Question

When does force of friction come into play?

Answer 1

1. Frictional force is a consequential force and comes into play when a force applied to a body slides the body over the surface of another body.
2. Frictional force between two surfaces is a contact force and comes into play when there is direct physical contact between the two surfaces.
3. Direct physical contact between the two surfaces and the relative motion between them is the basic prerequisite for the development of friction.

Hence, the frictional force comes into play when the two surfaces in contact experience a relative motion.