First slide

Law of conservation of angular momentum

Question

A boy standing on a rotating table with heavy spheres in his hands, suddenly brings his hands close to his body. The angular velocity of the table

Easy

A

Remains unchanged
B

Becomes zero
C

Decreases
D

Increases

Solution

L = I ω
⇒ ω = L / I
Since net external torque on the system is zero, its angular momentum (L) about the axis of rotation remains conserved. When the boy brings his hands close to the body, moment of inertia I of the system decreases. Since W α 1 / I, angular velocity increases.

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