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

The angular momentum of the system (ball and bar) is conserved about the hook because there are no external torques acting on the system about the hook. The only forces acting on the system are gravity and normal forces, which pass through the hook and do not create any torque about it. Therefore, the angular momentum of the system is conserved. Choice A is incorrect because the ball exerts a force on the bar during the collision and there is a torque acting on the system due to this force. Choice B is incorrect because the hook does not exert any torque on the system. Choice D is incorrect because the collision between the ball and the bar is not perfectly elastic, so kinetic energy is not conserved. Choice E is incorrect because the linear momentum of the system is not conserved during the collision because of the sticking together of the ball and the bar.

A Metal Bar Is Hanging from a Hook in the Ceiling