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

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

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

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

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

ANSWER: 
The period.

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

ANSWER: 
Suppose that you have a two-dimensional coordinate system and the coordinates of some point are (x,y); now, if you rotate this coordinate system the new coordinates of the same point are (x',y') where x' depends on both x and y, and y' depends on both x and y. That is, the coordinates become mixed up with each other. Now suppose that one observer moves in the x direction of some coordinate system; this observer carries a clock which measures a time t' and measuring sticks which measure a distance x'. If (x',t') are compared with (x,t), analogous quantities measured by a second observer not moving in the coordinate system, it turns out that x' depends on both x and t, and t' depends on both x and

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

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

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

ANSWER: 
The reason is that the two equations are essentially unrelated. The force equation applies to two point charges separated by a distance d. The plates of a capacitor are not point charges. You also have some misceptions about capacitors. The capacitor carries a charge Q. This means that one plate of the capacitor carries a charge Q and the other a charge -Q such that the total charge is zero. The plates of a capacitor therefore exert an attractive force on each other. It is pretty easy to compute the force one of the plates of a parallel plate capacitor feels because the electric field between the plates is nearly uniform. The field is given E=4

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

ANSWER: 
Because it is nuclear-powered.

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

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

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

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

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

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

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

ANSWER: 
For starters, absolute zero is not a fundamental constant. No matter what fundamental constants are, how units are operationally defined, absolute zero will always be that (unattainable) state of matter with zero kinetic energy. Examples of fundamental constants are the universal constant of gravitation, G, which quantifies the strength of gravity, and the permittivity of free space,

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

ANSWER: 
Aha, you make the classic mistake made by thousands of physics students going before you

• Look at the object. What are the forces on it? Its weight w down and a force b which the water exerts on it, up. (We call b the buoyant force.) Newton's first law (N1) requires b-w=0, so b=w.
• Look at the container and water. What are the forces on them? Their weight W down, a force N from the scale they are sitting on, up, and a force f which the object exerts on the water. Newton's third law tells us that f=b and points down. N1 now tells us N-W-f=0=N-W-w, and so N=W+w. There is the answer to your question—the scale reads the total weight. But let's look at another possibility.
• Look at the container and water and object all together. The forces are the weight, W+w down, and the force of the scale N up. The buoyant force does not come into it at all because the forces the water and object exert on each other are internal forces and cancel out (or, b-f=0). So N1 tells you that N=W+w. Again, there is your answer—the scale reads the total weight.

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

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

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

ANSWER: 
It does not take the same amount of fuel to accelerate the second 1 mph. It is a little tricky to talk about a car where it is actually the force of friction of the tires which push the car forward to accelerate it. That is were the work is done on the car, a forward force F which is constant if you are accelerating at a constant rate. Now, if the car starts at rest and increases its speed at a constant rate to speed v, how far, d, has it gone? Well, simple kinematics tell us that d=

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

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

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

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

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

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

ANSWER: 
See earlier answers.

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

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

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

ANSWER: 
Because we evolved in this environment and therefore there is a balancing pressure from inside. Every cell has an internal pressure of approximately one atmosphere. Think of a bottle which you have put a cork in

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

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

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

ANSWER: 
This is the kind of conundrum you can get into when dealing with wave/particle duality. A photon of a given energy and momentum has a one to one correspondence with electromagnetic waves of a given wavelength and frequency. Energy and momentum of a photon are related as E=pc; frequency and wavelength of a wave are related as f=c/

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

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

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

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

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

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

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

ANSWER: 
The short answer is that, if air friction is negligible, it makes no difference what the mass is. Ignoring air friction, we can write the equations for the x and y positions of the marble as a function of time t (assuming that x=y=0 at t=0). Here, x is the horizontal position and y is the vertical position. These equations also include the angle

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

ANSWER: 
I don't know how you got your answers, but it is impossible to get any answer by simply knowing the mass and the speed because the speed does not determine the acceleration (which is rate of change of speed). So you need to know or approximate the either the time it takes the puck to stop or the distance the puck moves before stopping. Then you can calculate the average force over that time or distance. This should be evident because if the goalie wears a thick foam chest protector (which causes the time or distance to be bigger than if he uses a hard shield), he will feel much less force. And, working in ounces and pounds is really hard

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ANSWER: 
This is one of the FAQs.

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

ANSWER: 
On each balloon there are two forces

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

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

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

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

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

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

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

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

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

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

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

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

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

ANSWER: 

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

ANSWER: 

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

ANSWER: 

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

ANSWER: 

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

ANSWER: 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ANSWER: 
I have previously answered this question.

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

ANSWER: 
I don't see why not.

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

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

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

ANSWER: 
See an earlier answer.

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

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

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

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

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

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

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

ANSWER: 
This is answered on my FAQ page.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

QUESTION: 
Does gravity effect a magnetic field?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ANSWER: 
I don't really know. You have to realize that he is going at a tremendously fast speed (like 18,000 miles/hr) and the parachute would probably not slow him enough before the air density got big enough to burn him up. Perhaps with a well-designed parachute it could be accomplished.

QUESTION: 
one section in my ice cube tray formed a 'mountain' of ice. it stands 9mm above the surface of all the other cubes. in that there is no water source to this economy fridge and water [to my knowledge] does not flow uphill, how is this possible?

ANSWER: 
I have previously answered this question.

QUESTION: 
I am studying circular motion, and I understand when something is swinging in a circle when attached to a string or rope that is above it, say attached to a ceiling. The gravitational force equals the y component of the tension, and you can set the equation for the centripetal force equal to the x component of the tension. Find both tension in the x and y directions, and you can solve for the vector tension. What is not so clear to me is when you are swinging something over head and the object is above the string or rope. I came across a problem in my text that has a ball being swung above your head such that the string made a 14 degree angle with the vertical. So instead of the string being hung from somewhere and the object attached to it being below it, the object is now above the string. I hope my description makes sense. Anyway, in the solution, it says that the tension in the y direction is equal to the force of gravity, mg. But wouldn't the tension in the y direction plus mg be equal to ma in the y direction?

ANSWER: 
After pondering your question for a while, it just occurs to me that maybe you are talking about the conical pendulum where the string traces out a cone and the object moves in a horizontal plane. For your second part, you cannot have a conical pendulum with a string, it has to be a stick. Then the stick would have to exert a force on the object which has vertical component up and equal to the weight to keep it moving in a horizontal plane; a string, which can only pull along its own length, could not do this. For your first (below support) problem, there is a unique solution. For the second (above support) problem, you cannot find the speed and the "tension" because the force the stick exerts will not be along its own length. So, presumably, the second problem asks you to find the force the stick exerts on the object for a certain speed.

QUESTION: 
I understand that gravity slows down time. So this had me thinking. What would the speed of time be like in the absolute middle of dead space (no matter whatsoever) compared to time on Earth? Would it be so fast that it would almost be like it was infinite? (I don’t know if that makes sense…) Would that be an absolute reference point for time? I can’t seem to find any answers, so thanks for your website and in advance for the consideration of this question.

ANSWER: 
The effect of gravity is very small, at least gravity in the vicinity of something of the size of the earth or the sun. So going to empty space (there is no such thing, of course) would have little effect on rate of time.

QUESTION: 
The force of gravity between two objects gets weaker as the distance between them increases. Why is it more correct to say the square of the distance?

ANSWER: 
I wouldn't call it "more correct", I would call it more quantitative. The first statement is a qualitative statement which tells you it gets weaker. The second statement is a quantitative statement which tells you how much weaker it gets; e.g., if you double the distance you quarter the force.

QUESTION: 
I seem to remember a law of physics from high school about the collision of two objects with the same mass and density which says something along the lines that the one traveling at the higher velocity will break the other one. Do I have this right or even close? What is the name of the law?

ANSWER: 
There cannot be any such law for more than one reason. First, the conditions for breaking depend on properties of the object other than its mass or its density; for instance, a glass ball would be more likely to break than a steel ball and not because of any density difference. Second, there is no physical difference whether ball A strikes ball B at rest or vice versa. All that matters is the relative velocity.

QUESTION: 
If a horse pulls on a calesa at rest, the calesa pulls back equally as much on the horsse. Will the calesa be set into motion?

ANSWER: 
I learned a new word—calesa; never knew that word before. Anyhow, what determines how an object moves is the forces on it, not the forces by it. Therefore if there is not some other force in the horizontal direction (like maybe friction from brakes being set), the calesa will accelerate in the direction the horse is pulling. What the calesa does to the horse is irrelevant in the motion of the calesa.

QUESTION: 
A hammer falls off a rooftop and strikes the ground with a certain KE. If it fell from a roof that was four times higher how would its KE of impact compare?its speed of impact?

ANSWER: 
Since gravitational potential energy is proportional to height, it would have four times the kinetic energy. However, since kinetic energy is proportional to the square of the speed, the speed would only be twice as great.

QUESTION: 
If you accelerate at a constant 1G, how long would your trip last ship time, for 100 years to pass on earth?

ANSWER: 
As I explained in an earlier answer, constant acceleration is not possible in special relativity.

QUESTION: 
If all objects are attracted by the force of Gravity to the center of the earth, why aren't air moloculesd sucked down to the floor?

ANSWER: 
They are, but they don't just go there and stay. Since the molecules are flying around with large velocities because of their temperature, they don't all just lie on the floor. However, there are more near the floor than there are near the ceiling. And, the higher you go, the fewer and fewer there are and eventually there are none—space. If there were no gravity at all, the air would all eventually leak out into space.

