When we push an object with only a small amount of force, it does not move at all. It means that the surface on which the object is resting exerts some force on the object which opposes the tendency of motion. A ball moving on the ground slows down or stop after some time because of the friction force. Hence, we can define the force of friction as the force which opposes relative motion or the tendency of relative motion of one surface over another surface in contact with it.

Cause of Friction
Friction arises on account of strong atomic or molecular forces of attraction between the two surfaces at the points of actual contact.

On account of roughness of surfaces, the area of actual contact is much smaller than the area of apparent contact. These areas are roughly in the ratio 1 : 104.

At the contact points, the molecules of the two surfaces are so close together that they exert very strong intermolecular force of attraction on one another.

As actual area of contact is very small, therefore, the pressure (= force / area) at the points of contact becomes too large. So that many of the contact points may deform plastically and becomes 'cold welded'.

When one body tries to move over the other, it involves rupturing of thousands of such tiny cold welds. This gives rise to opposing force of friction.

Characteristic of Friction Force
1. Friction force is independent of the area of contact.
2. The maximum value of friction force depends on the normal contact force between the surfaces.

Types of friction force
(i) Static friction:
Static friction acts on a body to oppose the tendency of relative motion.
For example if 5 N force is required to move a block, and horizontal force F is applied on the block and value of F is gradually increased from F= 0 N to 5 N the block will not move till F = 5 N upto 5 N the friction force is balancing the applied force. So the maximum value of friction force. is 5 N, which is called the limiting value of friction. Before reaching the limiting value, static friction is variable because, say F = 2 N block will not slide, so the static friction is 2N and when F = 3N then static friction in 3 N. So we can say that static friction is self adjustable force, which is equal and opposite to the applied force upto the limiting value.

(ii) Kinetic or sliding friction: Force of kinetic friction is the force that comes into picture, opposing the relative motion when one body is actually moving over the surface of another body. It is found that the force of kinetic friction is independent of the area of contact, and its magnitude is constant for the given surfaces. It does not depend on the velocity of the body.