Static and Kinetic Frictional Force

The magnitude of frictional force has two forms, namely static frictional force and kinetic frictional force.

The frictional forces can be explained using the applied force versus the frictional force graph. In the case of static frictional force, the applied force is exactly counterbalanced by the frictional force.

Hence, the net horizontal force is zero and the object remains at rest. Within the static frictional region, the graph is linear. Once the applied force exceeds the maximum value, the object accelerates and the frictional force decreases.

Now, the kinetic frictional force takes over and the object begins to move. Within the kinetic frictional region, it is easier to keep the object moving than to start moving it.

Hence, for any given pair of surfaces, the magnitude of the kinetic frictional force is always less than that of the maximum static frictional force.