# 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?

For rotation of the disc torque is required. If friction is absent at B due to tangential force (Torque) at A disc will rotate in the same place but won’t move forward. So, friction is necessary to make the disc to move forward.

A. Friction opposes motion. So, frictional force is opposite to the direction of linear velocity at point B, in rightward direction. Sense of frictional torque, before perfect rolling begins is perpendicular to plane of disc in the outward direction.

B. Perfect rolling means the linear velocity at point B is zero. So, frictional force is also zero.

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