question_answer 29)

Explain why friction is necessary to make the disc in Fig. 7.108 given in previous question roll in the direction indicated. (i) Give the direction of frictional force at B, and the sense of frictional torque, before perfect rolling begins. (ii) What is the force of friction after perfect rolling begins ?  

Answer:

                To roll a disc, a torque is required which in turn requires a tangential force to act on it. As the force of friction is the only tangential force acting on the disc, so it is necessarily required for the rolling of a disc. (i) Frictional force at B opposes the velocity of B. Therefore, frictional force is in the same direction as the arrow. The sense of frictional torque is such as to oppose the angular motion. By right hand rule, both  and  act normal to the plane of paper,  into the plane of paper and  out of the paper. (ii) Frictional force decreases the velocity of the point of contact B. Perfect rolling begins when this velocity is zero. Once this is, the force of friction is zero.