Why does friction does not depend upon area of contact?

Question

Why does friction does not depend upon area of contact ? Now we know that friction=(coefficient of friction)*Normal.So also is, why does friction arise.Is it because of the irregularities of a surface or because of the electrostatic forces in between the surfaces?

Answer 1

I will quote Klepnner here,

It may seem strange that friction is independent of the area of contact. The reason is that the actual area of contact on an atomic scale is a minute fraction of the total surface area. Friction occurs because of the interatomic forces at these minute regions of atomic contact. The fraction of the geometric area in atomic contact is proportional to the normal force divided by the geometric area. If the normal force is doubled, the area of atomic contact is doubled and the friction is twice as large. However, if the geometric area is doubled while the normal force remains the same, the fraction of area in atomic contact is halved and the actual area in atomic contact hence the friction force remains constant.

Answer 2

Why does friction does not depend upon area of contact ?

Because, even though surface gripping would scale with contact area, so does other parameters that are in the friction formula. Area therefore appears more times in the expression and turns out to cancel out completely, leaving the formula as $f=\mu n$.

For low normal forces, one would expect friction to be proportional to contact area $A$ and also to the pressure $q$ that presses the surfaces together (the normal pressure):

$$f \propto Aq$$

Normal pressure is normal force over area, $q=n/A$, so plugging this in causes $A$ to cancel out.

why does friction arise.Is it because of the irregularities of a surface or because of the electrostatic forces in between the surfaces?

Both. You can both have mechanical interlocking - as velcro - but also adhesion of one material onto the other - like glue - which tries to prevent sideways sliding. Friction is then the force needed to rip this adhesion free again.

I find it useful to think of the surfaces as very rough with "stickey" peaks and valleys. Peaks can grip into valleys and when pressed together like that they don't want to slide