# Why does anything that accelerates heat up?

1. Sep 3, 2007

### Skhandelwal

Friction may be a factor but is it the only reason? Even w/ friction... why do things when accelerating heat up?

Thx.

2. Sep 3, 2007

### kaisxuans

no,i dont think so. alot of things happen during the process when two objects rub against each other. but friction is the most known thing that happens and contributes the highest percentage of the activities goimng on.
and what is the meaning of your 2nnd question?
-why do things when accelerating heat up?-
why do things what?:rofl:
(visit my blog!)

3. Sep 3, 2007

### G01

I assume you are asking why things (such as a race car) heat up when accelerating. This is because of air friction and friction in the engine, tires, etc. All the energy dissipated by the friction will be converted to heat. In fact, the friction will still heat the object even if it is not accelerating. The air friction is proportional to the objects velocity, not its acceleration. In other words, there is still friction, accelerating or not, and either way it causes kinetic energy to be converted to heat. Was this what you were looking for?

EDIT: I forgot to mention that in my example with the car there will be heat energy released from the burning of the gasoline inside the engine. This is probably the biggest source of heat for the engine. If you are more concerned with the body of the car, then that probably gets most of it's heat due to the air resistance.

Last edited: Sep 3, 2007
4. Sep 3, 2007

### Skhandelwal

Pretty much but I wonder, since heat is energy and when the heat gets released, it tries to set up equilibrium by moving to the cool place. However, if the energy is coming out of the bonds that broke between the atoms/molecules, then the things coming out, shouldn't it be what bonds are made up of instead of energy?(what is bonds made up of?)

5. Sep 3, 2007

### kaisxuans

if it is then what GO1 says is true. heat is mostly caused by friction and air friction is what cause things to heat up.:rofl:
(visit my blog!)

6. Sep 3, 2007

### kaisxuans

ok im really getting confused here. what are you trying to say?
is it that bonds are a contributing party in the process of heating up in friction?
however, it is the energy that the bonds produce that can cause the friction process to happen.
not the bonds by themselves!
(visit my blog!)

Last edited: Sep 4, 2007
7. Sep 3, 2007

### G01

You seem to be a little confused about bonds. A molecule is held together by the electrostatic force. It is kind of like how we are held to the Earth by gravity. There is no physical substance connecting us to the Earth. The same is true with the atoms in a molecule. They are held there by a force and it requires some energy to overcome this force. When two atoms combine to form a molecule, this energy that enabled them to be free can be released, usually as heat. Again though, this isn't why objects heat up when moving in air. That is mainly due to air resistance.(The objects kinetic energy is converted to thermal energy by the friction from the air.)

Last edited: Sep 3, 2007
8. Sep 3, 2007

### kaisxuans

YES that is what im trying to say but did not reaily knew how to put it!
thx GO1

9. Sep 4, 2007

### rcgldr

Not everything, take a can of "freeze it", and press the nozzle, the gas inside accelerates outside, and both the gas inside and outside cool off.

Similary, imaging a two sided chamber, one half in near vacuum, the other at normal pressure. Open a small valve between the two chambers, and there will be overall coolling while the air accelerates towards the lower pressure side.

10. Sep 4, 2007

### Loren Booda

Tidal forces act through spacetime curvature between any two gravitating masses. As the bodies' relative acceleration changes, their shape changes, and resultant material friction generates heat.

This cataclysm by geometrodynamic resonances is more famously demonstrated in the volcanism of the mostly molten Jovian moon Io, situated near the Roche limit within which satellites destruct.

11. Sep 4, 2007

### Danger

Something that should perhaps be pointed out here is that friction is unfairly blamed for some heating phenomena. With very high-speed aircraft or things entering Earth's atmosphere from space, the heat of compression from the bow wave is actually far more contributory than air friction is.

12. Sep 5, 2007

### pervect

Staff Emeritus
Please give me an example of something that is accelerating heating up without friction?

Offhand I am not aware of such a thing. You can heat things up with a blowtorch, for instance, but while that meets the "heating things up" part of the question, it doesn't have anything to do with the "accelerating" part of the question.

Note that for instance the moon of Jupiter that was mentioned, Io, has both tidal forces and friction

Heat is a form of energy, so it has to come from somewhere. One source of heat is the kinetic energy of a body which is slowed down by frictional forces. The energy of the system has to be conserved, so the kinetic energy of the moving body is converted into heat by the frictional force.

There are other sources of energy that can be used to heat things up, like the blowtorch I mentioned.

I really can't think of anything that would involve acceleration and heat other than friction offhand.

13. Sep 5, 2007

### Maxwell's Demon

I thought that all matter radiates some quantity of energy when subjected to pressure, such as an accelerating force.

For example, if we start with a gas-filled piston/cylinder, then apply a force to the piston, wouldn’t the compression force increase the internal pressure, causing the system to emit some quantity of energy? If so, then wouldn’t this also apply to all bodies, since nothing is perfectly rigid?

