1. If the bodies slip over each other, the force of friction is given by
fk = μ kN
fk is the force of friction
N is the normal contact force
μ k is the coefficient of friction between the surfaces
2. The direction of kinetic friction on a body is opposite to the velocity of this body with respect to the body applying the force of friction.
3. If the bodies do not slip over each other, the force of friction is given by
fs = μ sN
fs = static force of friction
μ s = coefficient of static friction between the bodies
N = normal force between them
The direction and magnitude of static friction are such that the condition of no slipping between the bodies is ensured.
4. The frictional force fk or fs does not depend on the areas of contact. It depends on normal contact force only.