Friction

When a solid object moves or attempts to move over the surface of another solid object its motion is always opposed by a retarding force. This force is called friction. Thus, friction is the force directed opposite to the direction of motion or attempted motion. The frictional force is always parallel to the surface in contact.

Origin of friction
Classical view
According to Classical view, when two objects are kept in contact, there forms an interlock between the irregular surfaces and to break the interlock, we need an extra force. This force measures the force of friction.

Modern view
According to modern theory, friction is due to intermolecular force of attraction between the surfaces in contact. When two surfaces are put together, the actual area of contact is very less than the apparent area of contact. The pressures at the contact points are very high and the molecules are pushed very close so that attractive forces between them weld the surfaces together at contact points (which is called cold welding). To break this attachment we need an extra force.

Types of Friction
Static friction
The friction force that comes into play between the two surfaces when one body tends to move on the surface of other body is called static friction.
The maximum value of static friction is called limiting friction.

Kinetic friction
The frictional force that comes into play between the two surfaces when one body moves on the surface of other body is called kinetic friction.
There are two types of kinetic friction. They are:

1. Sliding friction
The friction that exists between two surfaces when one body is sliding on the other is called sliding friction. For example: friction between the wood block and the road when the wood block slides on the road.

2. Rolling friction
The friction that exists between two surfaces when one surface is rolling over the other is called rolling friction. For example: friction between the tyre of moving vehicle and the road.

Laws of friction

1. The frictional force opposes the relative motion of two surfaces.
2. The frictional force is parallel to the surfaces in contact.
3. The frictional force is directly proportional to the normal reaction.
4. The frictional force is independent of the area of contact.
5. The frictional force depends upon the nature of the two surfaces in contact and their state of roughness.
6. The kinetic friction is independent of the relative velocities of the surfaces.