Question

A man standing on a platform holds weights in his outstretched arms. The system rotates freely about a central vertical axis. If he now draws the weights inwards close to his body, then choose the incorrect statement.

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

Answer 1

Answer: (B)

When a man standing on a rotating platform holds weights in his outstretched arms, draws the weights inwards close to his body, then angular velocity increases. According to law of conservation of angular momentum, $L=I\omega =$ constant, therefore when angular velocity increases, then moment of inertia of the system will decrease, so that $L$ remains constant.
Thus, statement given in option is incorrect, rest are correct.