Question

A disc is rotating with angular speed $\omega$. If a child sits on it, what is conserved :-

• A. linear momentum
• B. angular momentum
• C. kinetic energy
• D. potential energy

Answer 1

Answer: (B)

When a child sits on a rotating disc, no external torque is introduced.
Hence the angular momentum of the system is conserved.
But the moment of inertia of the system will increase and as a result, the angular speed of the disc will decrease to maintain constant angular momentum.
[$\because$angular momentum = moment of inertia $\times$ angular velocity]