Question

Why is rolling friction less than sliding friction and is really friction not dependent on the contact area ?

Answer 1

Hint: Study the static friction and kinetic friction coefficient and the different quantities in which the coefficient of static friction and kinetic friction depends. Frictional force is a contact force when two bodies come into contact frictional force generates between the surface of contact of the bodies.

Complete answer:
 We know that frictional force is contact force when two bodies are in contact with each other; the force acting between the surfaces of them is the frictional force.
Rolling friction: rolling friction is the type of friction in which one of the bodies is in motion and the body rolls on the surface.
Sliding friction: rolling friction is the type of friction in which one of the bodies is in motion and the body slides on the surface.

We know that frictional force is proportional to the normal force and the coefficient of friction. Now, this coefficient of friction depends on many factors. This coefficient of friction depends on the geometry of the surface, the type of the surface etc. Now, frictional force is nothing but the inter- atomic attraction between the bodies. The more the bodies interact with each other the more is the value of coefficient of friction.

In rolling friction the interatomic interaction between the bodies is less since the surface in contact always changes with time and the forces between the atoms break again and again with time. On the other hand, in case of sliding, the surface in contact is always constant, so the atoms that are interacting with each other are the same and the attraction force between them does not get disrupted with time.

So, the forces between the atoms are greater than in case of rolling. So, this is the reason why rolling friction is less than sliding friction. The magnitude of frictional force does not depend directly on the contact area for a fixed body and surface, but the coefficient of friction depends on the geometry of the body. So, the coefficient of friction depends on the contact area between the surfaces indirectly.

Note: The magnitude of frictional force is the product of the coefficient of friction and the normal force. The surface of contact or the distance between the atoms depends on the normal force since, the more the normal force on the surface is the more closer it comes in the contact of the surface too. So, normal force tends to increase the frictional force.