefore no net force. I believe that dark matter advocates believe that it is concentrated in galaxies and it is "hot", that is it has a high speed so that it does not just build up around and in stars and planets.

QUESTION: 
here's my question which I read in an article from the early days of space flight: A manned spacecraft is circling the moon at an altitude of 100 miles above the surface, and the astronaut on board noted the "angle of depression of the horizon was 23.8 degrees. With this known data, the astronaut was able to determine the radius (or diameter) of the moon. I drew a diagram of the moon and the orbiting spacecraft with the known height of craft above the moon and the depression angle of 23.8 degrees. Thinking that this would be a simple case of similar triangles, etc. I found that I could not determine the radius of the moon from the given data

ANSWER:
The figure to the right shows what you need. The angle A in this figure is, you will agree, your 23.8⁰. You can see that cos23.8⁰=R/(R+100). Solving, I find R=1076 mi, pretty close to the known radius of 1079 mi.

QUESTION: 
I have been instructed to ask about photons following a long discussion with my teenage sons. Is it not possible that a photon is an energy more akin to a sound wave than a particle? Is it possible that the photon is just an energy moving through an, as yet un-identified, substrate ? I thought possibly the universe could be like a , bad analogy, bowl of thin cornflower mixture or wallpaper paste with the cornflower, say, instantly able to take the property of light and pass it on. Holds us all apart, no vacuum, no photon as such: just a property of energy moving through the substrate.

QUESTION: 
I do not understand the pictorial representation of an electromagnetic wave. Does the wave symbolize the trajectory of a stream of photons? Do the photons actually travel in an up and down wave motion? Or does the wave just symbolize energy oscillations? Or is it a cross-section of a disturbance in the electromagnetic field? Or heaven forbid, does the wave depict a probability? In other words, if the picture of the wave was on a graph, what would the axes of the graph represent? It's amazing that I'm having such a difficult time finding the answer! I am already familiar with the double-slit experiment and how light can act as both a stream of particles and a wave. I'm not sure how to interpret that either.

ANSWER:
In the 19th century light came to be understood as being waves of electric and magnetic fields. The picture I draw in that earlier answer shows what is called a one-dimensional, plane-polarized, monochromatic electromagnetic wave and, though simplified and idealized, is what you should think of when you think about light as a wave. It would be less confusing for you if you stopped right there. Alas, nature is not quite so simple. It turns out that there are experiments which can be done which can only be understood if light is not a wave but rather a stream of particles (photons). Examples of such experiments are the photoelectric effect (light knocking electrons out of a metal), Compton scattering (light scattering from electrons), and any light emitted from atoms. Light indeed is a wave and it is also a particle, a situation called wave/particle duality. If you design an experiment to prove that light is one or the other you will certainly succeed. It is each and it is both. This way of thinking about something is antithetical to how we usually think about nature, ambiguity is usually considered unscientific, and that is why you are having trouble getting your head around this.

QUESTION: 
With reference to binding energy and mass defect how do you explain why both fission and fusion releases energy.

ANSWER:
See an earlier answer.

QUESTION: 
please settel this argument if i was in a moving veichal that was moving at 30mph and i got up to walk what speed would i be traveling

ANSWER:
You always have to ask speed with respect to what. If the bus moves relative to the ground with speed 30 mph and you walk forward relative to the bus with speed 10 mph, someone on the ground will see you moving with a speed of 40 mph relative to him. If you walk 10 mph toward the rear of the bus, your speed relative to the ground will be 20 mph (moving in the same direction as the bus).

QUESTION: 
is the speed of light limited by inertia?

ANSWER:
Light, having no mass, has therefore no inertia. The speed of light is just what it is and that is determined solely by electromagnetism. Maybe you meant to ask about the speed limit for material objects being the speed of light? There you could say that inertia provides the limit because as you approach the speed of light the mass (i.e. inertia) approaches infinity.

QUESTION: 
When 64Co27 (where 64 is the mass number of cobalt and 27 is the proton number) undergoes gamma decay it produces 64Co27 and energy which is in the form of the gamma ray. I don't get why the binding energy per nucleon has increased?, there has been no change in mass number?

ANSWER:
You have to conserve energy, right? The photon carries off an energy E, the original nucleus has energy M*c², and the final nucleus has energy Mc². Therefore, Mc²+E=M*c². This means that the final nucleus has less mass than the original nucleus, M<M*, which means that the final nucleus is more tightly bound, which means its binding energy per nucleon is greater. The binding energy per nucleon is determined by the structure of the nucleus, not the number of nucleons it has.

QUESTION: 
I am working on an educational speech about the earth's motion around the sun, and how the primary factor in the seasons is not the distance to the sun, but rather the time the atmosphere in a hemispheres is exposed to the sun's energy in a given day. As I thought about the atmosphere, I had to remind myself that our atmosphere has mass, and thus that mass is attracted to the earth's surface by gravity. There is no invisible wall holding it all in. But, as I thought about it, the density of the atmosphere decreases as the altitude above the earth's surface increases, and it made me wonder -- at the very top-most reaches of the upper atmosphere, is the earth trailing a sort of "soap bubble" of oxygen, hydrogen, and nitrogen atoms? And, are some of those atoms drifting far enough away to "fall off" into space?

QUESTION: 
In a junkyard what is doing the work when an electromagnet picks up the car. Many people say it is the magnetic field but others say it is the electric field? Is there any real way of knowing?

ANSWER:
The magnetic field never does work. I have fully discussed this in a recent answer.

QUESTION: 
If I'm trying to calculate mass of all of the light that comes out of the light bulb per second, then I first have to find the mass of each photon which is given by the formula E=hf, then I have to include the amplitude- which will show me how much of those light quanta (photons) will come out per second. Which formulas do I have to use to find out the amount of photons coming out? Which distance are the photons from each other? And is there some proportionality between frequency and amplitude that is between mass of each photon and numbers of them coming out? And finally can you increase frequency without increasing amplitude?

FOLLOWUP QUESTION: 
Light has it's mass as long as it travels right?

ANSWER:
Wrong, light never has mass.