QUESTION: 
When you're riding in an elevator (or the space shuttle) you're accelerating away from the center of the earth. Gravity accelerates you toward the center of the earth. Acceleration is a vector quantity. So, why, when you're accelerating up at 9.8 m/s^2 do you feel 2g instead of 0g?

ANSWER: 
The way we do this kind of problem is to use Newton's second law (N2). When you are in an elevator, what forces are there on you? Only two, your weight mg, straight down and some force F which the floor of the elevator exerts up on you. N2 says that the total force on you equals your mass m times your acceleration a, so F-mg=ma.  If a=g, then F=2mg. Since F is how you judge your apparent weight, you feel twice as heavy as usual.

QUESTION: 
What causes the creation of plasma? Is it two or more nuclei being slammed together and heated to more than 200,000,000 degrees Fahrenheit or is it something else?

ANSWER: 
If you have a solid and heat it, it eventually becomes liquid. If you have a liquid and heat it, it eventually becomes gas. If you have a gas and heat it, it eventually becomes a plasma; to create a plasma, the average kinetic energy per atom must be larger than the ionization energy of the atom so that an inelastic collision can result in the removal of an electron from the atom.

QUESTION: 
With all the hassle that has been lately around 2012 - the end of mankind, I'm intrigated by the subject. I'm quite curios what is your scientific opinion regarding this subject: Will it be 2012 the end of mankind ? If yes why ? If not why ?

ANSWER: 
My scientific opinion is that it is astrological nonsense. See my earlier answer.

QUESTION: 
I’m having a problem determining the difference between Nuclear Fusion and Nuclear Fission. Listed below are some information that I’ve found. Could you clarify the major differences between the two, especially when it comes to matter that is heavier or lighter? Nuclear fusion - is the process by which multiple like-charged atomic nuclei (two or more joined together) join together to form a heavier nucleus. It is accompanied by the release or absorption of energy, which allows matter to enter into a plasma state. Nuclear fission - is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons (in the form of gamma rays).

ANSWER: 
The important concept here is that if you find a reaction which results in less mass after the reaction than before the reaction, you will release energy because E=mc² and energy is conserved in an isolated reaction. Hence, if you lose mass, that energy must appear elsewhere. Where it usually appears is in kinetic energy, that is thermal energy of the reaction products. Iron is the most tightly bound nucleus, that is the mass of the average nucleon (a nucleon is a generic term for neutron or proton) is smallest. As shown in the figure, average nucleon mass steadily increases if you get heavier or lighter than iron. Hence, splitting a nucleus twice the size of iron or heavier results in mass loss and therefore energy gain; this is called fission. Similarly, fusing two nuclei half the size of iron or smaller results in mass loss and therefore energy gain; this is called fusion. Fusion is the source of energy for stars and hydrogen bombs. Fission is the source of energy for nuclear power plants and conventional nuclear bombs.

QUESTION: 
Can you please explain how physics is used with character animation and animated films (disney, pixar, etc)? I am just wondering as I want to be an animator, but am struggling with physics.

ANSWER: 
The object (one, anyway) of animations is to make the scenes look real, believable. So description of motion (kinematics) is important to understand, I would think. The way things move and respond to forces in the real world is determined by the laws of physics and therefore those same laws apply to a simulated real world (animation).

QUESTION: 
I read an answer of yours that ended with "Because water is essentially incompressible, the density of the water does not change as you go deeper and therefore there would be no depth at which a sinking object would stop sinking." I've never really thought about it, but now I'm wondering "If not from density, where does the increasing pressure (like what you feel in your ears) come from?"

ANSWER: 
The pressure in a fluid is due to the weight of all the fluid above it. Atmospheric pressure in the air is due to the weight of all the air above it; because air is compressible, the higher the pressure, the higher the density. Similarly, deep under water the high pressure is due to the weight of all the water above you and the deeper you go the bigger it gets; however, since water is essentially incompressible, the increased pressure does not result in increased density.

QUESTION: 
Once an artificial satellite is placed in orbit above Mars it stays in orbit, why does this happen or does it simply follow the same principles that apply to it for staying in orbit above earth?

ANSWER: 
The laws of physics are exactly the same in the vicinity of Mars as they are on earth. The mass of Mars is different and so the periods of stationary orbits will differ from what they would be around earth, but it is all determined by classical gravitation and classical mechanics. Mars has two natural moons as well, Phobos and Deimos.

QUESTION: 
I am a high school physics teacher and am having some difficulty with part of the Compton Effect. I am trying to come to grips with whether the collision between the x-ray and the electron is elastic or inelastic? From what I have been able to find on the subject, when the x-ray collides with the 'whole' atom it results in an elastic collision and the x-ray leaves with the same frequency with which it came in with. On the other hand, when the x-ray collides with something closer to its own mass (an electron), it results in an inelastic collision and the x-ray is ejected with a lower frequency and energy. Any help you could provide would be greatly appreciated. I hate to think that I am not teaching it correctly and sending my students out into the world with misconceptions imparted to them by me.

ANSWER: 
The Compton effect is elastic scattering of photons from some mass. Elastic does not mean that the energy of the incoming particle remains constant, it means that the sum of the energies of the incoming particle and the target remain constant. Assuming the target is at rest before the collision, after the collision it will recoil and carry away some of the energy which the photon brought in and the only place it can get this energy is from the photon. It is maybe easier to see this by thinking about classical particles. If a BB (photon) hits a bowling ball (whole atom), the bowling ball is almost at rest after being hit and therefore the BB has approximately the same energy (and speed) after the collision. If the BB (photon) hits a marble (electron), the marble will be moving after the collision so the BB must have lost energy (and speed). Both processes are elastic. An inelastic collision is one in which the total energy before and after are not equal.

QUESTION: 
why do you hear your friends talking in he hallway even though you can't see them around a corner?

ANSWER: 
Two reasons: reflection of sound from walls and diffraction of sound around the corner. See this link for more details.

QUESTION: 
I am curious about something that I just read in an earth science book about the shape of the Earth. Since the polar circumference is less than the equatorial circumference, the Earth is an oblate spheroid. The authors attribute this shape to centrifugal force caused by the Earth's acceleration. However, I thought that centrifugal force was not a real force, but was instead due to a mass's inertia resisting the acceleration. Am I misunderstanding the concept? Thanks for helping me resolve this conundrum.

ANSWER: 
(This is sort of a long-winded answer, but I want to have an explanation of centrifugal and fictitious forces I can later refer to.) Centrifugal force is what we call a fictitious force. In an accelerating system, Newton's laws are not true. For example, if you are inside a big rotating drum you feel a force pushing you into the wall, right? Well, actually you do not; what you feel is the wall pushing you toward the axis of the drum. Your brain is trained mostly in nonaccelerating systems (moving with constant velocity) and so it wants to use Newton's first law which says that if an object is at rest the forces on it must add to zero; to make Newton's first law true (you are at rest in your system which is accelerating), your brain invents a force equal and opposite the centripetal force. This is called the centrifugal force and it does not really exist. A fictitious force is one we invent to force Newton's laws to be true in an accelerating system. Just because it does not exist does not mean that we cannot do physics with Newton's laws using it. If the accelerating system is a centrifuge, it works, doesn't it? The heaviest stuff is "pushed out" by the centrifugal force is true even if centrifugal force is just something we made up to make calculations easier. The rotating fluid earth is a little complicated, but I think that it is pretty intuitive: a ball of putty (or pizza dough) made to rotate faster and faster will stretch out into a pancake and you can say it is the centrifugal force which is causing it. A simpler case of "centrifugal stretching" is the following. A spring is attached to a nail at one end and a mass at the other. If you make it rotate about the nail, the spring will stretch out just the right amount so that the force of the spring will provide the necessary centripetal force. But if you make it go faster, a larger force will be required and so the spring will have to stretch more to do that. That example is easy to see without centrifugal force, but if you view it from the perspective of the rotating mass, you will say the centrifugal force pushes you out.

QUESTION: 
I would like to know if the protons and neutrons of metals such as nickel move about and vibrate relative to each other, or to they stay put (relative to each other)--while the atom vibrates and moves based on heat, chemical and magnetic affects, and other factors. If no one knows the answer to my question, if it's not known by the scientific community, then this is okay. Just say, "no one knows...the models we have today don't tell us."

ANSWER: 
If you mean do the atomic nuclei (wherein reside all the protons and neutrons) move in response to the actions you note, then, yes, absolutely.

QUESTION: 
More quantum entanglement! The measurement of one side of the pair is said to "put" the other one into the same state (say spin up or spin down). But I don't see that the spin being indeterminate (prior to a measurement) prohibits the entangled pair from having "synchronized" spins, that occurred at the event that produced the entangled pair. How do we know that is not the case as opposed to the "put" action at a distance?

ANSWER: 
I think you have the idea right. The use of the word "put" seems to be bothering you. The point is that the "entangled" particles are just that, what one "is" is determined by what the other "is"; call that "synchronized" if you wish. The pair is really a single system so making a measurement on one "collapses" the system to the required "synchronized" state.

QUESTION: 
I noted in some physics books that centrepital acceleration is inversely proportional to R (radius) while in others (and sometimes in the same text) that it is directly proportional to R. In the first case the book uses (a=v2/R) while in the second case the book refers to the formual (a=w2R). I feel something is wrong in this logic, as what relates a to v and R is basicaly a definition of the centrepital accelelration. Can you help?

ANSWER: 
The "catch" here is that both velocity (v) and angular velocity (ω) depend on R, v=Rω. So the two equations you quote are the same: a=v²/R=(Rω)²/R=Rω². It is incorrect, therefore, to say that the centripetal acceleration is proportional to R (or 1/R) unless ω (or v) is constant.

