# What is friction caused by?

Rockazella
At the smallest scale (quantum level I suppose) what is friction caused by?

ive never thought of that

i suprised no one has tried to answer this question yet ill try and take a crack at it even though I am probably way off, perhaps it has to do with the same bonds keeping everything together just on a much smaller scale for example when you pick up a glass your hand kind of becomes the glass, i really have no clue but you have struck my curiosity.

bleh

Jonathan

I saw a show about nanomachines and I think they said that friction is negligible on that scale. They also said that they can't use a lubricant even if they wanted, because the gears and stuff are molecule size and so is the lubricant, and it would be like dropping a wrench into a running engine. (DO NOT try this at home!)

Originally posted by Rockazella
At the smallest scale (quantum level I suppose) what is friction caused by?

Hate to say it, but (as of a couple years ago, at least) nobody really seems to know. I read an artical in SciAm about two or three years ago, about research into the field of friction as a force. The researchers were baffled by the appearent lack of interest this field of investigation was shown. String theory and particle accelerators get a great deal of attention and deservedly so, but the question of "what is friction?" could have enormous and imediately practical ramifications. Nevertheless, the question remained unnanswered at that time. I have heard of no new developements since.

Originally posted by LURCH
String theory...get a great deal of attention and deservedly so

Christ, I'm glad to hear someone around here feels that way.

Mentor
There really isn't any mystery behind friction. It doesn't happen on an atomic level, it is strictly a matter of how rough or smooth two materials are and how their roughness' fit together that determines friction.

When two bodies A & B are in contact, the atoms from Body A that "touch" body B interact through electromagnetism. That is the force responsible for friction. The nucleus of atoms in body A don't touch the nucleus of atoms in body B. Electromagnetic interaction keep them apart. You might know that atoms are composed mainly of empty space and Electromagnetism is basically what prevents you from "falling through the floor".

That said, the actual "force of friction" between body A & B is the result of the configuration of that system. Imagine two plates A & B. A is laying on the ground and B is on top. The formula is F = u * Fn

F = Force of Friction (direction opposite to movement or resisting the movement)
u = Friction Coefficient
Fn = Force perpendicular to movement (Mass of B * Gravity)

u: is influenced by the materials & their rougnness. There is two flavor or u (Static & Dynamic). Static friction coefficient is always higher than dynamic coefficient (which can be explained by the momentum of the moving body)

Fn: depends on the mass of the body on top and gravity.

Example: 2 steel plates 1 KG. Force of friction is:

Dry Steel Plate (u = .8)
F = .8 * (1 * 9.8) = 7.84 Newton

Lubricated Steel Plate (u = .16)
F = .16 * (1 * 9.8) = 1.568 Newton

Note that this is an engineering calculation so it is approximate.

See here for list of friction coefficients

Hope this helps

Alain

PeteGt
The question is why does friction occur?

Friction even between two apparently "smooth" surfaces is caused by attraction between the actual atoms. Now, the addition of a rough surface also adds intertia and force distribution along the two surfaces.

On a quantum level friction isn't usually an issue between two electrons or protons and neutrons. Mostly because the force is due to charge interactions. Which causes attractions and repulsions, which in turn, causes macro friction.

Hope this is somewhat a qualitative theory for you. :-)

Pete

Rockazella
Well that's a variety of explanations as well as some contradictions ....but, I am satisfied.

Thanks

Mentor
Originally posted by Rockazella
Well that's a variety of explanations as well as some contradictions ....but, I am satisfied.

Thanks
I'm not 100% sure, but I think what PeteGt was talking about isn't really friction, but simply magentic forces. They don't follow the normal mechanical rules of friction as Jal laid out. Does that clarify any?