# Coefficient of Friction and Normal Force

by Abraham
Tags: coefficient, force, friction, normal
 P: 69 I know that the force of friction is the (coefficient of friction) x (normal force). My question is, why isn't area involved? That is, why wouldn't a larger surface have more friction than a smaller surface, if the normal force is the same? PS: Sorry, just in case this is in the wrong section
 Sci Advisor HW Helper P: 1,276 There is no reason why the coefficient of friction can't implicitly depend on area.
HW Helper
Thanks
PF Gold
P: 26,122
 Quote by nicksauce There is no reason why the coefficient of friction can't implicitly depend on area.
erm … I've no idea what that means.
 Quote by Abraham I know that the force of friction is the (coefficient of friction) x (normal force). My question is, why isn't area involved? That is, why wouldn't a larger surface have more friction than a smaller surface, if the normal force is the same?
Hi Abraham!

Because the larger the surface, the more its weight is spread out.

The friction per area depends on the pressure between the surfaces …*the harder they're pressed together, the more friction you'd expect.

Pressure is force divided by area.

Halve the area, and the pressure is halved, so the friction per area is halved, so the total friction is still the same.

HW Helper
P: 1,276

## Coefficient of Friction and Normal Force

 Quote by tiny-tim erm … I've no idea what that means.
The OP's first point (it seems) was that F = uN, implies that F does not depend on area. I was saying that this was false, as u could implicitly depend on area, as u also implicitly depends on many other things (material, roughness of surface, possibly temperature). You covered the rest, by explaining how F should not depend on area.
P: 5
 Quote by tiny-tim erm … I've no idea what that means. Hi Abraham! Because the larger the surface, the more its weight is spread out. The friction per area depends on the pressure between the surfaces …*the harder they're pressed together, the more friction you'd expect. Pressure is force divided by area. Halve the area, and the pressure is halved, so the friction per area is halved, so the total friction is still the same.
if area is halved then pressure wud double (force remains same throughout-normal reaction force).
could u put forward ur point properly
PF Gold
P: 2,283
 Quote by nicksauce There is no reason why the coefficient of friction can't implicitly depend on area.
The coefficient of friction in the standard friction model is dependent on the material properties mainly and to the environment to a certain extent, not the area.

CS
PF Gold
P: 2,283
 Quote by nicksauce The OP's first point (it seems) was that F = uN, implies that F does not depend on area. I was saying that this was false... You covered the rest, by explaining how F should not depend on area.

F does not depend on the area.

CS
PF Gold
P: 2,283
 Quote by chandangang if area is halved then pressure wud double (force remains same throughout-normal reaction force). could u put forward ur point properly