Question

Assertion : A girl sits on a rolling chair, when she stretch her arms horizontally, her speed is reduced.
Reason : Principle of conservation of angular momentum is applicable in this situation.

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (A)

If friction in the rotational mechanism is neglected, there is no external torque about the axis of rotation of the chair and hence $I\omega$ is constant, where $I$ is moment of inertia and $\omega$ is angular velocity. Stretching the arms increases $I$ about the rotation, results in decreasing the angular speed $\omega$. Bringing the arms closer, body has the opposite effect.