MCAT Physics Question 26: Answer and Explanation

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Question: 26

11. Josh, who has a mass of 80 kg, and Sarah, who has a mass of 50 kg, jump off a 20 m tall building and land on a fire net. The net compresses, and they bounce back up at the same time. Which of the following statements is NOT true?

  • A. Sarah will bounce higher than Josh.
  • B. For Josh, the change in speed from the start of the jump to contacting the net is
  • C. Josh will experience a greater force upon impact than Sarah.
  • D. The energy in this event is converted from potential to kinetic to elastic to kinetic.

Correct Answer: A

Explanation:

Sarah will not bounce higher than Josh. Assuming that mechanical energy is conserved, Sarah and Josh will start with a given amount of potential energy, which is converted into kinetic energy, then elastic potential energy, then kinetic energy again with no loss of energy from the system, eliminating choice (D). By this logic, both individuals should return to the same starting height. Josh starts with

of potential energy. At the moment he hits the net, all of this potential energy has been converted into kinetic energy. Therefore,

eliminating choice (B). Josh will experience a greater force upon impact because the net exerts a force proportional to weight; the higher the weight, the larger the force exerted by the net, eliminating choice (C).

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