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) 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. 16) If the torque on an object adds up to zero A) the forces on it also add up to zero. B) the object is at rest. C) the object cannot be turning. 0) the object could be accelerating linearly but it could not be turning. E) the object could be both turning and accelerating linearly. A heavy boy and a lightweight girl are balanced on a massless seesaw. If they both move forward so that they are one-half their original distance from the pivot point, what will happen to the seesaw? Assume that both people are small enough compared to the length of the seesaw to be thought of as point masses. A) It is impossible lo say without knowing the masses. B) It is impossible to say without Knowing the distances. C) The side the boy is sitting on will tilt downward. D) Nothing will happen; the seesaw will still be balanced. E) The Side the girl is sitting on will tilt downward.

Explanation / Answer

15) c. It is an inelastic collision. The two objects stick together and kinetic energy is not conserved

16) e. Torque is independent of linear motion. It could be turning (revolving about its own axis), but only at a constant rate. It could be translating at a constant or changing velocity.

17) d. initially the torques are balanced, so m1gr1 = m2gr2 . If both distances are halved, the torques will still balance

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