Question

How can friction cause motion?

Answer 1

Hint: Recall that friction is essentially a force that is always directed opposite to the motion of an object. Use this intuitive understanding to deduce the role played by friction in different types of motions and states of motion. Think in terms of how friction might play an indirect role if at all it is responsible for some form of motion.

Complete answer:
A resistive force that opposes relative motion between surfaces, friction converts kinetic energy arising from the motion of one object in contact with the other into thermal energy or heat. Though friction is a resistive force, it is not necessarily an undesirable consequence. Friction is essential in providing traction and grip to automobiles in motion, we are able to start a fire by rubbing two pieces of wood together, we can ignite matchsticks due to friction between the head of the match and the rubbing surface on the matchbox, we are able to stand on the ground, or hold things in our hands, all due to friction.
Friction is usually classified as static friction and kinetic friction. Static friction is the force of friction acting between two surfaces that have no relative motion with respect to each other, i.e., they are at rest with respect to each other. Kinetic friction is the force of friction acting between two surfaces that are already in relative motion with respect to each other. Static friction helps us get a grip with the ground to push against the ground in order to move forwards, without which we’d end up slipping. Kinetic friction acts as a driving force when it comes to motion of a wheel, say in a bicycle where the friction acting backwards enables the road to propel the wheel forwards. In some cases, the energy or heat produced by friction can be harnessed to be used to drive another system that produces motion.
Thus, friction may either indirectly cause motion or just facilitate or aid motion by imparting traction and grip, but it never directly causes motion since it is inherently a relative motion-opposing force.

Note:
Kinetic friction is classified into sliding friction and rolling friction. Sliding friction is the resistive force acting between two objects that are sliding against each other, whereas rolling friction is the resistive force that opposes the objects that are in relative rolling motion. Remember that the friction acting on an object is directly proportional to the area of contact between the object and the surface over which it executes motion and arises as a result of relative motion between the two surfaces. Therefore, the weakest of all friction that we discussed above is rolling friction since at any given time, the point of contact between the two surfaces is minimum in comparison to the other forms of friction.