Question

A ring and a disc of different masses are rotating with the same kinetic energy. If we apply a retarding torque $\tau$ on the ring, it stops after completing n revolutions in all. If same torque is applied to the disc, how many revolutions would it complete in all before stopping

• A. 4 n
• B. 2n
• C. n
• D. $\frac{n}{2}$

Answer 1

Answer: (C)

n

Answer 2

Because kinetic energy $K_R$ and retarding torque x are same, therefore in accordance with the relation, less in kinetic energy = work done by torque $=\tau.\theta$ $=\tau.2 \pi n$ So, both ring and disc stop after completing equal number of revolutions n.