Question

How does friction depends on the nature of the surfaces in contact??

Answer 1

The friction force depends on two factors:
a) The materials that are in contact. The two materials and the nature of their surfaces. Rougher surfaces have a higher coefficient of friction and are hence difficult to slide apart. This makes sense in terms of a model in which friction is described as arising from chemical bonds between the atoms of the two surfaces at their points of contact. Very flat surfaces allow more atoms to come in contact
b) The force pushing the two surfaces together. Pushing the surfaces together causes more of the asperities to come together and increases the surface area in contact with each other.