QUESTION: 
why is current a qualitative concept

ANSWER: 
Do you mean electric current? It is not a qualitative concept. Electric current is the amount of electric charge per unit time. 1 Ampere=1 Coulomb/second.

QUESTION: 
My question is about Vacuum. it is the absence or air or any matter? there is a vacuum particule or something like that?

ANSWER: 
Normally vacuum refers to a volume from which all gas has been removed. If there is a solid object inside this volume, we would still say there is a vacuum but the object would not be part of it. There is nothing called a vacuum particle. However, our modern understanding of a vacuum is that there are virtual particles (like particle-antiparticle pairs) popping into and out of existence, existing only for as short a time as allowed by the uncertainty principle; this is referred to as vacuum polarization. So, in some sense, there is really no such thing as a completely empty space. Maybe you have heard the old saying "nature abhors a vacuum".

QUESTION: 
I was standing at the bottom of the stairs in my house (which has a downstairs and upstairs heating zone) and felt cold air gently falling down from above (we have not yet turned on the upstairs heating zone). Then I thought of the phrase "hot air rises" and it did not ring true. Almost the opposite! Cold air "sinks". I was wondering if the answer really had to do with the idea of displacement instead of convection.

ANSWER: 
This sounds more like an issue of semantics than of physics. Warm air rising displaces the colder air and forces it down, the only place it can go. So, does cold air sink? Sure, but it is doing so because it is displaced by rising warm air.

QUESTION: 
Can you tell me what sort of voltages can be achieved by a thermocouple when the temperature difference is only about 100° C ?

ANSWER: 
Typically a few millivolts. Read the Wikepedia article on thermocouples.

QUESTION: 
If I run my computer with a 300W power supply, it will produce a lot of heat (as well as doing useful work). Will it be just as efficient at heating my house as running a 300W electric heater? Or will the 300W electric heater somehow produce more heat per unit of energy put into it? If the computer is less efficient, then where does the extra energy go?

ANSWER: 
A 300 W power supply means that it can supply up to 300 W if called upon to do so, not that it is continuously consuming 300 W. Usually, it supplies much less than that. Not being 100% efficient, not all power it consumes is supplied at its output; rather some power is wasted in heating itself up. Therefore, if a 300 W power supply is running at full power, it is consuming more than 300 W of power from its source. In any case, it will certainly supply less heat than a 300 W electric heater which converts nearly all of its input power to heat.

QUESTION: 
Does a planet that is spinning on it's own axis exert more or less gravity (on its inhabitants) than a planet, of the same mass and distance from the sun, that is still ? What would happen to people and matter on earth if it stopped rotating on its own axis? Would we get heavier?

ANSWER: 
As I have said repeatedly, the weight of something is the force the earth exerts on it and therefore independent of its motion. However, your apparent weight will be changed if the earth rotates on its axis. The apparent weight is the weight which would be registered by a scale you are standing on but it is not a measure of your true weight if you are accelerating (as you are in a rotating system). For example, if you were in an upward accelerating elevator you would appear to weigh more but would not. It depends on where on earth you are located; at the poles there would be no effect (true=apparent weight) and at the equator you would have maximum effect. For more details about how to calculate this see an earlier answer. As you will see there, if the earth stopped rotating you would appear to be about 0.35% heavier on the equator.

QUESTION: 
if you push on a tennis ball and a bowling ball with the same force which ball with roll faster? Why?

ANSWER: 
Newton's second law states that if a force F is exerted on a mass m, its acceleration a will be given by a=F/m. Therefore, pushing with equal forces for equal times will result in a greater final speed for the less massive ball (the tennis ball) because it had larger acceleration.

QUESTION: 
Why is it more difficult to spin a metal cylinder around an axis perpendicular to its length than parallel to it?

ANSWER: 
Why is a more massive object more difficult to get moving than a less massive object? Because of inertia. In translational physics (moving along a line) inertia depends only on mass. In rotational physics, inertia is determined not only by the mass but also by the geometry. The quantity that plays the role of mass (that is the quantity which quantifies resistance to rotation) is called moment of inertia. The farther the mass is from the axis around which you are trying to rotate it, the harder it is to rotate it. For an axis along the symmetry axis of the cylinder, most of the mass is close to the axis and so it is easy to rotate. For an axis through the center but perpendicular to the symmetry axis, much of the mass is much farther from the axis of rotation and so it is harder to rotate.

QUESTION: 
Why do objects appear black? A pure black object would absorb all light so could not be seen since no light would be reflected.

ANSWER: 
If an object were truly black (absorbed all light striking it), you would not be able to see any detail on it. For example, if there were a lump on it, you would not be able to see it because your eyes would have to form an image of the light coming from the lump but there isn't any such light. However, you would be able to ascertain its shape because your eyes could image the light passing by its edges. Of course, there is no such thing as a perfect absorber and in the real world there is always come light coming from a "black" object.

QUESTION: 
Is there any way to visualize electromagnetic field lines similar to Faraday's experiment using iron shavings to visualize magnetic field lines? For example, is there a powerderized substance that could be tossed in the air that would expose the outline of an electromagnetic field?

ANSWER: 
Grass seeds or semolina seeds in oil work for showing an electric field (I think you must mean electric, not electromagnetic). They become electrically polarized like iron filings become magnetically polarized and align with the electric field.

QUESTION: 
How do I figure out my weight when I am impact a hard surface at different speeds? Example If I am traveling at x mph, how much will I weigh when I hit the ground from h height?

ANSWER: 
Weight is the force with which the earth pulls on you. It makes no difference how fast you are going or whether you are hitting the ground or anything else. Your weight is your weight.

QUESTION: 
It's very cold in my house, so I turn off the heat when I leave the house. When I come home, I turn the heat back on and it heats the house for a very long time. Is it more efficient to leave the heat on all day (so it must only heat the house a small amount each time the temperature drops below a certain point) or turn the heat off when I leave the house and let it heat the house for a very long time only after I get home?

ANSWER: 
You should turn your heat down when you leave the house or off if there is no danger of freezing pipes, killing houseplants, etc. The physics behind this is simple: the rate at which heat moves from a warm space to a cold space is proportional to the temperature difference between the two. All the furnace does is replace lost heat and the less heat lost, the less energy will be consumed. The problem is, of course, that it might be unpleasant waiting for the house to get comfortable. I think the best thing to do is get a programmable thermostat so you can set it to come on an hour before you get home and an hour before you get up in the morning. Then just leave it off all day and all night when you are in bed under some good comforters.

QUESTION: 
Why are there two definitions of gravity?One states that gravity is an artifact of a change in direction of mass in the curves of spacetime.The other states that gravity is a force generated by the theoritical " graviton particle".At lectures I have seen physicist interchange these definitions,which confuses the audience,and become very angry when questioned about the inconsistencies between the two.I have never had a proper answer to this question and.in fact, have been called an idiot by lectures when I bring this question up.

ANSWER: 
Really, called you an idiot? And for just asking for clarification? Such a speaker is an arrogant jerk, in my opinion. Anyhow, I would judge him as the idiot because there is no evidence that gravitons even exist and to put them on equal footing with the theory of general relativity is ludicrous. There is a serious problem in physics: we have been unable to reconcile gravitational theory (general relativity) with quantum mechanics, i.e. there is no successful theory of quantum gravity. If there were a theory of quantum gravity, there would be a quantum of the gravitational field, much like the photon is the quantum of the electromagnetic field, and this would be a graviton. A graviton has never been observed and there is no successful theory of physics which has a graviton as a component.

QUESTION: 
e=mc2.... why the speed of light? What is significant about the speed of light in determining the amount of energy in matter? How does multiplying mass by the speed of light squared give one an amount of energy? What does the speed of light have to to with energy? WHY THE SPEED OF LIGHT?

ANSWER: 
This kind of question implies that some particular equation should stand alone and somehow be comprehensible on its own without anything else. In fact, the famous E=mc² is just one small part of the theory of special relativity and must be derived within that theory. Starting with the postulates that the laws of physics must be the same in all inertial frames of reference and that the speed of light is the same for all observers, all of special relativity follows. (Actually, the second postulate is simply a special case of the first as I have discussed in earlier answers.) You may read a brief derivation in an earlier answer.

QUESTION: 
Could the accelerating expansion of the universe be due to electrostatic / magnetic repulsion? After all, gravity is the weakest of forces and the normal expansion (leading edge) of the universe makes the intergalatic / gravatic attraction weaker as time goes on. The weaker the gravatic attraction, the more the other effects become more noticeable. If true, this would not counter the proposed dark matter / forces, it would just be another aspect of what might be happening to accelerate the expansion.

ANSWER: 
As my disclaimer says, I am no astrophysicist. However, I find it impossible to believe that electromagnetism could possibly be responsible for "dark energy", the term usually attached to this accelerating expansion. The reason is simply that the universe would have to have a huge net charge and there is no evidence to suggest that this might be. Keep in mind that even though gravity is the weakest force, it need not be the least influential if there is a huge amount of mass around, as there is. My own feeling is that we do not understand gravity as well as we think we do and that is the reason for the anamolous dark matter and dark energy.

QUESTION: 
Imagine two electrons, side by side, are moving along at a constant velocity. If these electrons were to fly past a person sitting on a chair, would that person see the electrons repel with a force of F=kqq/r^2 or would there be an additional magnetic force of attraction between the electrons? Would only the person in the chair experience the magnetic field generated by the moving charges?

