Question

A sphere A moving with a speed u and rotating with an angular velocity $\omega$, makes a head-on elastic collision with an identical stationary sphere B. There is no friction between the surface of A and B. Disregard gravity. Then which of the following statements is/are true ?

• A. A will stop moving but continue to rotate with an angular velocity $\omega$.
• B. A will come to rest and stop rotating.
• C. B will move with a speed u without rotating.
• D. B will move with a speed u and rotate with an angular velocity $\omega$.

Answer 1

Answer: (A), (C)

The correct options are
A A will stop moving but continue to rotate with an angular velocity $\omega$.
C B will move with a speed u without rotating.

When the two identical particles make a head-on elastic collision, they exchange their velocities after the collision.

$⟹$ A will come to rest and B will move with velocity $u$ after the collision.

Friction force between the surface of the spheres provides necessary torque and thus is responsible for the change in their rotational motion. So due to lack of friction force between the surface of A and B, they perform the same rotational as initial.

$⟹$ A will continue to rotate with angular velocity $w$ whereas B will not rotate.