AP Physics 1 Question 122: Answer and Explanation

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Question: 122

2. A swimmer is able to propel himself forward through the water by moving his arms. Which of the following correctly states the applicant and recipient of the force responsible for the swimmer’s forward acceleration?

  • A. The force of the surrounding water on the swimmer’s arms
  • B. The force of the swimmer’s arms on the swimmer’s torso
  • C. The force of the swimmer’s arms on the surrounding water
  • D. The force of the swimmer’s torso on the swimmer’s arms

Correct Answer: A


A-Choice C is not correct because if the swimmer is accelerating, the responsible force must act on the swimmer, not on something else. That force can act on any part of the swimmer's body. But a force provided by the swimmer himself on the swimmer himself won't accelerate him-that's like pulling yourself up by your own bootstraps. Choices B and D do not consider a force external to the swimmer. The answer is A: the Newton's third law force pair to the force of the swimmer's arms on the water.