ANSWER: 
This is actually a pretty complicated, advanced problem. The fields of moving point charges are not as simple as you would guess from what you know about electric fields of stationary point charges and the magnetic fields of current-carrying wires. The answer to your question is that the electric force is not what you state because the electric field of a moving charge is not the same as that for a static charge. Also, there will be a magnetic field and therefore each moving charge will experience a force due to the magnetic field of the other. If you really care, here are the fields for a point charge moving in the x direction with speed v.  Let E₀ be the electric field in the rest frame of the point charge and E be the electric field in the lab frame. Then E_x=E_0x, E_y=γE_0y, E_z=γE_0z where γ=1/√[1-(v²/c²)]. The magnetic field may be calculated from B=-(vxE)/c². Notice that if v<<c, the force is approximately the force which you state, i.e. E is little changed and the magnetic force is very small compared to the electric force.

QUESTION: 
It is my understanding that quantum physics states that the existence of some subatomic particles is probabilistic (sometimes you can locate an individual particle sometimes you can't). The lack of ability to ascertain both the velocity and location of some subatomic particles has led some to assert further dimensions and even multiple universes. I am wondering if these theories (i.e. string and multiverse) could be challenged if we come up with equipment that can get both velocity and location of these particles. Do these theories work with or without such equipment? How?

ANSWER: 
The problem is not that we don't have accurate enough equipment; the problem is that these quantities are actually unknowable no matter how carefully you measure. As you know if you read this site, I have little time for string theories and am unaware of their having come into being because of the uncertainty principle. However, there is nothing to "challenge" because these theories do not make any predictions.

QUESTION: 
Can sunlight be distilled- Passed through a prism where certain wavelengths would be directed to heat water, generate electricity, grow plants, etc. more efficiently?

ANSWER: 
An interesting choice of words, "distilled". Yes, you certainly can do that. But I do not believe you would gain anything at all and certainly lose. Unless the light with less than ideal wavelengths for some task actually caused a negative effect, that is inhibited the task, you would lose their contributions. Much more important is to concentrate the light, to gather light from a big area to get more of it.

QUESTION: 
i noticed that if it takes light 1 second for it to reach about 300 million meters. this means that all we see around us is not present but the past? because when we see a supernova blow up and the news reached earth we are seeing it as it happened thousands of years ago. so, when i am seeing lets say a cabinet at 10 meter we are seeing it in the past? i am really confused...

ANSWER: 
Yes, anything you see you are seeing in the past, technically. However, anything within a few thousand meters is being seen such a tiny time later that your eyes or brain could never process and observe such a time delay. For all practical purposes, everything you can see (excluding anything not on earth like stars, planets, the sun, even the moon) are happening simultaneously and right now. There is one interesting exception. For a GPS to work accurately times must be extraordianrily accurate; therefore corrections are made for times of transit and relativistic effects on time.

QUESTION: 
i don't have a teacher and i'm studying myself. It's because of that i can't go to a high-school in Islamic Republic. I wanted to know if you could help me please with my questions. Well, i have faced with a problem that says: We have a box with two light strings on two opposite sides of the box. We hang the box from ceiling by one of the strings. (It means that the system is vertical.) The problem says: if we pull the lower string suddenly, only the lower string will be cut from the box (system); but if we pull it calmly and continuum, the upper string will be cut from the the ceiling; so all of the system (except ceiling) will fall.

ANSWER: 
The string has a maximum tension it can have without breaking. Before you do anything, the lower string has a tension of zero and the upper string has a tension equal to the weight of the box, W. If you pull down on the lower string with a force bigger than the maximum tension, it will break. If you pull down with a force F smaller than the maximum tension, the lower string will not break; but the tension in the upper string will be F+W and if this is bigger than the maximum tension, the upper string will break.

QUESTION: 
if an object say golf ball, fell from say a plane how long would it fall before it would stop getting faster or would it maintain its speed gain.

ANSWER: 
Let's say it is a hovering helicopter because something dropped from an airplane already has a large forward velocity which complicates things. You will find a lengthy discussion about this problem at this link. I have lifted these two graphs from that website. The first graph shows that it takes about 10 s for terminal velocity (about 34 m/s down) to be (almost) reached. The second graph shows that the golf ball would have fallen about 250 m in 10 s.
 

QUESTION: 
if you shoot a bullet out of a gun, straight down off a high building, will it get more speed or stay at the same speed?

ANSWER: 
...or maybe slow down? If you drop a bullet (or anything) off a high place it experiences two forces, its weight (gravity) down and air resistance up. The air resistance gets bigger the faster it falls. Eventually the bullet will reach a speed where the weight and air resistance are equal and it stops speeding up. This speed is called the terminal velocity. If you shoot the bullet straight down with a speed larger than the terminal velocity, the upward force on it from air resistance will be bigger than the downward force of its weight and so it will slow down. The terminal velocity of a bullet is about 100 mph whereas the muzzle velocity is about 1000 mph (these are just rough numbers). So the bullet will slow down and, if the building is tall enough, it will slow down a lot.

QUESTION: 
I am doing a project for 8th grade technology. the objective is to stop an egg from braking upon impact. I have a list of what i can use and the budget i can spend. here is the Q: if i put paper towels folded up tightly around the egg, (not wrapped around it but just to keep it snug while laying down on the wood base) will this be enough to support the egg from cracking on impact? (30 degree angle from top of room to bottom) or should i have it so the egg hits one layer of paper towels and slides into another?

ANSWER: 
It sounds like your idea is doomed to failure, particularly when you stipulate that your paper towel will be wrapped tightly. I will not tell you how to do it but will give you the general physical principal to think about. Suppose that you jump off the roof of your house and land on a cement driveway. It hurts a lot. The reason is that the driveway exerts a force up on you and that force is big. The reason it is big is because you stop very quickly and the force is inversely proportional to how long it takes you to stop, that is the force necessary to stop you in 1/10 second is ten times bigger than the force necessary to stop you in 1 second. So the trick is to make the stopping last as long as possible. So, if you put a big thick foam pad in the driveway you will not get hurt because the time to stop you becomes much longer. Your tightly wrapped egg will stop quickly and probably break.

QUESTION: 
What is the phase difference between applied and induced emf in an inductor ? If it is 180 then why don't they cancel each other

ANSWER: 
Almost by definition, the phase in a perfect inductor (zero resistance) is 180⁰. They do not cancel because they are not of the same magnitude.

QUESTION: 
Am considering a formula for the "stop height" of a self rising object say a balloon, am of notion that a self rising balloon wouldn't do that for ever, i want to know what formula would be used to calculate the stop height given considerable parameters. Similarly, the formula for "stop depth" of a self sinking object in an ocean bed of water, am still of the notion that it stops sinking at a depth far down.

ANSWER: 
I assume you are talking about a rigid object, something which does not change volume. Very high altitude balloons get much larger as they rise and this would be too complicated to answer without more information about the material from which they were made and other details of their design. For a rigid balloon of volume V and mass M the average density would be ρ=M/V. As you go higher in altitude, the density of the air gets smaller. When the density of the air becomes equal to ρ, the balloon stops rising. Because water is essentially incompressible, the density of the water does not change as you go deeper and therefore there would be no depth at which a sinking object would stop sinking.

QUESTION: 
How can I calculate the volume of air which would have to be introduced into the vacuum created at the trailing end of a cube moving through air in order to mitigate the turbulance? For example, a motor home travelling down the highway displaces the air in front and at the rear airnaturally wants to fill the vaccuum as quickly as possible. I want to assist that process.

ANSWER: 
There is not a vacuum there, but likely a bubble of still (with respect to the RV) air. This bubble probably helps the vehicle be more aerodynamic. Here is an interesting ancecdote about a related issue: Several years ago the Car Talk guys were asked for their opinon of those nets sold to replace the tailgate in pickup trucks to reduce drag. They said they thought it was a great idea. The following week they issued a retraction; an engineer from GM had called them to say that the tailgate is an essential part of the design and causes there to be like a "box of still air" like an extension of the roof of the cab which facilitates flow of air over the truck. Removing the tailgate actually increases drag.

QUESTION: 
This question is regarding kinetic energy Lets say a man whose weight is hypothetically "1kg " travels down an airport travellator. The moment he steps onto the travellator, he accelerates from rest to 3m/s. Thus, to the observer X standing outside the travellator, his kinetic energy is 4.5 J On the travellator, he walks at an additional 2 m/s to the other end.Thus, to the observer Y on the travellator, he gains an additional 2 J.Since the kinetic energy of a stationary person on the travellator is 4.5J, the observer Y reasons that the man's total kinetic energy is 4.5J+2J = 6.5J However, Observer X sees the man's velocity is now at 5 m/s, thus, his kinetic energy should be 12.5J.

ANSWER: 
Kinetic energy is not an invariant quantity. Different observers (as you have clearly found) do not observe the same kinetic energy of something. Nor do they measure the same speed. They do all measure the same acceleration, however.

QUESTION: 
I'd like to understand the concept of "north". How is "north" determined? Can it be determined without reference to some other position or circular reference (i.e. "north is the opposite of south, north is the "top", north's magnetic field points "downwards" - what is "down"). Is the definition of north Earth-centric, our solar system-centric, and will it work in other solar systems, including those that have more than one sun?

ANSWER: 
It sounds like you are interested in geographic north as opposed to magnetic north. The earth rotates on its axis which is a line passing through the center of the earth. If you look at earth from one direction along the line of this axis, it goes clockwise; if you look from the other direction, it goes counterclockwise. Choose the case where it is going counterclockwise; the place where the axis pierces the earth is called the north pole. North is the direction along the surface of the earth pointing to that pole. Clearly, this refers to what north means on earth. A similar definition could be used for any body having a rotation about some axis. If viewed from far in space above the north pole of earth, nearly everything in our solar system has counterclockwise rotation (including the orbits of the planets).

