friction  It  is  the  force  that comes into play between two bodies  in physical contact and when an external force is applied on  one  of them. The direction of the frictional force is opposite to that of the external force.

The  frictional force is not a constant. A body resting an a table does not move, when pushed with a small force. As the force is increased, the frictional force also increases up to a limiting value,  called  the  limiting friction. If the external force is beyond  the limiting  friction, the body will accelerate. In the moving condition also there is a frictional force on the body. If the external force is made equal to the frictional force the body will  move with uniform  velocity.  The frictional force on the moving  body is  a constant, called the kinetic friction. It is observed  experimentally  that frictional force F is equal to the product of coefficient of friction, m and normal reaction, R

F = m R

The  normal reaction for a body moving on a horizontal surface is product of mass of the body and g. m depends on the nature of both surfaces.

When  a body rolls over another as in the case of motor car wheel  on the road, the rolling friction resists the motion. The value  of  the  rolling friction is much less compared to sliding friction  discussed earlier. The friction between two surfaces can also  be  reduced by substituting rolling friction for sliding friction.  The  ball  bearings  between the wheel and axle is one example.

Friction  can  be  reduced to some extent by making the surface smooth,  but because it is caused by the adhesive forces between  the molecules  of  the  surface  in contact, it cannot be eliminated  altogether. A layer of lubricant reduces the adhesive bond  strength  by  replacing  it  with weaker bonds. The science of friction and lubricants is called tribology.