Question

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 $10\, rad/s$. If $I$ is the initial moment of inertia of the dancer, the new moment of inertia is

• A. $2I$
• B. $3I$
• C. $I/2$
• D. $I/3$

Answer 1

Answer: (A)

$2I$

Answer 2

Here, angular momentum is conserved. Initial angular momentum = Final angular momentum $I \times 20 = I' \times 10$, where $I'$ is new moment of inertia $\therefore I' = 2I$