When two bodies are kept in contact, electromagnetic forces act between the charged particles at the surfaces of the bodies. As a result, each body exerts a contact force on the other.
The perpendicular component of the contact force is called the normal contact force and the parallel component is called friction
Laws of friction
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.
If the applied force is small, the friction force will be equal to it. As the applied force is increased friction force also increases to a maximum limit and if additional force is applied the bodies start slipping.
4. The frictional force fk or fs does not depend on the areas of contact. It depend on normal contact force only.