• Kinetic Friction

The force of friction that comes into play when a body is in a state of steady motion over the surface of another body is called kinetic friction (fk ). If two bodies are in contact and moving relative to one another, then the friction between them is called kinetic friction (fk). Kinetic friction, also known as dynamic friction or sliding friction. The magnitude fk of the kinetic frictional force is given by

fk = μkFN

Where,  μk is the coefficient of kinetic friction, and FN is the magnitude of the normal force. Above figure helps to explain the main features of friction. The block in this drawing is initially at rest on a table, and as long as there is no attempt to move the block, there is no static frictional force (see part "a" of the drawing).

If a force applied to the block  is small, the block still does not move. It does not move because the static frictional force exactly cancels the effect of the applied force. The direction of is opposite to that of , and the magnitude of equals the magnitude of the applied force, fs = F. Increasing the applied force by a small amount still does not cause the block to move. There is no movement because the static frictional force also increases by an amount that cancels out the increase in the applied force (see part "b & c" of the drawing). If the applied force continues to increase, however, there comes a point when the block finally “breaks away” and begins to slide. The force just before breakaway represents the maximum static frictional force that the table can exert on the block (see part c of the drawing). Any applied force that is greater than cannot be balanced by static friction, and the resulting net force accelerates the block to the left (see part "e" of the drawing). Imagine, for example, you are trying to slide a heavy box across a rough floor—you may push harder and harder on the box and not move it at all. This means that the static friction responds to what you do - it increases to be equal to and in the opposite direction of your push. But if you finally push hard enough, the box seems to slip suddenly and starts to move. Once in motion it is easier to keep it in motion than it was to get it started, indicating that the kinetic friction force is less than the static friction force. If you add mass to the box then you need to push even harder to get it started and also to keep it moving. Furthermore, if you oiled the floor you would find it to be easier to get the box started and keep it going.

The maximum static frictional force fsmax would be the same, no matter which side of the block is in contact with the table. 