If you are interested in the magnetic north pole, it is a little confusing. A bar magnet has a magnetic field (which it is too involved to talk about in detail here) which points out of the north pole and into the south pole of that magnet. The earth is also observed to have a magnetic field which looks like a giant bar magnet. Magnetic north is defined as the direction to which the north pole of a magnet points. However, north poles are attracted to south poles and repelled from north poles and so the magnetic north pole of the earth is actually a south pole. The magnetic and geographic north poles are not exactly at the same place on earth.

QUESTION: 
Hello, my question is about the photo-electric effect which was used as an early example of the particle nature of light/photons: The particle analogy is that the photon is like a billiard ball colliding with another, the electron, and knocking it loose. The photon has to contain enough energy to knock it loose. Even if you knocked it with a large number of photons of less energy (frequency), it won't work. In other words increased intensity of light will not work, as one might expect from the classical wave nature of light. I like this visualization to explain the particle nature of light. But what prevents a large number of low energy photons knocking into the electron at the same time, in other words why do we assume just one photon can try to knock out one electron?

ANSWER: 
Essentially, probability prevents it. With sufficiently high intensity it is possible for two or more photons to participate in ionizing an atom. Until the advent of high intensity lasers, sufficiently high intensities were not available to have any observable multiphoton ionization of atoms.

QUESTION: 
Why is a body which falls freely in a weightless state???????

ANSWER: 
It is not in a "weightless state". Its weight is still there since weight is just the force the earth exerts on the body and it does not disappear just because the body is falling. However, if you are falling along side the body it appears to be weightless. For example, if you are in a falling elevator you would feel weightless because you would no longer feel the floor pushing up on your feet, your muscles would no longer have to support your internal organs, etc. The same thing applies to a craft orbiting the earth which is just in a special kind of free fall. Weightlessness is a misnomer for these situations.

QUESTION: 
You mentioned the principle of equivalence in an earlier answer. Let's explore that principle this way: For the first case, assume you are in a room that has the necessary supplies to keep you alive for a long time. Now, assume that the room is on the earth – you could stay in that room for many years. Now, for the second case, assume that the room is being accelerated at 9.8 m/s away from a gravitational field. According to the principle of equivalence, you wouldn't know the difference between the two cases. However, if you are in the accelerating room for many years, you would eventually be going very fast. Disregarding relativistic effects (which, of course, cannot be ignored, but I'm doing so to make my point), after about a year at this acceleration, your speed would be approaching light speed. So, my question is, would you continue to feel the same effects as you would in a stationery room in a gravitational field, no matter how long you were in the accelerating room? Or, would the fact that you were approaching light speed have other relativistic effects which would make your experience different than if you were in the stationery room?

ANSWER: 
As you point out, constant acceleration is problematic in special relativity. In fact, from a stationary observer's point of view it becomes impossible at higher speeds (see earlier answer). In the frame of the accelerating room, though, no relativity need be taken into account because everything seems normal here. You simply need to redefine acceleration in relativity to maintain the constant apparant gravity. See a nice essay by Baez; the constant acceleration is that measured by an inertial frame moving with your speed at a particular instant. To read several of my earlier answers relevant to your question, see the FAQ entry.

QUESTION: 
The magnetic force acting on a charged particle can never do work, because at every instant the force is perpendicular to the velocity. The torque exerted by a magnetic field can do work on a current loop when the loop rotates. Could you please explain how these seemingly contradictory statements can be reconciled?

ANSWER: 
If you put a current carrying wire in a magnetic field, it will have a force exerted on it, right? So we do not need to get into torque doing work but can just talk about a current carrying wire which we can use a magnetic field to move (do work on) a wire. But here is what is really happening: the magnetic force on the electrons moving in the current causes them to migrate toward the edge of the wire. So there will be an electric polarization of the wire (called the Hall effect) and there will be an electric field inside the wire itself and this field will point towards the region of excess electrons. This electric field pulls the positively charged ions in the wire toward the excess electrons and it is this electric field which does the work.

QUESTION: 
Is there any particular object or system which doesn't abide by Newton's first law of motion?

ANSWER: 
Newton's first law (N1), often called the law of inertia, is often used to define what is called an inertial frame of reference (IFOR). An IFOR is one in which N1 is true. So, suppose we can find a frame of reference in which N1 is true. Any other system which moves with constant velocity with respect to this one is also an IFOR. But, any frame which accelerates relative to the IFOR is not an IFOR and N1 will not be true. For example, if you are at rest in your car it is an IFOR and if you are moving down a straight road with constant speed it is an IFOR. But if you are are at rest and press the accelerator you feel like there is a force pushing you back in your seat but there is no such force; you just feel the seat pushing forward on you since it is the force which accelerates you. But, in your frame of reference, inside the car, you are at rest but there is one unbalanced force pushing you forward; N1 is not true.

QUESTION: 
If earth is escavated along its diameter (on any where) and an object is dropped in to it, what will happen? I thought that the object should stop at the centre of the earth, but i read somewhere that it will come back to the top in 85 minutes. If it is so, please tell me why and how?

ANSWER: 
If you neglect air friction, it oscilates back in forth to the other side of the earth with a period about what you state. It will actually be going the fastest when you get to the center. To see a full discussion in several answers, link here. (You could have found this question on the FAQ page.)

QUESTION: 
When all the oil is used as fuel and burned off, will the Earth be lighter? And if so will there be consequences of a lighter Earth and no oil underground?

ANSWER: 
Burning is chemistry and one of the tenets of chemistry is that the mass before (oil) equals the mass afterwards (CO, CO₂, etc.).

QUESTION: 
our senses tell us that a table is solid but science tells that a table is made mainly of empty space.how can these two views be reconciled?

ANSWER: 
Please see an earlier answer.

QUESTION: 
If you were to inflate one soccerball with air, and another with helium to the same presure, and hit them with exactly the same amount of forse, in an enclosed soccer field (so tempeture, weather, and air resistance will be the same with both balls) witch would travel further?

ANSWER: 
It is not sufficient to specify equal forces, you must also specify that the force acts either over the same distance for both (equal work done) or over the same time interval for each (equal impulse). In either case, the lighter ball gains more speed and thus goes further. Helium.

QUESTION: 
Since perpetual motion machines are considered impossible because they defy the laws of physics, where does the electron get it's energy to move around within an atom. Is an atom not a perpetual motion machine?

ANSWER: 
A machine is generally regarded as something which has energy output, can do work. An atom does not meet this criterion. Further, you should not think of an atom as an electron going around in a circular orbit; this is vastly oversimplified. The electron should be viewed as a cloud whose density at any point determines the probability of finding the electron there at some time. In fact, if the hydrogen atom is correctly done quantum mechanically, the average velocity of the electron is zero.

QUESTION: 
It is easier to catch a soft tennis ball than hard cricket ball moving with same velocity.why?

ANSWER: 
The force you exert on a ball to stop it is equal to the mass of the ball times its acceleration. This is Newton's second law. A cricket ball has a larger mass than a tennis ball, so for equal accelerations it would be harder to stop. But the acceleration is important also. The tennis ball, being softer, compresses while being stopped and so the time to stop it is larger than for the hard cricket ball, thereby having a smaller acceleration, therefore needing a smaller force.

QUESTION: 
My question relates to how objects with differing mass (and thus, different gravity strength) affect each other when they are all simulatenously in each other's gravitational fields? My hypothetical example is as follows: You have an object with relatively small mass, a marble; an object with relatively larger mass, a bowling ball; and then an extremely massive object, like a planet. What happens if the marble and the bowling ball were both equidistant from the planet and also a measurable distance from each other? As the two objects "fell" towards the planet's stronger gravity, would they also be slightly attracted towards each other on the way "down"? Would the initial distance between the two objects gradually decrease, and if so, would the marble move a greater distance towards the bowling ball than the bowling ball would move towards the marble, due to the bowling ball's stronger gravity? Or does the presence of a superior gravity simply override any attraction between smaller objects?

ANSWER: 
Any object with mass both causes and feels gravity. In your example, the earth feels a gravitational force from both the marble and the bowling ball, the bowling ball feels a force from both the earth and the marble, and the marble feels a force from both the earth and the bowling ball. However, the mass of the earth is so big that you would be hard pressed to observe any acceleration other than that toward the center of the earth. If you (roughly) work out what the accelerations are, you get:

• Bowling ball and marble each have a component of their acceleration toward the center of the earth of about 10 m/s².
• Bowling ball has a component of its acceleration toward the marble of about 10^-13 m/s².
• marble has a component of its acceleration toward the bowling ball of about 10^-10 m/s².

I assumed the marble and bowling ball were about 1 m apart.

QUESTION: 
What is the difference between saying "the sun revolves around the earth" and saying "the earth is the center of the universe"?

ANSWER: 
Well, of course they are both wrong so they have that in common. Saying that the sun revolves around the earth simply says nothing about the rest of the universe. Maybe the sun goes around the earth but the earth goes around the center of the milky way galaxy and the rest of the universe is going around the Andromeda galaxy; in this scenario the sun goes around the earth but the earth is not the center of the universe, the Andromena galaxy is. Why did I spend time on this?!

QUESTION: 
Consider a baseball pitched with a spin around the vertical axis. To be precise, let the ball’s initial direction be Northward and the direction of its spin be clockwise when observed from above. Because of aerodynamic effects, the spinning ball will...