FOLLOWUP QUESTION: 
if photon is massless then how could it then itraract with normal matter such as electrons... and also how could massless object be travaling at all? it'll be impossible to do work on it (photoelectric effect) electron requires energy to move electron and electron emits photon whenever it is changing it's orbit into lower one. How can you think of photon then if it doesn't has mass? How momentum of photon is measured then if on in P=M/V

QUESTION: 
If I have a wire with a current flowing through it, the magnetic field B generated by the current can be found using B= (mu*I)/(2piR), with R being the radius away from the wire. As R approaches zero as you get closer to the wire, does the magnetic field strength approach infinity? Or does it simply max out at the radius of the wire?

QUESTION: 
Someone told that newton is wrong in his second equation of motion as it gives an impossible situation in the following statement: 1- F = M x a ok now we have a car with 500 KG mass and constant velocity 50 mph the car hit some wall what force will be applied on the wall ??? as the velocity is constant the acceleration would be zero and substituting in the 2nd law F = 500 x 0 =0 ???which is impossible

QUESTION: 
what makes sunset and sunrise red?

ANSWER:
See an earlier answer.

QUESTION: 
can u explain me wat parity is and why it is not conserved...

ANSWER:
Parity is a symmetry having to do with how a function behaves under reflection. In particular, if f(x,y,z)=f(-x,-y,-z), f is said to have even parity and, if f(x,y,z)=-f(-x,-y,-z), f is said to have odd parity. An example of a function with even parity would be f=x²+y²+z²; an example of a function with odd parity would be f=x+y+z; an example of a function with no parity would be f=x²+x. Parity nonconservation refers to the wave function of a system having a different parity after some interaction. Beta decay is an example of a situation where parity is not conserved. It is much too involved to try to explain the theory of beta decay here.

QUESTION: 
When an airplane tilts, why doesn't water fall out of a full cup . Am i only imagining this and is the tilt less than what i observed out of my window.

ANSWER:
A plane seldom tilts (banks) without turning. You are then moving in a circle and the (fictitious) centrifugal force keeps the surface of the water from being horizontal. If you are tilted upwards steeply but going with constant speed in a straight line, your water will spill if you are not careful to hold the cup horizontally.

QUESTION: 
is it true that the decay of one field causes the build up of another?

ANSWER:
Yes. Two of Maxwell's equations contain the fact that time varying electric fields cause magnetic fields (Ampere's Law) and time varying magnetic fields cause electric fields (Faraday's Law).

QUESTION: 
how does the electric force changes into magnetic force by giving certain velocity to the observer i.e. how are these 2 forces frame dependent?

ANSWER:
Actually, electric and magnetic fields are not different things, they are manifestations of a single field, the electromagnetic field. When you transform from one frame of reference to some other moving frame, you change the mixture of the electric and magnetic parts of those fields. For example, a point charge at rest has a pure electric field; but if you view it from a moving frame of reference, it has both an electric and magnetic field. It is far too involved to show the details here. The reason that we still talk about electric and magnetic fields in elementary physics courses is that, historically, it took a long time to realize their connections. The electromagnetic field is not a vector field, it is a tensor field, a field which has more than 3 components.

QUESTION: 
I understand why they say the speed limit of a rocket engine is based on the maximum velocity of escaping gasses, but when I consider it this way I get lost. Where does my thinking go off the rails? In space everything is relative, especially velocity. So let's imagine that we have a spacecraft in open space with no points of reference to determine velocity. The spacecraft may be travelling a million miles per hour, but there would be no way to tell. The only point of reference is the spacecraft itself. So relative to the spacecraft, its engine is at a standstill, and when it is fired the escaping gases must expand and in doing so must provide pressure against the spacecraft, and consequently must provide accelleration. If not why not? How would the engine "know" it was already at any particular velocity and couldn't go any faster?

QUESTION: 
Many sites and books discuss space-time curvature as the reason for gravitational force. They use an analogy of a bowling ball sitting on a bed and causing a depression due to its mass. Rolling a marble across the bed will roll down the depression accelerating towards to the bowling ball and eventually hitting the bowling ball and coning to rest. I don't under this analogy because the ball accelerating is due to the earth's gravity, but in space, the moon does not roll down the earth's space-time curvature and run into the earth. Also, if gravity is weaker the further out we go, then how is it possible that Sun's gravitational force can keep pluto in orbit when Jupiter's mass should overpower the Sun's gravity effect since it is closer to pluto? Why doesn't pluto orbit Jupiter, Saturn, Uranus or Neptune? Likewise, why doesn't mercury fall into the Sun since it is so close? If gravity from the sun is the reason that earth remains in an elliptical orbit, then what started the initial inertial momentum of earth? Lastly, if gravity was not a force and only space-time curvature, then how do we explain that space is curved all around us and that is the reason that a ball drops to the floor when we let it go and comes to rest on the earth?

ANSWER:
First, the bowling ball on the bed (often a trampoline) is not meant to be literally true, it is a way to illustrate an idea. See an earlier answer. Regarding your question about why Pluto is not in orbit around Jupiter instead of the sun, it is simply that, although Pluto is certainly closer to Jupiter sometimes*, the gravitational force from the sun is still much stronger because the sun has so much more mass; the gravitational force of the sun is always much larger. In terms of your "bed" model, the sun is a sumo wrestler and Jupiter is a baby, so whom you gonna roll toward? (*I say sometimes because there will be times in their orbits when they are on opposite sides of the sun.)

QUESTION: 
I need to keep quarter gallon of water in trunk of my car (asume same temperatures as outside of the car) not frozen at all times (Water is kept in plastic and/or glass bottle. How much electric energy do I need if during any 24H period temperatures are between 10 to-10F. How much less electric energy would I need if water is contained in a thermos?

QUESTION: 
Is there an explanation for a light clock for dummies?

QUESTION: 
i have heard several times that einstein claims that an object's mass increases as its velocity's magnitude increases. but einstein also says that all motion is relative, and there is no absolute state of rest. what, then, is the frame of reference for the calculation of the increase of mass of an object? we cannot use the earth because the earth is moving; that would be like trying to calculate the speed of a car from a different car moving at an unknown speed.

