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) the angular momentum of the system (ball and bar) is not conserved because the hook exerts a force on the bar.
C) 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.
D) both the angular momentum of the system (ball and bar) and its kinetic energy are conserved.
E) both the linear momentum and the angular momentum of the system (ball and bar) are conserved.

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) the angular momentum of the system (ball and bar) is not conserved because the hook exerts a force on the bar. 
C) 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. 
D) both the angular momentum of the system (ball and bar) and its kinetic energy are conserved. 
E) both the linear momentum and the angular momentum of the system (ball and bar) are conserved.

Quizplus · Accepted Answer

Torque is the force that causes rotation, and torque depends on the force and the distance from the pivot point. In this case, the only forces acting on the system are gravity and the collision with the ball, both of which pass through the pivot point (the hook). Therefore, there is no net torque on the system, and angular momentum is conserved. The hook may exert a force on the bar, but this force does not produce a torque because it passes through the pivot point. The conservation of kinetic energy is not mentioned and cannot be inferred from the given information. The conservation of linear momentum is also not mentioned, but it is not relevant for this question as the answer specifically asks about angular momentum conservation.

A Metal Bar Is Hanging from a Hook in the Ceiling