ANSWER: 
...be deflected east. The general idea is shown in the figure to the right. The wake is deflected down in the picture for the ball moving to the right. The reason for the force is essentially Newton's third law. If the air is being deflected down, the ball must be exerting a force on it; if the ball exerts a force on the air, the air must exert an equal and opposite force on the ball. This force is called the Magnus force. There are also Bernoulli forces because the air goes faster over the top than the bottom and therefore there is lower pressure at the top, but I believe that this is much less important than the Magnus force (although most elementary physics texts say that this is the reason for the curve ball). (To apply this picture to your specifics, imagine you are looking from below and north is to the right; then east will be in the direction of the force.)

QUESTION: 
I have two very similar bikes, both almost perfectly balanced front to rear (with and without rider) with very similar geometry and parts, and one of them oversteers slightly, but for the life of me I can't figure out why the other one understeers, causing the front tire to wash out dangerously. The only way I can get the misbehaving bike to break traction more evenly is to hang my weight way over the back of the bike, but this is of course impractical and seems like a huge change considering how similar the bikes are. What makes a bike over/understeer and what can I do to the rebellious bike to make it safe to ride on fast trails (short of reducing rear tire and therefore overall traction)? Thanks in advance for significantly reducing my REM sleep (and likelihood of breaking a wrist/collarbone)!

ANSWER: 
There is no way that I could figure this out; there are two many variables. Perhaps you can read up a bit on bicycle stability and apply it to your particular situation.

QUESTION: 
If we were to build a giant rail ultimately pointed skywards, we would be able to give the space shuttle a certain speed before it even ignited its rockets. This obtained speed would mean less rocket fuel would be necessary to achieve escape velocity. This reduction of mass would mean less required energy overall. Assuming that the apparatus used to speed up the shuttle has less mass then the saved fuel (a very bold and baseless assumption, no doubt), is there a physical explanation as to why we don't use this model?

ANSWER: 
The total amount of energy necessary to put a particular payload in a particular orbit does not depend on where you get the energy. I guess you want to use some other device or method to get the shuttle moving along your rail, but that will take just as much energy as the way we do it now. And, such a "rail" would be pretty expensive to build, probably.

QUESTION: 
To travel in a circle at a constant angular velocity a body must be constantly accelerated. Yet its angular velocity and hence its kenetic energy does not increase. Where does the energy go?

ANSWER: 
You actually told me the answer. If the energy does not change, it does not "go" anywhere. It is an example of energy conservation where, if no external forces do work on something, its energy remains constant. There is an external force here (whatever is keeping it moving in a circle, tension in a string, friction of tires for a car, etc.) but this force (called the centripetal force) always points toward the center of the circle. Since the particle always moves tangentially on the circle, the force and motion are perpendicular and no work is done. So, here is an important lesson: even if there are forces acting on something they do not necessarily change its energy.

QUESTION: 
A man on a skateboard is said to be travelling at constant motion until he hits a rock, his velocity goes from 12m/s to 2m/s what is the force that he hits the rock with? How do you work this question out?

ANSWER: 
This sounds suspiciously like homework, but it can't be because you cannot find the answer. In order to calculate force you need to know the acceleration and you cannot calculate acceleration from the change in velocity; you need to know the elapsed time. If this took 1/10 of a second the force would be much bigger than if it took 10 seconds. Furthermore, to calculate the force you need the mass of the thing which is accelerating (you would probably want the mass of the skateboard + the man. If this was homework, your instructor played a nasty trick on you.

QUESTION: 
If a solid metal pole were made to be one lightyear long and it was pulled at one end, would it take over a year to feel the tug at the other end, or would the fact that it is a solid object cause the tug to be felt instantaneously at the other end?

ANSWER: 
You could have found this answer on the FAQ page. The answer is here.

QUESTION: 
Suppose a ship is moving at 3/4 the speed of light. If an object aboard the ship were to be launched forward at 1/2 the speed of light relative to the ship, then would the object be moving faster than the speed of light or would the object for some reason never reach the speed of light?

ANSWER: 
I have previously answered this question. The object, for your scenerario, would have a speed of about 91% the speed of light.

QUESTION: 
Is it possible to create artificial gravity? I know we have yet to figure out why we have gravity to begin with but if we get that answered it would seem logical that the next step would be to create it on our own. Also in your expertise, would you have any insight on how artificial gravity could potentially be utilized? Possibly for space travel?

ANSWER: 
What do you mean that you "know we have yet to figure out why we have gravity"? If you mean we do not understand gravity, you are wrong; we have a very good theory of gravity called general relativity. One of the things that general relativity says is that there is really no difference between gravity and acceleration. For example, if you are in an elevator in empty space with an acceleration equal to the acceleration due to gravity, there is no experiment you can do which will have a different result from if you were sitting on earth. This is called the principle of equivalence. So one way to make "artificial gravity" is to simply be in an accelerating spaceship. However, it is not really practicable to have a spaceship always accelerating in a straight line because you would have to be continually burning fuel. On the other hand, something going in a circle is constantly accelerating so if you make a giant donut-shaped craft which is rotating, you could stand on the inside of the outer wall and have "artificial gravity". The acceleration depends on the radius of the circle, though, so it would have to be very large or else the gravity at your head would be much different from the gravity at your feet. Certainly if we ever have long distance space travel, a craft of this sort will be needed for a tolerable environment. I have little hope for space travel becoming a reality unless a colony of people is ok with leaving and never coming back. The distances are unimaginably big and very large speeds are difficult to achieve in reasonable times without crushing everybody on board. And, at very large speeds there would be a big problem with radiation shielding.

FOLLOWUP QUESTION:
I suppose I should have been more specific on my question. You were wondering what I meant by "When we figure out gravity". I was actually referring to what causes gravity and not so much how it works. I know we have a good understanding of it's mechanics but as to why there is gravity in the first place was my real question. To my knowledge we still dont know why gravity exists but I could be speaking out of ignorance.

ANSWER: 
We do understand how it works. Mass bends spacetime and gravity is the result. The often used (instructive if not an exactly a true picture) is a bowling ball sitting on a trampoline; a marble will then roll toward it because it deforms the space it is in. General relativity is discussed in several previous answers. Although an extremely successful theory of gravity, it is probably not the last word because there is no quantum theory of gravity.

QUESTION: 
There is frequent mention in various publications that states that a spaceman's clock would run slower than that of an observer on earth. However, from the spaceman's perspective (assuming he was not accelerating) he would apparantly be at rest and whilst the earth was receding. This being the case, the earthbound clock would be running slower than his. The net result seems to be that when he eventually returns to earth, he will be the same age as his twin brother and not younger as is often suggested. Or am I missing something?

ANSWER: 
What you are referring to is the twin paradox which is not really a paradox at all. The key to understanding is understanding the asymmetry between the two such that they can come back together to compare their clocks. See my earlier discussion of the twin paradox.

QUESTION: 
Apollo 13 the movie. 'We just put Isaac Newton in the driver's seat'. could you please justify this statement made by Jim Lovell

ANSWER: 
When you go for a drive in your car, you burn fuel the whole way. In space travel, you coast most of the way. But in space, there is often very influential gravity you can use to pull you, push you, or steer you. I do not know the exact context of your quote, but Apollo 13 used gravity to swing it around the moon and then gravity to pull it back to earth and some stage of this is what is referred to by the quote because Isaac Newton is considered the first physicist to appreciate what gravity is.

QUESTION: 
Isn't space exploration a terrible waste of taxpayer dollars?

ANSWER: 
I have a stock answer to this "waste of money" question which I have written earlier in reference to the CERN supercollider.

QUESTION: 
I want to be able to convince my father without a doubt that there is a valid way of finding the distance between earth and very distant stars or nebula. If you could explain in detail or perhaps in plain english, I would appreciate it. The next question is somewhat more complex. I was wondering if there was a rate at which gravity attracts. For instance, if the moon were to disappear suddenly. How long would it take the earth to be affected by the lack of the moons gravity? Instant? A a day? A minute? Would it be faster then light? Is that even possible?

ANSWER: 
Please adhere to site grounrules for single, well-focused questions. I have previously answered your second question. Regarding your first question, there are three ways to measure the distance to the stars.

• The first is parallax; this method works for fairly close stars (a few thousand light years). Hold your finger in front of your face and move your head back and forth. Your finger appears to move back and forth against the distant background. This is parallax and by doing careful measurements you could deduce the distance from your finger to your eyes. If you observe a star not too far away at two different times of year when the earth is at opposite ends of its orbit, it will appear to move against the background of much more distant stars.
• The second is to use stars of known brightness and measure their apparent brightness. If a light bulb has a known brightness and you measure its light reaching you, you can deduce its distance. The bulb, for example, when viewed from one mile away will be four times as bright then when viewed from two miles away.
• Finally, you can use red shift of light from the most distant objects. It is known that the farther something is away from us, the faster it is moving away. The principle is the doppler effect where the wavelength of waves (light in the case of stars) is longer. The every day example of this is a train whistle having a higher pitch when moving toward you and lower when moving away.

You can read about this in more detail here.

QUESTION: 
What's the physics behind ballpoint pen?

ANSWER: 
Check out HowStuffWorks.

QUESTION: 
Based on the general fear that is going around about the world ending in 2012.... Is it really the end of time are we all going to die on 12/2012?

ANSWER: 
Well, do you believe in astrology? I recommend, for a good balanced view of the evidence, a recent New York Times article.

QUESTION: 
If you are trying to calculate the rate at which atoms of a particular fission product are being produced. Are you looking for the activity of the fission product or the number of fissions per second?

ANSWER: 
The overall fission rate would not give information about any particular fission product.

