# Friction force and contact area [duplicate]

It's usually stated that the friction force is independent of the area of contact (Amontons' Second Law).

I've always thought that this shouldn't be true, because the atraction between molecules would be higher and there would be more peaks.

I've read that this is important for rubber surfaces. Is it important for nonelastic materials, like wood? Are there any quantitave studies of this phenomenon?

## marked as duplicate by Ben Crowell, Waffle's Crazy Peanut, ja72, Qmechanic♦May 28 '13 at 17:54

• It is only true in certain limits. Limited distortions, no permanent distortions of either material and the like. – dmckee May 27 '13 at 23:46
• Possible duplicates: physics.stackexchange.com/q/16213/2451 and links therein. – Qmechanic May 28 '13 at 8:32
• @Qmechanic I was also asking for numerical results and how good is Amontons model compared to reality, but it seems that it's not very studied. – jinawee May 29 '13 at 13:10

The real area per asperity is roughly proportional to pressure, i.e. $F/A$, and the number of asperties in contact is proportional to area $A$. So when you multiply these together you find the real area of contact, and hence the friction, is just proportional to the applied force.