AP Physics 1 Question 150: Answer and Explanation

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Question: 150


An object of mass m is attached to an object of mass 3m by a rigid bar of negligible mass and length L. Initially, the smaller object is at rest directly above the larger object, as shown in the figure. How much work is necessary to flip the object 180°, such that the larger mass is at rest directly above the smaller mass?

  • A. 2πmgL
  • B. 4mgL
  • C. 4πmgL
  • D. 2mgL

Correct Answer: D


D-Consider the system consisting of the objects and Earth, with the location of the 3m mass being the zero of gravitational energy. The initial gravitational energy of the system is mgL. After the rotation, the final gravitational energy of the system is 3mgL. That extra gravitational energy of 2mgL came from the work done on the system, meaning choice D. If you want instead to think of work on the objects as force times distance, remember that the force of Earth on the objects acts straight down, not along a circle. So the distance term to use here is just L, not πL.