QUESTION: 
Does energy create momentum to create force?

ANSWER:
I have no idea what you are asking! Momentum is caused by impulse, a force exerted over some time. Energy is created by work, a force exerted over some distance.

QUESTION: 
If I carefully arrange two permanent magnets in such a fashion as to levitate one of the magnets due to mutual repulsion, will the distance between the two eventually go to zero? What is the nature of the energy transfer that occurs when the "experiment" is performed?

ANSWER:
If you "carefully arrange" them, the energy to place the levitated magnet just right comes from you. Once they get there, they will stay there forever (assuming the magnets do not become demagnetized somehow). But, the magnetic fields themselves never do any work. For more discussion on that, see the following answer.

QUESTION: 
two permanent magnets will repel each other indefinitely
often with great force
not hard to make a permanently magnetically levitated platform
Where does the energy to perform the work come from?

QUESTION: 
I was reading about the Large Hadron Collider and their ALICE experiment with the collison of lead ion particles. In the experiment the collison of these particles creates temperatures of 100,000 times hotter than the center of the sun. How can the collider itself handle such high temperatures? It would seem any metal, or substance the collider would be made of would simply melt (along with any instruments measuring the collison). What is the collider made of - or what condition makes the collider able to withstand such high temperatures?

QUESTION: 
Why do racecars have bigger tires? My first thought is that a racer would want to reduce the surface area between the tire and the ground to reduce static friciton so that he or she needs less acceleration to overcome the static friction; however, in class today we learned that friction does not depend on surface area with some exception, tires possibly being one of the those exceptions. Why would a racer want to increase static friction when it resists movement of the car in the opposite direction? I think my teacher said that static friction resists motion but also accelerates it. If this is true, how does it does it also accelerate in the object?

QUESTION: 
My teacher did a demonstration in class where there was a mass of 1 kg suspended by a string above it and then he attached a string below the mass. For the first demonstration, he pulled quickly to show that a rapid acceleration on the string below the mass would break that string. For the second demonstration, he showed that if you gradually accelerate the pulling of the string below, the string holding the mass would break instead. Could you please explain why this is the case, including the interaction between the two tensions between the strings? I understand that a greater acceleration means a greater force, but I have difficulty understanding why the gradual pull breaks the upper string and what "breaking the tension of the string" means.

ANSWER:
If you pull up gradually with a force F and the mass M is at rest, then the forces on the mass are its weight, -Mg, the lower string, -T_lower, and the upper string T_upper=F. Newton's first law states that 0=F-Mg-T_lower. So, the tension in the upper string is greater than the tension in the lower, F=Mg+T_lower. So, as you gradually increase F, the tension in the upper string will reach the breaking point first. I assume that in the second case, the lower string starts out slack and the mass is pulled upward by the upper string. When the lower string suddenly gets taught, there is suddenly a very large acceleration in the downward direction which can only be provided by the lower string. Therefore, there is suddenly a large tension in the lower string which exceeds the breaking strength and breaks it.

QUESTION: 
in photoelectric effect (keeping intensity and frequency constant) as we gradually increase the +ve potential the photoelectric current increases.but after a certain value of current(saturation point) the current does not increses eventhough potential is increased..i want to know why the current is not increasing??if all the electrons reach the positive plate then how does the current still exist without charge flow?

ANSWER:
A certain intensity of photons will give a certain number of electrons per second ejected. So the current coming off the cathode is some constant. With zero or small voltage to the cathode, some of the electrons do not make it (they hit the walls of the enclosure or scatter off the diffuse gas that might be present). But as you increase the voltage, you capture more and more of them until finally you get them all and you can't get any more than all of them.

QUESTION: 
if a bullet was travelling at 823meters per sec and hit an object that stopped it dead how much force per square cm would be exerted onto the target?

ANSWER:
Here is the question which I get in one form or another which indicates how poorly understood the concept of force is! First, you want to ask what the force exerted by the bullet is, not the force per cm². If you were able to get the force, you could divide that by the cross sectional area of the bullet to find the effective force per unit area; but you cannot get the force because it depends on how quickly the bullet stops. If you mean by "stopped it dead" that it stops instantaneously, then the force would be infinite. The average force is the change in momentum (mass times velocity) divided by the time to stop. So, you need also the mass of the bullet. Suppose the bullet had a mass 0.02 kg, then the change in momentum (0.02x823) is about 16 kg m/s. If it stops in 0.01 s the average force is 1600 N=360 lb, if it stops in 0.001 s the average force is 16,000 N=3600 lb.

QUESTION: 
I understand the general principles of spacecraft in orbit around the Earth (viz., gravity continues to act on an object in orbit, but because it is in freefall - continually "falling over the horizon" - the object is weightless). I'm confused about one thing though. If terminal velocity in a vacuum (which for all intents and purposes applies to low earth orbit) is 9.8m per second per second, how come the acceleration of the object towards the Earth does not continue to increase such that the object cannot remain in orbit but instead falls to Earth? If terminal velocity is reached or is stabilized, then I understand how orbit is maintained, but there is clearly something I'm missing. Obviously objects do orbit, so I'm unclear on the physics behind the orbital motion vis a vis gravity.

QUESTION: 
Is the electric field of any charged particle moving in a straight line dependent in its velocity? If it is, and it is just a constant velocity, would that make the electric field equal to zero?

ANSWER:
Yes, but this does not become noticeable until very high velocities (comparable to the speed of light). The electric field gets smaller in the direction parallel or antiparallel to the velocity and stronger perpendicula to it. The picture to the right shows the field for a negative charge with speed 90% the speed of light; if the charge were at rest, the field lines would be of equal intensity in all directions. You cannot transform the field to zero.

QUESTION: 
is it possible,in any way,shape, or form that time travel is possible?

ANSWER:
From what we know, it is possible to travel forward in time (e.g. the twin paradox), but not backward.

