Q: A circular platform is mounted on a vertical frictionless axle. Its radius is r = 2m and its moment of inertia is I = 200 kg m2. It is initially at rest. A 70 kg man stands on the edge of the platform and begins to walk along the edge at speed. v0 1.0 m/s relative to the round. Find the angular velocity of the platform.
Sol: angular momentum of man = angular momentum of platform in opposite direction.
m v0 r = I ω
⇒ ω = 0.7 rad /s .