Question

A uniform disc is rolling on a horizontal surface. At a certain instant $B$ is the point of contact and $A$ is a stationary point at height $2R$ from ground, where $R$ is radius of disc :

• A. The magnitude of the angular momentum of the disc about $B$ is thrice that about $A$.
• B. The angular momentum of the disc about $A$ is anticlockwise.
• C. The angular momentum of the disc about $B$ is clockwise.
• D. The angular momentum of the disc about $A$ is equal to that about $B$.

Answer 1

Answer: (A), (C)

The magnitude of the angular momentum of the disc about $B$ is thrice that about $A$.
The angular momentum of the disc about $B$ is clockwise.
Angular velocity of the disc about any point on itself is constant.
Moment of inertia of a disc about its central axis is given by, $I_o=m{R}^2/2$
Moment of inertia of a disc about an axis passing through a point on it's circumference and parallel to it's central axis is, $I_c=I_o+m{R}^2=3m{R}^2/2$
Therefore angular momentum of disc about is thrice than that about .
All points on the disc about points and rotate clockwise. Therefore, angular moments are clockwise.

Answer: A,C