A student spins around while standing on a frictionless platform. His arms are o
ID: 1549957 • Letter: A
Question
A student spins around while standing on a frictionless platform. His arms are outstretched in front of him and he holds a weight in each hand. When he pulls in his arms...?
A. his moment of inertia decreases and his angular speed increases
B. his moment of inertia decreases and his angular speed decreases
C. his moment of inertia increases and his angular speed increases
D. his moment of inertia increases and his angular speed decreases
E. his moment of inertia increases and his angular speed remains the same
Explanation / Answer
The angular momentum of the system is conserved before and after pulling the hand because the external torque acting on the system is zero. The angular momentum of the system is the product of moment of interia and angular velocity of the system. When he pulls his arm, the moment of inertia decreases because the distance of weight from axis of rotation decreases and thus the angular velocity of the system increases because angular momentum of the system is conserved. So the correct option is A.
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