What causes the friction force?

Answer 1

Frictional force arises due to irregularities present on the surfaces in contact. Even if the surfaces in contact appear smooth to naked eye, but at microscopic level their unevenness is cleary visible. When two surfaces are in contact, the uppermost layer of atoms or molecules on the surfaces intercept eachother due to weak inter-molecular attractive forces and the normal reaction force. These forces give rise to frictional force which is also dependent on coefficient of friction.

Answer 2

The coefficient of friction mentioned in the other answer actually depends on the microscopic unevenness. More unevenness = higher coefficient of friction. The coefficient depends on the unevenness, not the other way around. The frictional force is really a surface average of how firmly these irregularities are forced to interact. A higher normal force causes more interaction and more resistance to sliding. We interpret this resistance to sliding as a frictional force.

Answer 3

Friction force is caused by the interactions between surfaces in contact that resist their relative motion. It is due to the roughness of the surfaces on a microscopic level, which creates resistance when one surface tries to slide over the other. The magnitude of the friction force depends on factors such as the roughness of the surfaces, the force pressing the surfaces together, and the type of material involved.