Which of the following is an example of a body exerting a force on itself?
- a person standing up from a seated position
- a car accelerating while driving
- both of the above
- none of the above
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.