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  4. 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.

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Q. 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.

System of Particles and Rotational Motion

Solution:

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.