where F is limiting friction and N , the normal reaction between the contact surfaces.

Laws of friction:

(1) The force of friction always acts in a direction opposite to that in which the body tends to move.

(2) Till the limiting value is reached, the magnitude of friction is exactly equal to the force which tends to move the body.

(3) The magnitude of the limiting friction bears a constant ratio to the normal reaction between the two surfaces.

(4) The force of friction depends upon the roughness/smoothness of the surfaces.

(5) the force of friction is independent of the area of contact between the two surfaces.