# Friction: Laws Of Friction

The laws of friction for two bodies in contact with each other are given as follows:

• If the two bodies in contact with each other are slipping over each other, or having relative motion with respect to each other, the friction that is involved is kinetic friction and is given by:

$F_K$ = $µ_k N$

where,

fk = kinetic friction

µk = coefficient of kinetic friction

N = Normal reaction force

• The direction of this kinetic friction on a body is opposite to the direction of the relative velocity of the body with respect to the other body.

Suppose body A is moving over body B in the positive x-direction, then the kinetic frictional force on body A will act in the negative x-direction.

• If the bodies are not having relative motion with respect to each other, the friction between them is static friction and is always less than or equal to the kinetic friction. It is important to remember that the value of static friction is not constant, as the case is with kinetic friction. The value of static friction depends on how much force is applied to the body. Suppose the maximum value of static friction (limiting friction) is 5N for a body kept on a certain surface. If you apply a force of 2N on the body, the value of static friction would be 2N, and the body will not move. The body will start moving when the force crosses the values of 5N.
• The kinetic frictional force and the static frictional force do not depend on the area of contact as long as the Normal Reaction force is the same.
• The maximum value of static friction is achieved when the body is just on the verge of motion. The friction at this point is also known as limiting friction. Limiting friction is always slightly greater than the kinetic friction, and is given by:

fmax = µs N

where,

fmax = limiting friction

µs = coefficient of static friction

N = Normal reaction force