(i) Explain why friction is necessary to make the disc to roll in the direction indicated. (ii) Give the direction of frictional force at `B`, and the sense of frictional torque, before perfect rolling begins. (iii) What is the force of friction after perfect rolling begins?

Explanation

As we know, torque is required to roll a disc. Torque can only be provided by a tangential force. As force of friction is the only tangential force so friction is necessary for rolling
(a) At point B, velocity is towards left and frictional force always opposes the velocity. So, frictional force must be towards right.
(b) As the frictional force decreases the velocity of point of contact B with the surface. Perfect rolling occurs when the velocity of point B becomes zero and force of friction will also be zero at this stage.