QUESTION: 
I have a question pertaining to a gas in a fixed volume: If I were to ionize (such as through an air ionization device) the gas, would the density change, and what would be a good equation for calculating the change? (Would this be a piece of the Ideal Gas Law that I am missing, or something else entirely?) And second, how would this interact witht he Ideal Gas Law if I were to also change the temperature?

ANSWER:
The only way to ionize it is to add energy; if you add energy you increase the mass and therefore the density (E=mc²). That said, the amount of mass increase would be unmeasureably small. The ideal gas law does not apply because you have a mixture of ions and electrons of different masses and both are highly interactive with the walls of the container.

QUESTION: 
If we have a long tank filled with water and a motorboat motor attached to the end of the tank with its propeller spinning rapidly under the surface of the water sending a water jet out into the tank, and given a long enough tank, is it possible the water jet’s momentum can dissipate before the jet--or some of the jet--hits the other end of the tank and thus preventing any further push on the wall opposite the motor? Or will there always be momentum of the jet hitting the opposite end of the tank(opposite the motor)? In short, when will there be a push on the wall opposite the motor not due to normal water pressure in the tank but rather stemming from the jet. How much?

ANSWER:
If you have a short tank water will be sloshing back and forth but, if you turn off the motor, it will eventually calm. The reason is that water is a viscous fluid and viscosity is friction in a fluid. So energy the motor puts into the water is eventually taken away by this friction. If the tank is long enough, any disturbances caused by the motor will eventually be damped out before reaching the other end. But, then, you didn't really expect to be able to tell at the end of a 1000 mile long tank whether there was a motor going at the other end, did you?

QUESTION: 
Suppose a deserted boat on the sea advancing in a straight line at a fixed speed due to it's inertia, the motor is shut off and nobody is on the boat to obey commands. Is it possible to "knock" it off course, to change the direction of the boat to modify it's vectors without adding energy to the boat ? If I ram into the boat with another boat, I would have knocked it off course but I would need to transfer energy from my boat to the boat I wish to knock off course. My question is that is it possible to modify a system without modifying it's stored kinetic or potential energy ? Is it possible to change the direction of the boat without adding energy to the boat ?

ANSWER:
Yes, you can change the direction of the boat without changing its energy. The idea is that you must exert a force perpendicular to its direction. The result is that you cause the direction of its velocity to change but not the magnitude of its velocity. Examples are a car going around a circular track with constant speed due to the (centripetal) force exerted by the friction of the tires, a charged particle in a magnetic field will not change its speed but will change its direction, etc. If you "ram" the other boat, you lose energy but it does not show up in the energy of the other boat, it is lost to frictional forces.

QUESTION: 
If gravity affects all matter in the same way, regardless of mass, then why does a beam of light not deflect the same way as other particles? For example, it is known that light bends due to gravity. Therefore, it is possible to take deep space pictures due to light bending around massive objects such as stars. Traditionally it is said that photons are "massless." However, if this were the case, they would not be able to be bent by gravity in the first place. So, to formulate my question: take for example a beam of light being projected by a flashlight. It would seem that the light should deflect toward the ground due to gravity. However, this is not the case. Why is it that the beam projects out as a straight line?

ANSWER:
First, you should read the FAQ links on this question. Regarding your question about the flashlight: the light does bend down, but the amount is so tiny that you cannot observe it. An earlier answer can give you a more quantitative idea of how small this bending is.

QUESTION: 
if you were to suddenly put a large object in space then the space would be pushed out to the sides (displacement) . because the object is now in the place were space once occupied ?? so space is just pushing back to its original area ???? is this what gravity is??? or bending of space?

ANSWER:
Your basic premise is wrong. The object occupies the space, does not "push it out".

QUESTION: 
Is there a way to vary or regulate the strength being produced by an electromagnet? If i was to run more current would it have a stronger polarity and if so, what would be the minimum amount of current I could use to give the magnet charge? I also am wondering by running magnets in succession for long periods of time if there is any risk? I have heard that you can over charge your magnets witch makes them like a bomb. I that true? If so, is there a a way to measure the polarity being produced to avoid an incident like this.

ANSWER:
The strength of the field may be increased by increasing the current. However, assuming this is a magnet with an iron core, the field will eventually reach a maximum limit called saturation. The field can be increased by increasing the current, but by a quite small amount for each increase in current. There is also the problem of hysteresis which means that if you turn the current back down to zero, the field will not completely go away because of the residual magnetization of the iron. There is no limit to how long you can operate a magnet. The only way it would "explode like a bomb" would be if the wiring overheated or something like that. Maybe you heard something about magnets at the accelerator at CERN exploding; these are superconducting magnets which have to be kept at extremely low temperatures and a malfunction resulted in an explosion of the cooling system.

QUESTION: 
i was reading up on Quantum Teleportation and i found the it is descused to sound more like a form of cloning than telrportation. it made it sound like of insted of you being telaported yourself, there would be a copy of u at the destnation, and not a vary good one. it also said that when you get copied the first u must no longer exist so that the second will. so is this what Teleportation really is a way to send not u but a copy of u somewhere else, or did i get the idea of how it might work?

ANSWER:
Here we have a relatively concise question which cannot possibly have a concise answer. For a thorough discussion of the "beam-me-up" problem, I recommend Lawrence Krauss's book The Physics of Star Trek.

QUESTION: 
I am doing an extended essay in physics where I am investigating how the volume of water in a plastic bottle affects the time taken for it to roll down an incline and then after leaving the incline a particular distance along an horizontal plane. My results show me that if you increase the volume inside the plastic bottle, the bottle takes less time in that it is faster. How would you explain this relationship in terms of different volumes of water inside the plastic bottle?

QUESTION: 
The sun sits there... burning. Converting H to He and it's been doing this for millions and millions of years. My question is, why does the sun burn so long instead of just blowing up like a nuclear bomb?

