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 ?
(a)
As the disc is moving in clockwise direction, the linear velocity at the point of contact will act in the left direction. In such a case, there has to be some opposing force acting at the point of contact in order to get the net velocity at that point to be equal to zero. The opposing force (frictional force) should act tangentially in the right direction at the point of contact.
Thus, the direction of frictional force at B is tangentially in the right direction. Accordingly, the frictional torque acts in the anticlockwise direction.
(b)
During perfect rolling, the linear velocity at the point of contact becomes zero. Therefore, the net force acting at the point of contact has to be zero in the horizontal direction. As only frictional force is acting at the point of contact, its value should be zero at the time of perfect rolling.
(a)
As the disc is moving in clockwise direction, the linear velocity at the point of contact will act in the left direction. In such a case, there has to be some opposing force acting at the point of contact in order to get the net velocity at that point to be equal to zero. The opposing force (frictional force) should act tangentially in the right direction at the point of contact.
Thus, the direction of frictional force at B is tangentially in the right direction. Accordingly, the frictional torque acts in the anticlockwise direction.
(b)
During perfect rolling, the linear velocity at the point of contact becomes zero. Therefore, the net force acting at the point of contact has to be zero in the horizontal direction. As only frictional force is acting at the point of contact, its value should be zero at the time of perfect rolling.
