AP Physics 1 Question 115: Answer and Explanation

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Question: 115

5. A 1-kg object is released from rest at the top of a rough-surfaced incline. The object slides without rotating to the bottom of the incline. The object’s kinetic energy at the bottom must be

  • A. Equal to the block’s gravitational potential energy when it was released, because total mechanical energy must be conserved.
  • B. Equal to the block’s gravitational potential energy when it was released, because the gain in kinetic energy compensates for the mechanical energy lost to thermal energy on the rough incline.
  • C. Less than the block’s gravitational potential energy when it was released, because the gravitational potential energy was converted both to thermal energy and to kinetic energy.
  • D. Less than the block’s gravitational potential energy when it was released, because the work done by the friction force must be greater than the block’s gain in kinetic energy.

Correct Answer: C

Explanation:

C-Mechanical energy is not conserved because of the work done by the nonconservative friction force provided by the rough-surfaced incline. The object starts with only gravitational potential energy, because it is higher than its lowest position and at rest. This gravitational energy is converted to thermal energy via the friction force, and to kinetic energy because the object speeds up.