# How friction depends on surface area

1. Dec 23, 2012

### s0ft

I'd read that friction is independent of the surface area of the bodies in contact. But somewhere in the internet I found that this explanation was just a good approximation and that friction actually depends on area. Can anyone explain a bit more on this?

2. Dec 24, 2012

### Woopydalan

Just look at the equations for the force of friction..nowhere is the surface area of the object included

3. Dec 24, 2012

### s0ft

You can look @ http://zebu.uoregon.edu/1999/ph161/friction.html
and there you will find written friction is nearly or mostly independent of surface area. That must mean there is something more to it than just normal force and a constant.

4. Dec 24, 2012

### AlephZero

There is a lot more to friction than what you learn in a first course in mechanics. The Coulomb model of friction (the static and dynamic coefficients of friction are constants and independent of the normal force, surface area, etc) is a simple model of friction that works pretty well for "rigid" objects with "hard" surfaces moving fairly slowly, in orher words the situation you have in lab experiments. It is also simple enough to use in hand calculations.

Some students seem to get the wrong idea that Coulomb's "law" of friction is the same sort of law as Newton's "law" of gravity or the ideal gas "laws". It isn't.

More complicated models of friction include the flexibility of the objects that are in contact, and possibly the atomic structure of the materials as well, but the only practical way to use those models is in a computer simulation, not setting up and solving problems by hand.

5. Dec 24, 2012

### s0ft

Just asking, would it be possible to derive the simple : friction = μN expression from Coulomb's law for electric charges?