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Question
Which of the following is an example of a body exerting a force on itself?
  1. a person standing up from a seated position
  2. a car accelerating while driving
  3. both of the above
  4. none of the above
Question by OpenStax is licensed under CC BY 4.0.
Final Answer
(d)
Solution Video

OpenStax College Physics for AP® Courses Solution, Chapter 4, Problem 2 (Test Prep for AP® Courses) (1:36)

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Video Transcript

This is College Physics Answers with Shaun Dychko. This question is a little bit tricky. Part B, a car accelerating while driving is the not tricky part. That clearly is not exerting a force on itself. If you draw a free body diagram of it, you'd have the ground here and a car and the car exerts a force backwards on the ground. This is the force on the ground due to the car. Then the ground, in turn, exerts a new third law counterpart of equal magnitude but opposite direction on the car. The car is is not exerting a force on itself. It's exerting a force on the ground, and then there's a reaction for us on the car from the ground. The tricky thing is Part A, a person standing up from a seated position. We're going to count this as no, not exerting a force on itself, but you could make an argument that it is. Here's a person and when they stand up, they're going to exert a force on the ground. Then the ground will in turn exert a force upwards on the person. That's the reason why you could say that it's not exerting a force on itself. But it really depends on how broadly you define your system. The back muscle will exert a force on the hips, say, and the hips will exert a force back on the back muscles. In that sense, it is exerting a force on itself. It's a little bit of a gray area, but we're going to have to go with D on this one. None of these are exerting a force on themselves.