I suppose this emission of energy would be brief, because the force of a constant acceleration would be compensated for by a new state of internal equilibrium…but a compound acceleration would be a different story.

And conversely, if the same piston were –pulled- rather than pushed, wouldn’t there be a drop in internal pressure, resulting in the –absorption- of energy?

14. Sep 6, 2007

### pervect

Staff Emeritus
I'm getting a better idea of what you're interested in, but this description is a bit garbled.

What you probably mean is that if you compress a gas, it heats up. This does have a lot to do with pressure, but it doesn't have anything to do with "radiating energy", and only a little bit to do with acceleration.

[add]In fairness, one could say that the issue has to do generally with transferring energy, but "radiating" is the wrong word here, technically radiation is one specific means of transfering energy, a means that is not related to this particular problem.

Thermodynamics isn't my main interest, but in order to compress a gas, you have to do work, for example you have to push on the piston.

Where does this energy go? It goes into the internal energy of the gas. The gas is a bunch of atoms or molecules moving around rapidly in random directions. The only way for a gas to store energy is in the motion (kinetic energy) of the molecules. And since temperature is a measure of the average kinetic energy of a molecule, and because the molecules are not created or destroyed, when you do work on a gas the stored energy must show up as heat.

The details of how the energy gets transfered is that the molecules hit a moving piston, and this causes them to speed up. So I suppose you could say that compressing the gas by moving the piston accelerates the molecules, but I didn't get that from your original question. "Radiating energy" really doesn't apply in any way that I can see.

It is also not universal that energy must be stored as heat - in a gas there is no other way to store the energy, but this is not true for instance of a spring. If you have an ideal spring, and you compress it, it will not heat up. The energy is stored in a different form - basically, its stored in the bonds between the atoms, bonds which do not exist in a gas.

The spring may heat up slightly if it is not perfectly ideal, one could generalize this to "friction" perhaps, though I'm not sure exactly how this happens - it's related to the fact that the spring isn't quite perfectly elastic. But while an ideal gas does heat up when you compress it, an ideal elastic body (or an ideal spring) will not.

Last edited: Sep 6, 2007
15. Sep 6, 2007

### Maxwell's Demon

Sorry if I’m coming through in garbles, I sometimes have trouble tuning this thing…

I don’t understand why you draw a line between ‘heat’ and ‘radiant energy.’ I’m under the perhaps deluded notion that anything which is ‘hot’ is emitting infrared radiation.

If we compress the gas in a cylinder by applying a positive acceleration to the piston, the temperature of the gas increases. If the initial temperature is at equilibrium with the environment, then this new condition isn’t, and we say the system has gotten ‘hotter.’ Within a short time, the energy difference between the interior and the exterior is transferred to the environment via kinetic energy transfer and photon emission, which is radiant energy, isn’t that right?

Certainly this seems clear in the extreme case, where the compression of the gas induces a high enough temperature increase to make the gas become luminous and the system ‘red hot,’ in which case we can actually see the photons being emitted because they’re of visible frequency.

Hmm, maybe my terminology is too literal. The increase in internal energy moves radially from the center, to the housing, to the environment…so I think the energy is ‘radiating’ from the center outward. Perhaps I’m wrong, but I assumed this involved the transfer of photons.

But the thing is, the increase in internal energy doesn’t stay internal. If it did, we wouldn’t feel the cylinder heat up at all (as in the case of a piston/cylinder composed of a perfect thermal insulator…which doesn’t exist as far as I know). The energy moves outward, heating up the piston/cylinder, and ultimately the energy joins the environment until a state of equilibrium is reached. Perhaps my phrasing is rotten, but this seems to be energy radiating to me.

But since we don’t inhabit an ideal world, the compression of matter via an accelerating force will transfer energy to the interior of matter, and then that energy will move outward into the environment as ‘heat,’ no?

Let’s take the closest thing we have to an ideal spring (I think): a perfect crystal. Let’s say it’s a stout kind of crystal, a diamond perhaps. We strike it hard with a titanium baseball bat (ah, for the love of science…). The impact induces a shockwave through the diamond, ie, which is an increase in internal energy. The diamond is now (to some small magnitude) ‘hotter’ than the initial equilibrium condition it had with the environment prior to impact. But it can’t hold this energy forever. It returns this energy to the environment, until they’re in thermal equilibrium again.

Please correct me if I’m wrong, but if we hit the diamond hard enough (and assuming it doesn’t shatter), the crystal lattice would vibrate quickly enough to emit visible light, which is radiant energy. If this model is accurate, then the same principles should apply in cases of much gentler accelerations. But the emitted photons would be of far too long wavelengths to be seen with the naked eye.

In the case of quartz, the wavelength of emission is often in the radio wave frequency range, which is quite useful.

But of course I may be going about this all wrong-headed. I’ve suffered from my share of mythconceptions in the past…