Physics of Friction
 

I am working to get a better handle on the reasons for heat generation from friction. I understand that we are tearing
apart molecular bonds which release heat (why does that release heat again?).

Now, thinking about this, it seems to me that this would be a similar mechanism to what happens when pressure is
increased, but what I am missing is how that relates to the speed that atoms are moving about in either case.

So, what I'm asking for is the short course in all of this. I would guess this should be a fun topic for a lot of folks
here as well.

Bit of a complex subject for a short answer, but a few pointers:

In a lubricated bearing it is mostly down to the dynamic viscosity of the lubricant, bearing area and velocity (rate of shear in the lubricant).

For solid to solid the dynamic mechanisms vary with crystalline and plastic materials, but work is imparted to the surface material as dislocation movement, bonding energy etc.
Static friction doesn't involve any work and relates mainly to surface topography.

Air friction (e.g. skin friction) is similar to the lubricated case, except that turbulent flow makes the sums a lot harder.

All this is from memory and some long ago work experience, failing a more expert response try searching under tribology for more detail.

Try the great Aussie self-help science forum for a few more ideas - http://www2.abc.net.au/science/k2/stn/

Oh, and if you can smell smoke, slow down.

Not sure what that was about though.

Thanks, still looking for more, tough to find!

Concerning the breaking of chemical bonds releasing energy, the following site may provide interesting information. I seem to remember some of this from O Level Chemistry, but to be honest, that was a while ago...
<A HREF="http://www.ultranet.com/~jkimball/BiologyPages/B/BondEnergy.html" TARGET="_blank">http://www.ultranet.com/~jkimball/BiologyPages/B/BondEnergy.html</A>

Londoner,

Afraid not. The bond energy that article refers to is the bond you cracked when you put a few drops of electrolyte in the beaker of water, then applied a DC voltage to two electrodes in the water. It busted Water into H2 and O2, as in

2H2O -&gt; 2H2 + O2

Friction would not attack those types of bonds except indirectly via chemical reaction (eg decomposition).

Embarrassed to say I don't have a better answer. Will drag out old textbooks and bone up.

EchoTango - I stand corrected. Thank you sir!
What about lattice energy? That's another thing that lurks in the dim recesses of my memory. If you're scraping off atoms (which I suppose you must be), that would affect the lattice structure. so... energy that is related to the lattice structure would (should?) be liberated.
Then again, I failed A Level Chemistry, so what do I know? :)

Londoner,

I think you are on the right track now. But I'm not going to offer anything more at this stage, in case I end up with foot-in-mouth disease.

EchoTango,

I think I'll pass the baton to someone else, after my recent outbreak of foot-in-mouth disease ;)

Friction is extraordinarily complicated and not well understood, especially not by me.

There seem to be two mechanisms at work, as I understand it. One is phonon exchange -- when you have two surfaces (or a gas and a surface) moving past each other, the atoms in each collide and in effect pluck at each other and set each other vibrating. These vibrations -- called phonons -- absorb kinetic energy from the moving atoms and are apparent as heat (which is, very crudely, atoms vibrating).

The other mechanism is the interaction between shells of electrons in the atoms of the surfaces/gasses. These have minute fluctuations in their density, and if you get a denser area in one shell it will repel the cloud in an adjacent atom's shell. This produces an area of positive charge, which in turn is attractive to the negative charge in the shell that caused the fluctuation in the first place (I think that's right -- seems a bit topsy to me, but that's electrons for you). These are called van der Waals bonds, and if you get atoms moving past each other the bonds once again tend to pull the atoms together and kinetic energy is removed from the motion of the system as they pull apart.

If you increase the rate of these inter-atomic interactions -- either by increasing the speed or increasing the density of the materials involved -- then the energy transfer will increase, friction will increase and heat will increase.

Perhaps. I don't understand how atoms 'collide'-- my knowledge of what actually happens at a quantum level is not good -- or whether this collision involves anything other than the van der Waals bonds. I do think that the electron model is attracting most interest at the moment, as it creates the possibility of surfaces where you can vary the coefficient of friction by changing an electric current flowing through them.

R

Thanks Self, that actually covered the the subject fairly well!