Static Friction: When a body in contact with a surface is at rest and no external force is applied on it. the force of friction on the body is zero. But when yon apply a force of less magnitude on the body horizontally, the body continues to be at rest due to the force of friction which acts tangential to the surface of contact, in a direction opposite to the direction of force applied. So, the friction offered by the surface keeps the object on it stationary. Although the object is not moving on the surface. it has a tendency to move due to the applied force. The friction in such case is called static friction. So, the static frictional force is a self-adjusting force which increases on increasing the applied force till the relative motion just starts. e.g. a boy standing on ice.
Rolling Friction: When a body rolls on the surface of another, the frictional force exerted by the surface on the rolling body is called rolling friction. e.g. Friction exists between rolling tyre and the road. The motion of the wheels is called rolling motion and force of friction exerted by the surface is called rolling friction.