ANSWER:
The trick to making a hydrogen bomb is to hold everything togther long enough for a significant amount of hydrogen to undergo fusion reactions. This is really hard because to make fusion you need very high temperatures and it is difficult to hold a high temperature gas together. And, you do not hold it together for long but long enough to put a lot of energy into the expanding explosion afterwards. This is the same reason it is so challenging to make a fusion reactor, just holding the hot plasma contained is not easy. So, what makes this not an issue for the sun? What the sun has going for it is its huge gravity (because it has a huge mass) which holds the whole thing together, even at a high temperature. The sun is no more likely to explode than are you able to just jump into outer space because the sun holds everything down. The reason it lasts a long time is that there is a huge amount of fuel.

QUESTION: 
Electricity is made of electrons. Atoms have electrons, protons and neutrons. We split atoms, and fuse atoms to cause a chain reaction, which releases more energy than any other chemical reaction that we can achieve currently. And we do this in order to boil water. To boil water quickly, to spin mechanical turbines, which rotate magnets inside wire coils to produce electromagnetic induction and siphon away electrons from that induction to be used as electricity. Why can't we just directly access the electrons in each atom. Why can't we simply siphon off all of the electrons from a 5 gram cube of uranium 235. What stops us from breaking down the atoms and just absorbing the atomic level electrons, even the protons since they have a positive charge.

QUESTION: 
Is it possible to find the density of protons, neutrons and electrons or just the nuclei?

ANSWER:
Unlike electrons in atoms, neutrons and protons in nuclei are pretty much tightly packed. Therefore, the density of neutrons and protons is about the same as for nuclei, about 2x10¹⁷ kg/m³. Electrons really do not have a meaningful size since they are elementary, so density is rather meaningless. You could calculate a density using the classical electron radius, about 3x10^-15 m, but this number does not really represent anything physical.

QUESTION: 
Is everything moving relative to everything else? Can anything be 'still'?

ANSWER:
There is no such thing as "absolutely still", that is one still is no 'stiller' than any other still. In classical physics, all inertial frames are equivalent. An inertial frame is one in which Newton's laws are true. Once you find one inertial frame, any other frame moving with constant velocity relative to it are also inertial frames and nothing makes any one of them special. However, lots of things are still if they happen to have zero velocity in your frame. I should add that, quantum mechanically, the Heisenberg uncertainty principle states that you cannot know both the velocity and position of a particle to arbitrary precision. Therefore, the only an object can be perfectly still is if you also are totally ignorant of where it is. But, that is probably not what you were asking about.

QUESTION: 
are cell phones harmless to ones health?

ANSWER:
I would be reluctant to say they are "harmless". But the recent hoopla over brain cancer and cell phones deserves some comment. The recent designation by the World Health Organization that cell phone use is possibly carcinogenic (added to a list which includes coffee and pickles), is based on a very weak study which asked people with brain cancer to recollect how much they used cell phones. Also, although cell phone use has increased astronomically in the past 20 years, there has been no corresponding increase in brain cancer rates. Finally, there is no proposed mechanism for cell phone radiation to cause cancer because unlike x-rays, for example, they do not have sufficient energy to alter DNA. A good discussion can be read on the New York Times. (I personally think excessive cell phone use causes stupidity just like excessive Facebook and Twitter!)

QUESTION: 
if a box drops from a moving plane and there is no wind which way does it drop

ANSWER:
There is never "no wind" because the plane is moving through the air. If you neglect air drag, the box will drop such that it stays directly under the airplane. To the airplane pilot the box will appear to drop straight down, always being directly below. However, air drag will never really be negligible in this situation. The box will hit the ground behind where the airplane is when it hits; the details depend on the speed of the airplane, the mass of the box, the geometry of the box, etc.

QUESTION: 
What is the speed of electric current through a copper wire?

ANSWER:
Electrons carry the electric current, so your question could mean either how fast are the electrons moving in the direction of the current or how fast does information move down the wire (like how long after the switch is turned on do I have to wait until the light turns on). The electrons in a conductor which carry the current are called conduction electrons and are free to move around; they do so in random directions in an isolated wire of copper; so, there is no net flow of electrons just as there is no net flow of air molecules in a room even though they are zipping about very fast. When you apply a potential difference across the ends of a wire (e.g. connect a battery or plug them into the wall) an electric field is established in the wire which pushes the electrons opposite the direction of the field (because they are negatively charged particles). As they move along, they bump into atoms in the copper and slow down but then the field speeds them back up. The net effect is that, on average, the electrons have some average speed in the wire called the drift velocity. The actual magnitude of the drift velocity depends on how much current flows, how thick the wire, etc., but it is quite slow. A numerical calculation is shown for a specific case in the Wikepedia article on drift velocity; the drift velocity for this example is about 1 meter per hour. But, the electrons in the whole wire start moving together almost instantaneously because the electric field which drives the electrons is set up at almost the speed of light.

QUESTION: 
Electrons, as negatively charged particles, not only rotate about the core of atom but also about their own axis (spin) Why this happens ?

QUESTION: 
If I wanted to build a antigravity chamber by using a sphere made of osmium or iridium suspended 1 meter above the Earth's surface to cancel out the Earth's gravity, how massive would the sphere need to be and how large considering that these elements are over 4 times more dense that the Earth's average density? What if I used neutron star matter?

QUESTION: 
If 2 waves interfere destructively, where does the energy go?

ANSWER:
This question was answered, for a double-slit experiment, in an earlier answer. For a much more detailed discussion, go here.

QUESTION: 
im currently undergoing an Extended Experimental Investigation at school. My friend and i are looking at doing something with saxons bowls. Our variables are to be the hole size and also the viscosity of the substance (oil, water, thinners). As you would know for any experiment there is a main purpose to find something. However, we are unsure what can be our main purpose. We are unsure of what we should be comparing to make this an A standard experiment. We were thinking to possibly look at the different rates at which they sink and compare that between the different substances as it is obvious that they will all be exponential growths.

ANSWER:
I did not know what a Saxon bowl was! A bowl with a hole in the bottom is placed in water and sinks when it fills; they were used by Saxons to limit the time of orations. They were also used, reputedly, by ancient Greek prostitutes to allocate time to customers.

