Question

A man standing on a platform holds weight in his outstreached arms. The system rotates freely about a central vertical axis. If he now draws the weights inward close to his body

• A. The angular velocity of the system will increase
• B. The angular momentum of the system will increase
• C. The kinetic energy of the system will increase
• D. He will have to expend some energy to draw the weights in

Answer 1

The correct option is D He will have to expend some energy to draw the weights in

$\tau =0=\frac{dl}{dt}$

$\Rightarrow L=constant$, Angular momentum remains constant.

By drawing arms in, $I$ is decreased.

$L=I\omega$

For $L$ to remain constant, $\omega$ increases.

$KE=\frac{L^2}{2I}$, $I$ decreases.

$\Rightarrow KEincreases$

The energy spent in drawing weight is converted into KE of rotation