Question

Two discs $A$ and $B$ of equal masses and radii are rotating about an axis perpendicular to the plane of rotation and passing through their centre. The speed of rotation of the discs is identical. A larger force needs to be applied on disc $B$ for it to come to rest. Which of the following statements about the rotating discs are true?

• A. Such a scenario is not possible
• B. The mass distribution of disc A is uniform
• C. The mass of the disc B is concentrated near it's center
• D. The mass of the disc B is concentrated away from it's center

Answer 1

The correct option is D The mass of the disc B is concentrated away from it's center

The force required to stop the disc B is larger. So, the moment of inertia of the disc B is larger. Moment of inertia of a body depends on the mass of the body and its radius of rotation. Both of these are equal for both the bodies. Hence the mass distribution of the two bodies must not be equal.

We know that the moment of inertia increases as the distance of the masses from the axis of rotation increases. It is also possible that the mass of the disc B is concentrated away from its centre which increases its moment of inertia.

Hence the correct option is (d).