Question

A ball having velocity $v$ towards right and having angular velocity clockwise approaches the wall. It collides elastically with wall and moves towards left. Ground and wall are frictionless. Select the correct statement about angular velocity of the ball after collision.

• A. No change in angular velocity.
• B. It becomes zero.
• C. Angular speed decreases.
• D. It will be clockwise.

Answer 1

Answer: (A)

No change in angular velocity.

As collision is elastic, so after collision ball moves towards left with speed $v$. As walls and ground are smooth, there is no tangential torque on the balls. Only normal forces and $mg$ force pass through the centre of the ball. So, their torques about its centre are zero. By $\tau =I\alpha$, angular acceleration is zero, hence angular velocity does no change.