QUESTION: 
I'm trying to make a simulation game in which the player or players change the angle and velocity of a projectile launched. To make it simple, i'm stating that "easy mode will be in a vaccum, so no air resistance. I've been able to calculate how far away it will hit away from the origin and how long it will take to get there... what I haven't been able to calculate is how you determine the parabolic curve it takes to get there. In other words, so far i have a game where you can't see the projectile, then a random explosion goes off wherever it impacts. i've gone through pages and pages of google searches and a college physics textbook. i wasn't able to find it in either of those. I also forund one formula for finding the X,Y coordinates, but when the angle < 19 or > 69, every coordinate becomes negative, but that can't be...

QUESTION: 
I was wondering whether objects that bend space-time and move through space-time would produce something like waves of space-time or gravity waves. I looked it up and found out that physicists were looking or trying to find a way by which to detect gravity waves. What exactly are these waves and what is "carrying" this wave. What is the medium?

ANSWER:
It is thought that an accelerating mass radiates energy (gravitational waves) similar to how accelerating electric charges radiate energy (electromagnetic waves). Gravity waves are waves which are carried by space-time itself. They have never been directly observed. They have been indirectly observed by measuring the rate at which energy is disappearing in binary systems of very massive stars. However, like all of science, they should be considered a hypothesis until they are directly observed.

QUESTION: 
Say a hole was dug that ran from the surface of the earth, thru the core of the earth and exits on the exact opposite side of the earth... so in a sense, one could see straight thru the earth to the other side. Say, someone fell thru that hole, they reach terminal velosity but since theres no resistance other than air, theres no way to slow down when they get closer to the core center of the earth. What are the likely affects and effects of gravity would that person experience once they reach exact core center of the earth? They wouldn't come to a dead stop once they reach core center would they? Or would they shoot past the core center, slow down then fall back towards the core center again, then repeat until they rest in the middle of the air at core center?

ANSWER:
Always read the FAQ page before asking a question!

QUESTION: 
I am confused about kinetic energy. Newtonian kinetic energy law KE = 1/2MV^2 says that when I drive my car at a certain speed and double it I have 4 times the energy compared to my un-modified speed. Supposed I was floating in space and I fire a gun to go at a certain speed (obviously moving backward away from the bullet), does that mean I need to fire 3 more bullets to double my speed or just 1? Keeping in mind that in this case the bullet's mass is negligible compared to my body so each bullet fired should intuitively create consistent force against my mass. I fail to see why I need to fire 4 bullets in total to go at a speed 2 times that compared to a speed achieved by firing only 1 bullet. But at the same time kinetic energy law says I will possess 4 times the energy is I double my speed. So this screams free energy to me if I fire 2 bullets only and possess 4 bullets worth of energy.

QUESTION: 
I cannot find the answer to this on the internet (at least not for a physics novice). Q: What is the relevance of light speed in the equation E=MC2? How do light speed fit in with Mass and Energy?

ANSWER:
What is the relevance of anything in a physical equation? Like many equations, this one was found by taking earlier equations and deriving this one. The speed of light follows along in the mathematics. There is a brief summary of the derivation in an earlier answer, but if you do not know anything about the theory of special relativity, you will probably have trouble following it. The qualitative idea is that relativity is based on the notion that the speed of light is a universal constant; this means that all observers measure the same speed for a light beam regardless of the observers' speeds. With this assumption we find that linear momentum must be redefined from what we use in classical physics. But, linear momentum may be related to kinetic energy of something and it turns out that the total energy of something is its kinetic energy plus a quantity which turns out to be its mass times the speed of light squared; we call that quantity the rest mass energy of the object.

QUESTION: 
Is it possible to fire a gun with absence of oxygen ? Why ?

ANSWER:
See an earlier answer.

QUESTION: 
We know that a pendulum attached to the ceiling of an accelerated car will make an angle with the vertical. I wish to know why this only happens when the car is accelerating and the angle returns to zero when there is no more acceleration. Some books use the concept of fictitious force to explain which is difficult to understand.

ANSWER:
If you view this from a coordinate system attached to the earth, the pendulum bob is accelerating (along with the car). Now, according to Newton's second law, if an object is accelerating in a particular direction, it must have a force on it and in that direction. What can possibly exert that force? Only the string. But, if the string is vertical, it cannot exert a force forward, so it swings such that the magnitude of the vertical component of the tension on the string equals the magnitude of the weight of the bob and the horizontal component is ma where m is the mass of the bob and a is the acceleration. That is all you need to know. For completeness, let us ask how we understand things if we use a coordinate system attached to the car, not the ground. In this coordinate system the ball is not accelerating even though there is clearly an unbalanced force on it (the horizontal component of the tension). The key is that Newton's laws of motion are not true in accelerating frames of reference. You cannot write F=ma because here F is not zero but a is. However, you can force Newton's second law to be true by introducing a force which, in this case, would be ma toward the rear of the car; then, there are two forces, one forward and one backward, which cancel each other out and the particle is therefore obeying Newton's first law, F_net=0 if the object is not accelerating. It is important to understand that this force does not really exist, that's why we call it a fictitious force. The best known example of a fictitious force is the centrifugal force, the force you think you feel when you are inside a rotating drum; actually, you feel the drum pushing you toward the middle (centripetal force).

QUESTION: 
I am building a 3D graphical simulator of atoms and sub atomic particles that allow people to view atoms in bhor as well as quantum models. Could you help me by answering short concise questions as you so prefer every now and then? At present I need to know how fast an electron travels in a ground state hydrogen atom in the s1 shell in meters per second.

QUESTION: 
There are all these stories latley of anti-hydrogen and how they got it to last >15min by cooling it down. This doesn't imply a stable atom since obviously the colder it is the longer it stays together before the orbital or force instabilities blow it apart. Since Hydrogen exist in a stable form in nature, have scientists ever created Hydrogen from the collision of protons and neutrons that was stable? It seems any science experiment should be able to reproduce the obserable before trying to understand the unknown.

