(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

To roll a disc, we require a torque, which can be provided only by a tangential force. As force of friction is the only tangential force in this case, it is necessary.
(a) As frictional force at B opposes the velocity of point B, which is to the left, the frictional force must be to the right. The sense of frictional torque will be perpendicular to the plane of the disc and outwards.
(b) As frictional force at B decreases the velocity of the point of contact B with the surface, the perfect rolling begins only when velocity of point B becomes zero. Also, force of friction would become zero at this state.