A 40-kg boy is standing on the edge of a stationary 30-kg platform that is free to rotate.The boy tries to walk around the platform in a counterclockwise direction.As he does:
A)the platform doesn't rotate.
B)the platform rotates in a clockwise direction just fast enough so that the boy remains stationary relative to the ground.
C)the platform rotates in a clockwise direction while the boy goes around in a counterclockwise direction relative to the ground.
D)both go around with equal angular velocities but in opposite directions.

Question

Quizplus · Accepted Answer

According to the law of conservation of angular momentum, the initial angular momentum of the system (boy + platform) is zero (since both are stationary initially). When the boy starts to walk counterclockwise on the platform, he begins to exert a torque on the platform. As the platform is free to rotate, it starts rotating in the clockwise direction to conserve angular momentum. However, the boy is still moving counterclockwise relative to the ground, creating a relative motion between him and the platform. Therefore, the correct choice is C, the platform rotates in a clockwise direction while the boy goes around in a counterclockwise direction relative to the ground.

A 40-Kg Boy Is Standing on the Edge of a Stationary