First slide

Law of conservation of angular momentum

Question

A ballet dancer, dancing on a smooth floor is spinning about a vertical axis with her arms folded with angular velocity of 20 rad/s. When she stretches her arms fully, the spinning speed decreases to l0 rad/s. If I is the initial moment of inertia of the dancer, the new moment of inertia is

Easy

A

2l
B

3l
C

$\frac{I}{2}$
D

$\frac{I}{3}$

Solution

Here, angular momentum is conserved.
Initial angular momentum: Final angular momentum
$I \times 20 = I^{'} \times 10$
Where 1 is new moment of inertia
$I^{'} = 2 I$

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