ANSWER:
Antihydrogen is an antiproton plus a positron. The properties should be similar to hydrogen because the masses are equal, only the charges are flipped. The difficulty in trying to make and study it is that if it encounters normal matter, the positron will annihilate with an electron and the antiproton will annihilate with a proton. So, the reason for keeping it cold is because low temperature means that it moves slowly so that it will not bump into a wall. I am sure it is also kept in as near to perfect vacuum as possible since you do not it colliding with any ordinary gas atoms. The cold has nothing to do with the stability of the atom; atomic properties are not dependent on temperature unless the temperature gets so high that collisions excite or ionize the atoms. I do not know why you would think that colliding a neutron and proton would make hydrogen since it is not made from those.

QUESTION: 
can we break a neutron??

ANSWER:
We don't have to because it falls apart on its own. The free neutron has a half life of about 15 minutes and decays into a proton, an electron, and a neutrino.

QUESTION: 
Ice sometimes forms in my freezer that has small columns like spikes sticking out of the ice cubes. What causes that to happen?

ANSWER:
See an earlier answer.

QUESTION: 
If you place a heat source in a room, and said heat source is independently capable of bringing the temperature to 30 degrees, would adding another identical source make the room hotter than 30 degrees, or simply increase how quickly the room gets to 30 degrees?

ANSWER:
I assume that these heaters do not have thermostats, that is do not shut off when the temperature reaches 30⁰. A heater gives off heat at a given rate; when the rate of heat coming from the heater equals the rate of heat leaking from the room, equilibrium is reached and the temperature remains constant. If there are two such heaters, when the temperature reaches 30⁰ there is more heat flowing into the room than out of it so the temperature continues to rise. Both your scenarios are right, the temperature will rise more quickly and reach a higher level with two heaters. Incidentally, I am assuming that the temperature outside the room is colder than 30⁰; if not, heat will flow into the room even with no heaters.

QUESTION: 
my question is regarding the dimensions. physical quantities do have dimension in the form [ LMT ] but what is this actually?

ANSWER:
Once you have operational definitions of length [L] (distance between two points in space), time [T] ("distance" between two events at the same point in space), and mass [M] (inertia, resistance to being accelerated), every other physical thing you might measure or observe may be expressed in terms of these three. These are usually called dimensions but are not the same things as when we talk about three dimensions, e.g.; each of the "three dimensions" are length. Sometimes we talk about space-time or four-dimensional space which adds time to three dimensions. Scientists usually prefer units in the SI system to measure [L], meters (m); [M], kilograms (kg); and [T], seconds (s). Some examples are

• area: [L²] m²

• volume: [L³] m³

• speed: [L/T] m/s

• force: [ML/T²] kg m/s² (called a newton, N)

• energy: [ML²/T²] kg m²/s²=N m (called a joule, J)

• power: [ML²/T³] kg m²/s³=J/s (called a watt, W)

Dimensional analysis is often useful in checking calculations. Supposed you derived that the energy of something was E=5mv³ where m is mass and v is speed. Then the dimensions of the left side are [ML²/T²] and the right side are [ML³/T³]. Since these are not the same, your derivation must have been wrong.

QUESTION: 
Why does the energy level of shells increase in terms of potential energy...I get that the atomic radius increases, however electric field of the nucleus decreases with distance and further more shielding by other electrons decrease charge felt by the electron from the nucleus...Therefore as distance is increases electric potential is decreasing so how does potential energy increase! Maybe the answer is that is just the way it is according to experimental testing...the physical world doesn't follow formulas exactly...especially my basic understanding of them...if I am missing something in my thought process please point it out to me.

QUESTION: 
How does dark matter interact with gamma-ray bursts?

ANSWER:
In my opinion, dark matter is a hypothesis, possible but essentially a fiction, until it is actually observed experimentally. As advocates of see it, dark matter interacts only gravitationally, that is it is perfectly transparent to electromagnetic radiation which is what gamma rays are.

QUESTION: 
I was trying to explain Time Dilation and the Twin Paradox to a friend and I am having trouble explain it to him. I have shown him experiments of precise clocks traveling at high speed being slightly slower than stationary clocks,but he always seems to come back to the same point. If 2 twins were floating in empty space then suddenly one of the twins was moved at the speed of light for 1/2 a year to the left and then moved at the speed of light for 1/2 a year to the right, the time that has passed is only 1 year yet the moving twin is supposedly younger than the stationary one. Also, if there are no reference points around then neither of them would know whether they are moving or stationary.

QUESTION: 
why does a ball bounce higher if the height in which it's dropped from increases? i know its something to do with the gravitational potential increasing but how does that have an effect?

QUESTION: 
How many joules of solar energy are consumed in the evaporation of water from the ocean (say 1kg)?

ANSWER:
The quantity you seek, latent heat of evaporation, depends on water temperature and salinity. It is approximately 2200 kJ/kg. You can get more specific information at this link.

QUESTION: 
in my school their is a great contrdiction between the motion of rocket in space will tell me just that when a rocket move away from the gravitational field of earth then will it use fuel to go farword because according to 1st law of motion it will not use fuel . so please tell me that which law should be obeyed by the rocket newtons 1st law or newtons 3rd law of motion???

ANSWER:
If there are no forces on the rocket, which is approximately true far from any planet or the sun, it will continue moving with constant speed in a straight line without burning its engine; this is Newton's second law. But, if you are close to a planet then the rocket experiences the gravitational force and to move with a constant speed in a straight direction your rocket needs to apply a force opposite to the direction of the weight of the rocket and equal in magnitude.

QUESTION: 
a large number of free electrons are present in metals .Why is there no current in absence of electric potential across it?

ANSWER:
The conduction electrons act like a gas where they move around in random directions. But there is no preferred direction and so, at any given time, there are just as many electrons moving in one direction as the opposite direction, no net flow. Your question is sort of like asking why there is no breeze in a closed room.

QUESTION: 
i understand that gravity is merely the force of 2 masses attracting. They both attract each other in relation to each other's mass. I ALMOST understand the gravitational constant, but just to be clear, is that just a number that is always the same? Like unto Pi? Something I could closely represent with 2.4 or 59.886 (probably both way off) for simplicity sake? Or is it a different value based on the mass of the 2 attracting masses?

