# Why does friction cause heat?

by wildkat7411
Tags: friction, heat
 P: 5 I understand that friction is the force that opposes motion and that rubbing two objects togethere generate heat. But what is the cause of this release of energy?
 Sci Advisor HW Helper P: 8,961 You are deforming the surfacse so stretching the bonds between atoms. When you move along those bonds now relax back to their original state and have extra energy left over = heat
 P: 1,084 I also have an indirect reason for there being a heat, possibly a more physics relevant one, even though mgb is right. something moving has energy. say you slide a piece of wood on the floor and you give the wood 100 joules of energy. eventually the floor's friction with the wood will make the wood stop. where did that energy go?
P: 5

## Why does friction cause heat?

 Quote by dacruick I also have an indirect reason for there being a heat, possibly a more physics relevant one, even though mgb is right. something moving has energy. say you slide a piece of wood on the floor and you give the wood 100 joules of energy. eventually the floor's friction with the wood will make the wood stop. where did that energy go?
It went where mgb said it went. The energy was transfered to the atoms, stretching their bonds. But when there is no oppsosing force agaisnt the bonds then they retract to their original length releasing energy into the air that we perceive as heat.
 Sci Advisor P: 5,443 I disagree with the above answers, I claim nobody knows why. Arguments regarding deformation of orbitals are not very rigorous; the electromagnetic force is conservative, and so any postulated deformation is *elastic*, and the energy is still available to perform work, as opposed to being dissipated.
 Sci Advisor HW Helper PF Gold P: 2,532 Is it available to do work? If I have a network of springs (not a bad model for a solid) and I do work to stretch one and release it, it seems like the energy will be immediately dissipated as vibrations throughout the material.
P: 880
 Quote by Andy Resnick I disagree with the above answers, I claim nobody knows why. Arguments regarding deformation of orbitals are not very rigorous; the electromagnetic force is conservative, and so any postulated deformation is *elastic*, and the energy is still available to perform work, as opposed to being dissipated.
I agree with your disagreement---but defer the reasons for explanation for a while.
 P: 99 whenever work is done or rather we can say that whenever we see any type of work being done, then we have energy as a production of that work. In this case energy is in the form of heat. If you have ever used a simple pendulum, have you ever wondered why does it stop oscillating after some time ? !
P: 2,159
 Quote by Mapes Is it available to do work? If I have a network of springs (not a bad model for a solid) and I do work to stretch one and release it, it seems like the energy will be immediately dissipated as vibrations throughout the material.
You can decide to consider the lengths of all the springs as your external variables. Then:

dE = T dS - Sum over i of F_i dX_i

So, the definition of heat is entirely subjective, it depends on what microscopic variables you have coarse grained over and what you decided to keep in your thermodynamic description of the system.
 P: 5,462 Friction is the most complicated and (perhaps) the least understood force, if indeed it is a single force acting. I can't however go along with the explanations offered. Take the mattress model. In such a model this does not account for the observed fact that friction between bulk solids is largely independant of both contact area and speed of passing. Of course friction between fluids and solids is dependant upon area. Then again friction coefficients greater than 1 have been observed which imply that something strange is acting since at $$\mu$$ = 1 the energy to physically lift the bodies apart is less than that to overcome friction. In fact, theory suggests the contact forces do more than just jiggle the molecules about a bit harder. It suggests that the actual regions of contact are quite small and isolated within the overall contact area and deform until their total area can support the reaction force. The contact regions then resist a lateral force until this force exceeds their shear strength. Since the area of normal stress (which is at yield) is the same as the area of shear stress $$\mu \quad = \quad \frac{{{S_{shear}}}}{{{S_{yield}}}}$$ As to the question of where does the energy go. It goes into the yield of the materials It goes into the surface energy of creating new surfaces as they shear It goes into a considerable amount of sound energy created within the bodies of both solids
P: 5,443
 Quote by Mapes Is it available to do work? If I have a network of springs (not a bad model for a solid) and I do work to stretch one and release it, it seems like the energy will be immediately dissipated as vibrations throughout the material.
It's not dissipated- you are describing *sound*. Call it phonons if you want.
P: 2,159
 Quote by Andy Resnick It's not dissipated- you are describing *sound*. Call it phonons if you want.
Mapes point (I think) is that the "phonon gas" will give you the complete microscopic description of the system. And my reply to that is that the whole point of thermodynamics is to describe a system with a huge number of degrees of freedom in terms of a few variables.

If you keep track of all the degrees of freedom, the entropy is always zero and heat does not exist.
 Sci Advisor P: 5,443 I'm not sure that's the *whole* point of thermodynamics- Thermodynamics concerns itself with the flow and transformation of energy in matter. But, your comment points out interesting connections between temperature, entropy, and heat. Your comment is certainly true for 0 K, but not for any other temperature. Phonons are often invoked to predict thermal conductivity (and other properties) in materials, which is not the same thing as heat. Phonon gases are analogous to particle gases by replacing 'pressure' with 'temperature'. http://www.springerlink.com/content/c5gk8m24g8506101/
 P: 5,462 Hello, The original question was about change in temperature, which means change in enthalpy since the objects remains substatially the same. This change is undeniable, regardless on any accompanying entropy change so I don't see the relevance of such considerations. However I have already pointed out the spring model cannot sustain the also undeniable observation of independance of contact area. The springs have to be broken not just stretched. Finally, all the destinations I mentioned, of the work supplied against the friction force degrade to heat. Hence the warming.
P: 2,159
 Quote by Andy Resnick I'm not sure that's the *whole* point of thermodynamics- Thermodynamics concerns itself with the flow and transformation of energy in matter. But, your comment points out interesting connections between temperature, entropy, and heat. Your comment is certainly true for 0 K, but not for any other temperature. Phonons are often invoked to predict thermal conductivity (and other properties) in materials, which is not the same thing as heat. Phonon gases are analogous to particle gases by replacing 'pressure' with 'temperature'. http://www.springerlink.com/content/c5gk8m24g8506101/
Yes, but note that the very notion of temperature (in the precise thermodynamic sense) already assumes that you have performed a coarse graining and are describing the system statistically. If you don't do this (and in practice you have no choice but to do this), then the system is always in some pecisely known microstate, the entropy is then the fine grained entropy which is zero.

A simple thought experiment: Consider a box with N elastic balls. It is assumed that the kinetic energy of the balls is exactly conserved. Suppose that the exact solution of this system is somehow known. Then we can play the same type of games as in Maxwell's demon's thought experiment, with the difference that in this case the demon succeeds as it knows in advance what the balls will be doing (it doesn't have to obtain any a priori random information and then later have to clear its memory).

This is known as Laplace's demon and it simply demonstrates that notions such as heat, entropy and temperature arise when we have a lack of information about the system.
 P: 2,293 Ok, this thread has me interested. I remember as a kid taking a metal coat hanger and bending in back and forth. The part where it is bending can get VERY hot. Also I was shown that if you take a rubber band close to your face, stretch it and place it against your lips it feels warmer than it was. If you then remove the band from your lips, let it contract, then retouch your lips it feels cold. Not offering anything special here, just some examples for thought on this topic.
 P: 880 well, I'd like to present my explanation of heat here, but 1) its against the rules and 2) it will be in my paper. Andy Resnick --It good to see someone being honest and not just throwing out dog 'ma'