Laws of kinetic friction :
1. First law : The force of kinetic friction (Fk) is directly proportional to the normal reaction (N) between two surfaces in contact.
Fk ∝ N
∴ Fk = µkN
where, µk = constant called coefficient of kinetic friction.
2. Second law : Force of kinetic friction is independent of shape and apparent area of the surfaces in contact.
3. Third law : Force of kinetic friction depends upon the nature and material of the surfaces in contact.
4. Fourth law : The magnitude of the force of kinetic friction is independent of the relative velocity between the object and the surface provided that the relative velocity is neither too large nor too small.