ANSWER:
Newton's universal law of gravitation is F=Gm₁m₂/r² where G=6.67x10^-11 m³kg^-1s^-2 and it is a constant which never changes. r is the distance between the masses m₁ and m₂. So, you see, gravity is incredibly weak unless one of the masses is very big.

QUESTION: 
There are 2 masses floating in an empty universe, they are 1 mile apart. One has a mass of 100 pounds, and the other has a mass of 200 pounds. Where is the point of impact in relation to both of their starting points? I understand it should be closer to the one of more mass, but is it exactly twice as close since it is twice as massive, which would put the point of impact at 2/3 between mass 1 and mass 2? Or is it a more complicated formula? Thanks.

ANSWER:
They collide at their center of mass. The distance of the center of mass from mass m₁ may be found from R=m₂r₂/(m₁+m₂) where r₂ is the distance from m₁ to m₂. For your example, R=100x1/(200+100)=1/3 mile from the 200 lb object. You realize, however, that the force is really tiny which means you would wait almost forever for the collision to occur. See the following question for a little more quantitative discussion.

QUESTION: 
I saw the experiment by an Astronaut in which bits of salt or sugar clumped together while in a bag of water, thus showing gravitational attraction. Made me wonder why we do not see similar occurances on Earth. Eg. Why doesn't a grain of sand on a pier migrate toward an aircraft carrier while docked?

QUESTION: 
Assuming everything else is equal (chemistry, length, width, microscopic defects, etc), why specifically is a thinner piece of glass less durable (i.e. more fragile) than a thicker piece?

ANSWER:
Imagine two springs, identical except that one has much thicker coils than the other. To stretch the thinner spring 1 inch will require a much smaller force than to stretch the other 1 inch. Glass is like a spring: you push it and it bends and when you stop it goes back to how it was. But, if you bend it too far, it breaks. It takes less force to bend thin glass to the breaking point than a thicker piece.

QUESTION: 
How old would someone be to themselves if they traveled 10 light years in 20 earth years? How old would they be to us back on earth? OR in other words - How long would a 10 lightyear trip feel to the occupant, and how long would it be for mission control at with the vessel going 1/2 the speed of light?

QUESTION: 
i am curious about the nature of electromagnetic waves...looking at the diagram of an em wave from maxwell's equation, i can't figure out...do the electric and magnetic fields (represented by vectors) continually rise and fall over time like a wave, or does the entire waveform (all vectors) occur at a single instant? It's probably a misconception, but i'd like any advice to help me visualize an electromagnetic wave. ty for your help!

ANSWER:
Look at an earlier answer.

QUESTION: 
According to my understanding about the force acting between two parallel straight conductors, i have a feeling that two overhead power cables must undergo attraction or repulsion. Then why dont we notice this attraction or repulsion?

ANSWER:
Suppose that the wires are separated by 1 m and each carries a current of 100 A. The force each meter of each wire would feel is about 0.002 N, about 0.01 oz. Magnetism is a pretty weak force.

QUESTION: 
Imagine, Earth like a big round stone bead. means a hole in the earth from one side to another, passing thru the center crust of the earth, the game is ....what will happen when one jumps in that hole ??? will it hurt him ? will he be able to reach the other end ? or will the hole act like a spring ? what will happen ???

ANSWER:
Always look at the FAQ page before asking a question!

QUESTION: 
let's say there is a star 7 billion light years away. we can see the star that was there 7 billion years ago. how long will it take for the photons to stop reaching us and make the star 'disappear'? once those photons arrive, shouldn't the star disappear or is there a constant stream of photons? shouldn't the star only be visible for a short periof of time?

ANSWER:
As long as a star lives it will shine. Suppose that the star in question had a lifetime of 5 billion years and we are seeing it right now at the halfway point of its lifetime. That means we can continue looking at that star for 2.5 billion more years before it burns out.

QUESTION: 
I'm not sure how to word this properly but here it goes. Radio waves can generally be assumed to travel at the speed of light. My question however, is if the distance from point a to be is a straight line and a wave doesn't follow a straight line (sine curve for simplicities sake), with changing frequencies and amplitudes isn't the "distance" traveled greater than the straight line distance?

ANSWER:
The wave travels in a straight line. The sine curve describes the shape of "what is waving", electric and magnetic fields for radio waves. Think of a sine-shaped stick which you move in a straight line. You should read my earlier answer on electromagnetic waves.

QUESTION: 
T Jefferson used his famous example of the "candle vs the flame" to describe what is and what is not "property". Here is the Set up: You have a candle with a burning flame on it. I have an unlit candle with no flame. If I extend my unlit candle into your candle's burning flame in order to light my candle and withdraw my candle at the moment it ignites, ..... Here is the Question: Have I slightly taken any energy away from your candle & flame, slightly added to the total overall energy, or is it all equal or balanced out "even-stevens" ?

ANSWER:
I don't understand what this has to do with property. Energy conservation says that the total energy of the two candles and all the gases they exhaust remains constant. This is very complicated, of course, because a candle burning is the conversion of chemical potential energy into heat, light, and sound energies. Until the second candle is lit, its energy is simply stored chemical energy. To light it requires adding some energy, it will not spontaneously start burning. If you light it, that ignition energy must come from somewhere, obviously the released heat energy from the first candle. I guess you could say that it is taking energy from the hot gases in the flame, not from the candle, but that would be splitting hairs. Imagining the two candles to be inside a box which isolates them from the rest of the universe, the total energy in the box never changes.

QUESTION: 
Two trains 200 km apart are moving at a relative speed of 50 km/h. A fly takes off from one train, flying straight to the other one at the speed of 75 km/h, touches and flies back to the first train. How far will the fly have gone when the trains collide?

ANSWER:
I suppose this might be somebody's homework, but I have waited long enough that the homework was probably due long ago! It takes the trains 4 hours to collide. Something going 75 km/hr will travel 300 km in that time.

QUESTION: 
Suppose a space ship traveling at relativistic velocities, say 0.9c, could overcome inertia and stop on a dime, how many g-forces