AP Physics 1 Question 177: Answer and Explanation

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Question: 177


Question below refers to the following information:

A small ball of mass m moving to the right at speed v collides with a stationary rod, as shown in the figure. After the collision, the ball rebounds to the left with speed v1, while the rod’s center of mass moves to the left at speed v2. The rod also rotates counterclockwise.

Is angular momentum about the rod’s center of mass conserved in this collision?

  • A. No, the ball always moves in a straight line and thus does not have angular momentum.
  • B. No, nothing is spinning clockwise after the collision to cancel the rod’s spin.
  • C. Yes, the only torques acting are the ball on the rod and the rod on the ball.
  • D. Yes, the rebounding ball means the collision was elastic.

Correct Answer: C


C-Choice C states the fundamental condition for angular momentum conservation, which is correct here. The ball does have angular momentum about the rod's center of mass before and after the collision, because its line of motion does not go through the rod's center of mass. Whether or not the collision is elastic has to do with conservation of mechanical energy, not angular or linear momentum.