Static Friction

Consider a man trying to push a couch on a slightly rough floor. When the applied force is zero, the couch is in equilibrium.

As the applied force increases, the frictional force increases linearly, keeping the couch in equilibrium. This shows the relation between the applied force and frictional force is linear.

When the applied force reaches a certain point, the couch is about to slip and the frictional force decreases slightly. It remains constant for some time, then decreases as the applied force increases.

The maximum value of static frictional force, known as the limiting static frictional force, is proportional to the normal force with a coefficient of static friction, which depends on the roughness of contacting surface.

At this point, the couch is on the verge of slipping and is in unstable equilibrium, known as the point of impending motion.

The angle made by the resultant reaction force with the normal force is known as the angle of static friction.