Physics
An ice skater spins with his arms outstretched. When he pulls his arms in and raises them above his head he spins much faster than before. Did a torque act on the ice skater? If not how could his angular velocity have increased?
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Yes, a torque acted on the ice skater when he pulled his arms in and raised them above his head. This torque caused a decrease in the skater's moment of inertia, which in turn, caused an increase in the skater's angular velocity. The decrease in the moment of inertia was the result of the skater transferring some of the mass in his body, which had previously been a source of inertia, to his arms and upper body, which had been further away from the axis of rotation and, therefore, had had less of an effect on the rotational speed. By moving his mass towards the axis of rotation, the skater reduced his moment of inertia, allowing more force to be applied to the rest of his mass, resulting in an increase in angular velocity.
