Question

Explain why friction is necessary to make the disc in Fig. 7.41 roll in the direction indicated.

(a) Give the direction of frictional force at B, and the sense of frictional torque, before perfect rolling begins.

(b) What is the force of friction after perfect rolling begins?Question 7.29 System Of Particles And Rotational Motion

Answer 1

Torque is required for rolling of the disc . which can be provided by a tangential force . friction force is the only tangential force acting on the disc , hence, friction is necessary to make a disc roll.

(A) frictional force acting at point B, opposes the velocity at point B ( towards left) hence, frictional force acting at point B is towards right .

Here , direction of frictional torque oppses the angular motion wo, whose direction is normal to the plane of paper in the inward direction , so, the direction of frictional torque is normal to the plane of paper in outward direction.



(B) the perfect rolling begins when velocity of disc at point B becomes zero . hence, force of friction also becomes zero .