QUESTION: 
If electromagnetic energy in the form of visible light spectrum (photons) travels from the sun to the earth at the speed of light, it wouldn't have any mass? (i.e., photons are massless, therefore have no energy?, E=mc^2, m=0, then E=0. If a photon doesn't have any mass and therefore energy to begin with, how has the conservation of energy principle upheld? I ask this because clearly electromagnetic energy from photons of visible light are converted into chemical energy in plants thru photosynthesis so it would seem that photons do indeed have energy and therefore mass. But if they have mass, how can they travel at the speed of light?

ANSWER: 
I have answered this question before.

QUESTION: 
A magnetic field can be created with a varying electric field. Hence, an electric field can be created with a varying magnetic field. In a betatron, where an electron is accelerated, the electric field remains constant while the magnetic field varies. Since, the electron is a charged particle, in motion, how does the electric field remain constant while the magnetic field fluctuates??

ANSWER: 
Imagine a donut with a magnetic field passing through the hole. If the magnetic field is increasing there will be an electric field around the inside of the donut and it is this which exerts the force on the electron to accelerate it. If the magnetic field increases at a constant rate, the electric field will remain constant. If you are worrying about the electric field due to the electron itself, don't—a particle cannot exert a force on itself.

QUESTION: 
What happens to water molecules when water is heated from 90 degrees Celsius to 110 degrees Celcius?

ANSWER: 
The same thing that would happen to any molecules—their average kinetic energy increases (they move around, on average, faster). Because you have specified particular temperatures it has a little more to it. If water is at 100 ⁰C the molecules have acquired sufficient speed to escape from the liquid and become gas (boiling) and only molecules which escape the liquid can have temperatures above 100 ⁰C.

QUESTION: 
Is there is a principle or theory that you cannot study an atom without changing the structure of said atom? And if there is, can you please tell me what that is?

ANSWER: 
Maybe you are referring to the Heisenberg uncertainty principle, one incarnation of which can be that the act of measuring something disturbs that something. On the other hand, the "structure of an atom" refers to all its properties and, although you might change an atom from one state to another when studying it, you do not change the atom's essential properties; I would say you reveal those properties.

QUESTION: 
I know that materials are more likely to fail when energy is delivered over a short period of time for a given total energy load. Similary if an oil drum is pushed along by a bus at .5 miles per hour for several miles it is less likely to fail structuraly then if the bus hits the drum doing 75 miles per hour--even if the total energy displacement is less. Is there a simple characterization or formula to express this? Is there a general rule that applies to all materials. I have asked a shock physicists this question and could not get a straight answer.

ANSWER: 
This is Newton's second and third law. There are a couple of problems with your example, one is the bus going with constant speed pushing the drum. If the drum slides smoothly or is on wheels, say, it will experience practically no force from the bus. Also, "total energy displacement" has no meaning in physics. Permit me to rephrase your question: Suppose I drop a hammer from 1 cm above your hand resting on the table; it doesn't hurt much. Suppose I slam the hammer down on your hand as hard as I can; it hurts a lot. Why? When the hammer hits your hand, it stops and so it experiences an acceleration. In order to stop the moving hammer, a force must be exerted on it and it is your hand which exerts this force. This is where Newton's second law comes in; the force F is given by F=ma where m is the mass of the hammer and a is the acceleration of the hammer. So, since the acceleration (the rate of change of speed) is much bigger for the fast hammer than the slow hammer, your hand must exert a much larger force on the fast hammer to stop it. Now, finally, this is where Newton's third law comes in; if your hand exerts a force up on the hammer, the hammer exerts an equal force down on your hand. I see on rereading your question that I have not quite yet answered it completely because I have assumed that your hand stopped the hammer in about the same amount of time for each blow because your hand was on the table. If your hand is not on the table and you are able to stop the fast hammer over some several feet, the acceleration is much smaller because the velocity change is over a much larger time; smaller acceleration again means smaller force.

QUESTION: 
An object on Earth accelerates at 9.8 m/s2. The object does this until it reaches "Terminal Velocity" which is when the object can accelerate no more because of the fluid friction with the air. If you put this object in a infinite complete vacuum with the gravity puling from one end. Will the vacuum make reaching Terminal Velocity impossible meaning that the object could reach the speed of light and beyond?

ANSWER: 
Terminal velocity normally refers to the maximum speed of an object moving through a fluid like air. It can also mean the minimum speed if you start the projectile with a speed greater than the terminal velocity. If there is no fluid, then it will continue accelerating forever as long as the same force (its weight) acts on it. It cannot, however, exceed the speed of light, so you could say that its terminal velocity now is the speed of light. You may be interested in more details regarding acceleration in special relativity in an earlier answer.

QUESTION: 
I have a question about faster than light communication. If a string spanned a great distance through space and it were tugged on one end, wouldn't the other end move at the same time allowing for data to be transfered, perhaps through morse code? I believe this would require an extremely dense string that wouldn't expand, but is is possible?

ANSWER: 
Your question has been asked before, so see an earlier answer. Your question is a little different since you are pulling instead of pushing, but the answer is essentially the same: no communication faster than the speed of light is possible and, for your proposal, it would be far smaller than the speed of light because the stretch in the string which would propogate the information about your tug would travel at the speed of sound.

QUESTION: 
1.What is the physical process of the conversion of mass into energy? 2.Is there a physical process by which energy can be transformed into mass?

ANSWER: 
Your question is like "how can we convert an orange into fruit?" Mass is simply a form of energy. Can mass disappear and become another form of energy? Yes, it happens all the time. When you burn coal you are essentially turning carbon and oxygen into carbon dioxide; where does the heat energy come from? CO₂ is slightly lighter than C+O₂. The amount is so small that you could not hope to measure it. However, in nuclear physics it is easy to measure the mass difference: a nucleus with Z protons and N neutrons has a mass significantly less than the sum of the individual masses. For examples of converting non-mass energy to mass, see an earlier answer.

QUESTION: 
How magnet(magnetic field) acts on the PC monitor.What happends?

ANSWER: 
I do not know if there is any effect on the modern LCD or LED screens. The older CRT screens are illuminated by an electron beam which sweeps across the screen. A magnet will deflect this beam. However, there is a thin screen with many tiny holes in it to assure that the electrons hit exactly on the spots intended. The metal of which this is fabricated can become magnetized if you hold a magnet near it and then this magnetic field will alter the path of the electron beam thereby messing up the picture. It is usually possible to demagnetize the screen with something called a degausser.

QUESTION: 
If there is no friction between two objects, and lets say object A is on top of object B and object B accelerates, will object A fall off and move to left? (assuming velocity is moving to the right in the X direction)

ANSWER: 
If there is no friction, the block will not move at all and will drop straight to the table where it will remain at rest.

QUESTION: 
what exactly makes CO2 a greenhouse gas? As a 'blanket' there's no convection to outer space. Does it allow solar radiation in, but not allow earth's radiation out?

ANSWER: 
To get a greenhouse effect, you need something which is transparent to visible light but absorbs infrared. Most objects (like the ground) absorb visible light and radiate infrared, so if there is an absorber of infrared blanketing the earth, that energy is trapped. CO₂ is transparent to visible light and absorbs infrared.

QUESTION: 
Glow-in-the-dark, Radium painted, watch dials were popular in the 40's and 50's (until Radium use was discontinued). Today, sum fifty years later, these watch dials no longer glow, even if most of the original "paint" remains. My question is; if one were to move one of these old watch dials, close to an energetically active radiation source, such as a medial isotope or Uranium 235, would the watch dial glow again? -at least while it remained close to the radioactive material? I know X-rays are different than beta emitters, but would the old paint glow if you wore the watch while getting an X-ray?

ANSWER: 
The phosphor is zinc sulfide. Its efficiency as a phosphor is complicated and is dependent on the impurities present. A little research on my part reveals that the reason that the dials no longer glow is not because the radium has decayed away (it has very long halflife), but rather because the alpha radiation had done sufficient damage to the crystal that the excited conduction electrons cannot efficiently get transported to the impurity sites. Therefore, nothing would make it glow; however, a "virgin" radioluminescent dial but without the radium could be made to glow with any external beta, x-ray or γ-ray source of sufficient energy. The definitive article on this seems to be here.

QUESTION: 
If a 400 lb man falls 3 feet to the ground and lands on one foot, How much force did the foot recive when it inpacted the ground.

ANSWER: 
There is no way to answer this question because it depends on how long it takes the man to come to rest. Of course, that is why you should let your knees bend when you land, to lengthen the time of impact. Or, if you landed on a pile of foam there would be much less force than if you landed on a concrete floor.

QUESTION: 
Concerning newtons third law. Please explain which is receiving the reaction when a person is jumping on a trampoline. Is the spring the object receiving the opposite reaction or is it the trampoline fabric?

ANSWER: 
Look at the person when he is in contact with the fabric: the forces on her are her own weight down and the force of the fabric up. No force on her from the springs because she is not in contact with the springs. Newton's third law says that if the fabric exerts a force up on her, she exerts an equal and opposite force down on the fabric.

QUESTION: 
If a fish tank containing only water weighs 50 pounds byt itself. You add a 5 pound fish, who swims around, but never touches the bottom. Does the fish tank increase in weight by 5 pounds?

ANSWER: 
As I always say, you have to be really careful how you ask this kind of question. The weight of something is the force that the earth exerts on it and, the way you have asked the question, you can put a ten ton truck in the fish tank (assuming it would fit) and the tank would still have the same weight. But, what you are probably asking is: if the tank is sitting on a scale, will the scale still read 50 lb after I put in the 5 lb fish? No, it will not, it will read 55 lb. If you are worried that the fish is not resting on the bottom and should therefore not cause the scale to deflect, here is the appropriate argument. The fish is hovering and something must be holding him up—the water, of course. Therefore, the water exerts a 5 lb force up on the fish. But, Newton's third law tells us that the fish must therefore exert a 5 lb force down on the water. So the water pushes down on the bottom of the tank and therefore on the scale by 5 lb more than it normally would.

