(a) Impending motion of the body is opposed by static friction. When external force exceeds the maximum limit of static friction, the body begins to move. (b) Once the body is in motion, it is subject to sliding or kinetic friction which opposes relative motion between the two surfaces in contact. Kinetic friction is usually less than the maximum value of static friction.
When there is no applied force, there is no static friction. It comes into play the moment there is an applied force. As the applied force F increases, static friction also increases, remaining equal and opposite to the applied force (up to a certain limit), keeping the body at rest. Hence, it is called static friction.
Static friction opposes impending motion. The term impending motion means motion that would take place (but does not actually take place) under the applied force, if friction were absent.
Thus, when two bodies are in contact, each experiences a contact force by the other. Friction,
by definition, is the component of the contact force parallel to the surfaces in contact, which opposes impending or actual relative motion between the two surfaces. Note that it is not motion, but relative motion that the frictional force opposes.
Consider a box lying in the compartment of a train that is accelerating. If the box is stationary
relative to the train, it is in fact accelerating along with the train. What forces cause the acceleration of the box?
Clearly, the only conceivable force in the horizontal direction is the force of friction. If there were no friction, the floor of the train would slip by and the box would remain at its initial position due to inertia (and hit the back side of the train). This impending relative motion is opposed by the static friction.
Static friction provides the same acceleration to the box as that of the train, keeping it stationary relative to the train.
Rolling Friction
