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A swimmer pushes on a pool wall with her feet and accelerates away from it. Why do the two forces of this action and reaction pair not cancel?
ABecause one of the two forces is much larger
BBecause the wall is far too heavy to move at all
CBecause friction removes one of the two forces
DBecause the two forces act on different systems
Answer & Solution
Correct answer: D. Because the two forces act on different systems
1. The swimmer pushes on the wall and the wall pushes back on her feet.
2. The two forces are equal in magnitude and opposite in direction.
3. They do not cancel because they act on different systems.
4. Taking the swimmer as the system, only the force of the wall on her feet is external to it.
5. The force of her feet on the wall acts on the wall, so it never enters her equation of motion.
6. Cancellation would need both forces acting on the same body, which is not the case.
_Source: OpenStax College Physics (CC BY 4.0), Ch 4 "Dynamics: Force and Newton's Laws of Motion", section 4.4 Newton's Third Law of Motion: Symmetry in Forces_
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