QUESTION: 
In my Science exam on Friday I was given a question that I was unsure about the answer to. I was hoping you would be able to assist me. The question was that if there was a balloon in a box with a needle attached to the top & another needle attached to the bottom, the balloon being secured with an elastic band what would happen if it was dropped from a heigh of 10m. Options were that it would be popped by top needle, bottom needle, or neither needle. I answered top needle based on Newton's law in relation to when a car accelerates fast after being at rest passengers are momentarily pushed forward but then pulled back into the seat, I figured the same would apply here. The balloon (when the box was dropped would fall towards the bottom needle but then due to the elastic band be pulled back up towards the top needle and popped. Can you advise me if this is right?

ANSWER: 
You are right. The balloon must accelerate down and the only thing able to provide the required force are the rubber bands. But, for the rubber bands to have a component of their tension down, the balloon must move up (in the box).

CORRECTION: 
I made an error in my answer. First of all, I had the wrong picture, namely I thought that there were two rubberbands stretched out to the walls of the box. But, more importantly, I failed to note the principle of equivalence which states that there is no experiment you can do to distinguish between being in an accelerating frame of reference or in a uniform gravitational field. In other words, as seen from inside the falling box, gravity can be ignored. As seen from outside the box, the balloon accelerates down because of its weight not as I said above "the only thing able to provide the required force are the rubber bands". Now, to answer the question correctly. The question is ambiguous about the elastic band and this is a problem. There are two possibilities: this is an air balloon and would drop to the floor if not secured by an elastic band to the ceiling; or this is a helium balloon which would rise to the ceiling if not secured to the floor by an elastic band. In either case, the elastic band will stretched before the box is dropped. When the box is dropped, gravity "disappears" for the balloon and it is pulled down if the band is attached to the floor or pulled up if it is attached to the ceiling. My thanks to the person in Portugal who spotted my error.

QUESTION: 
Why is it compulsory to make a photo diode reverse biased?

ANSWER: 
It is not compulsory, it just works better that way. The photocurrent flows in the reverse direction and having a forward bias would have there be a steady current flowing in the direction opposite the photocurrent. Having a reverse bias greatly improves both response time and signal to noise ratio. See the Wikepedia article.

QUESTION: 
Is there a saturation point for energy(temperature gained by a body)??

ANSWER: 
I really do not know what you mean by "saturation point for energy". If you mean is there a limit to how much energy you can add to something, then the answer is no. If you mean is there a limit to how much energy you can add to something without it "changing it", then the answer is yes. If you have a block of iron, you can keep adding energy until it melts; then keep adding energy until it vaporizes; then keep adding energy until it ionizes; etc. So, there are limits before phase changes occur.

QUESTION: 
If I was standing on Mars, would I be able to see the same constellations I see on Earth?

ANSWER: 
Yes. The distance from Earth to Mars is very small compared to the distances to the stars and therefore there would be an insignificant change.

QUESTION: 
Where do radio waves end and light begin, if in physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not?In physics, if the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not, would radio waves at very high frequencies be light?

ANSWER: 
This is really semantics, not physics. Light usually refers to the visible part of the electromagnetic spectrum and sometimes also ultraviolet and infrared. Radio waves are usually not called light, nor are x-rays, gamma-rays, microwaves, etc. We often refer to the speed of light as the speed of any electromagnetic wave since all travel with the same speed in vacuum. There is no clear rule about where one type of electromagnetic wave ends and another begins.

QUESTION: 
My question regards finding the angular velocity. If you are given the radius of a cylindrical spool and the time it takes to pull something off it, how would you go about find the angular velocity of the spool?

ANSWER: 
The first question you sent was too specific and likely homework. This is more general, so I will answer it. If you pull string off at a constant rate, say X m/s, that means that X m of circumference of the spool passes a point in a second. Therefore, the tangential velocity of the spool is X. But, that means that the spool has turned through an angle of X/R radians in one second or has a rate of X/R radians/s which is the angular velocity.

QUESTION: 
i am iraqi student.... i have a question about RESONANCE FREQUENCY i want to learn the fundamental explain of it..i want to learn more about RESONANCE FREQUENCY...

ANSWER: 
Think of a physical system which wants to oscillate, vibrate. It has what is called a "natural frequency". For example, a particular pendulum will oscillate with a period of ten seconds. Now, suppose you push on that system. If you push with a period which is far from the natural frequency, the system will vibrate but only with a very small amplitude. If you push on it with a period near the natural frequency, it will vibrate with a large frequency. That is called resonance, the fact that a force with period close to the natural frequency will have a large response.

QUESTION: 
Why is it when I look in a mirror, my left and right sides are reversed, but I am not reversed upside-down? I was reading a book, Odd Perceptions by Richard L. Gregory who writes that that no one knows for sure. That was back in 1986. Is there an explanation in the 21st century? Just curious.

ANSWER: 
It is ridiculous to say that "no one knows for sure". Goes to show you—don't believe everything you read. Read my earlier answer.

QUESTION: 
I often hear from prominent ecologists the statement that "All (sometimes adjusted to MOST) energy on earth comes from the sun." I do not believe this. Gravity produces energy when something falls (hydroelectricity). The movement of tides does the same. So, my question is, where does the energy on earth come from, and how much can be attributed to the sun vs. other forms?

ANSWER: 
Most cases of "energy from gravity" would be erroneous because in order for something to fall it has to be raised up. So, hydroelectric energy comes from water falling, but how did it get up there? The water evaporated out of the sea (read sun) and then fell as rain in the mountains. Your tidal example is one where the sun is not responsible (although the sun does play a minor role in tides); the moon lifts the water and then it falls. Also, geothermal energy comes from the ambient heat in the earth. And, radioactivity and other nuclear energy comes not from our sun but from some other star where the elements were created (as did you). There is no quantitative way we can neatly separate solar from nonsolar energy here on earth. I think that what is most important is which forms of energy can we tap using current technology. I would agree that, by that criterion, solar or solar-orginated (like petro energy sources) are the pretty clear winner.

QUESTION: 
If you had two positive point-charges, seperated by a given distance, and you drew a graph of the potential on the line between them, would the graph look like a squashed Gaussian curve, with a maximum at the centre point?

ANSWER: 
No, actually the potential would be minimized halfway between and would look like a "U" with the potential rising at the charges.

QUESTION: 
Why do you think Physics is such a difficult course for some students?

ANSWER: 
From my many years teaching, I can tell you that there is no single reason. One important reason is that students entering a physics course have often been poorly taught in mathematics and are unprepared for applying mathematics to something real. I have often been astounded by how college students can be unable to do even simple arithmetic. Another thing, which is getting worse all the time as testing is becoming the focus of so much primary and secondary education, is that memorization of facts and equations is not the key to success in physics; it sounds corny, but your physics teacher is trying to teach you how to think, not how to do physics. Often the problem is a deep-seated psychological aversion to things quantitative; how often have you heard somebody say "I'm just not a math person"? Or, I get so sick of going to a party and, when asked what I do, have a response like "you must be sooooo smart".

QUESTION: 
Often dreaming of black holes connecting to some parallell uinverse was ok to reflect on however my daughter and I were observing a graphic of a quasar and how there is a polar emmision of energy. From what I understand we view black holes as a dense body swallowing all in it's orbit without a trace assuming all matter consumed goes somewhere or somewhen. My question is could the matter consumed by a black hole be expelled at it's poles the same way as a quasar in the form of dark energy thereby contributing to the expansion of the universe?

ANSWER: 
As I explain on the home page, I am not an astrophysicist. But I do know that mass does not disappear when it goes into a black hole. As the black hole captures mass it becomes more massive. Regarding your question about whether a black hole might be a source of "dark energy", I can only tell you that both dark energy and dark matter are things which are not at all understood and your guess is as good as any, I guess!

QUESTION: 
In Einsteins Theiry of Relativity there is a part that explains time dialation. If you were on a space craft going near the speed of light could you mechanicaly adjust the clock on board the craft to display the time on Earth?

ANSWER: 
Sure, you could design such a machine but it wouldn't really be a clock, would it? It is like asking if we could build a clock which showed one hour passing during each 10 hours.

QUESTION: 
How do we communicate with things like the hubble telescope/ satellites/ the mars rover (e.g. is it with radio waves)? Is this communication instantaneous? If the hubble telescope was really far away (say, 1 light year), how long would it take to tell it what to do/ receive an image from it?

ANSWER: 
Communication is done with radio. It certainly is not instantaneous, no communication is; the fastest speed of communication is the speed of light. To get or receive any information from something one light year away takes one year.

QUESTION: 
Why are iron, nikkel or cobolt more likely to magnetize as opposed to copper or aluminum?

ANSWER: 
Every electron is like a tiny magnet. In most materials in nature, all the electron magnets are pointing in random directions and the material has only very weak magnetic properties. In a very few materials, called ferromagnetic materials, the crystal structure of the material has a tendency for the outer electrons in one atom to align themselves with the electrons in neighboring atoms and so very large numbers of tiny magnets all line up with each other and the whole thing looks like a magnet. One additional subtlety is that a piece of iron may not be a very strong magnet because the alignment happens over small regions called domains (small like maybe grain-of-sand-small, not atom-small) and the domains all have random orientations. But when you expose the iron to a magnet, those aligned with the external magnet grow at the expense of those not aligned, so the iron becomes magnetized and attracted to the external magnet.