A metal bar is hanging from a hook in the ceiling when it is suddenly struck by

Question

A metal bar is hanging from a hook in the ceiling when it is suddenly struck by a ball that is moving horizontally (see figure). The ball is covered with glue, so it sticks to the bar. During this collision A) the angular momentum of the system (ball and bar is conserved about the hook because only gravity is acting on the system. B) both the angular momentum of the system (ball and bar and its kinetic energy are conserved. C) both the linear and the angular momentum of the system (ball and bar) are conserved. D) the angular momentum of the system (ball and bar is conserved about the hook because neither the hook nor gravity exerts any torque on this system about the hook. E) the angular momentum of the system (ball and bar) is not conserved because the hook exerts a force on the bar.

Explanation / Answer

D)  the angular momentum of the system (ball and bar) is conserved about the hook because neither the hook nor gravity exerts any torque on this system about the hook. Because

The only external forces acting on the ball and bar are gravity and the hook. Neither exerts a torque during the collision.

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