Q.

A ballet dancer, dancing on a smooth floor is spinning about a vertical axis with her arms folded with an angular velocity of 20 rad/s. When she stretches her arms fully, the spinning speed decrease as 10rad/s. If I is the initial moment of inertia of the dancer, the new moment of inertia is

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a

2 I

b

3 I

c

 I / 2

d

I / 3

answer is A.

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Detailed Solution

Here, angular momentum is conserved.

Initial angular momentum = Final angular momentum I ×20=I'×10, where I' is new moment of inertia

  I'=2 I

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A ballet dancer, dancing on a smooth floor is spinning about a vertical axis with her arms folded with an angular velocity of 20 rad/s. When she stretches her arms fully, the spinning speed decrease as 10rad/s. If I is the initial moment of inertia of the dancer, the new moment of inertia is