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A circular platform is mounted on a frictionless vertical axle. Its radius R = 2 m and its moment of inertia about the axle is 200 kg m². It is initially at rest. A 50 kg man stands on the edge of the platform and begins to walk along the edge at the speed of 1 m s⁻¹ relative to the ground. Time taken by the man to complete one revolution is

1. πs
2. 3π/2 s
3. 2πs
4. π/2 s

Correct answer: 2πs

Solution

By conservation of angular momentum, the angular velocity of the platform is determined by the man's motion. The relative angular velocity of the man with respect to the platform is calculated, and the time for one revolution is found to be 2π seconds.

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