Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 5 - Think and Explain: 45


Yes. No.

Work Step by Step

The upward support force from the floor is equal and opposite to the person's weight. There is zero net force on the person, which explains the person's constant velocity (zero acceleration). However, these two forces ARE NOT part of an action-reaction pair of forces. That’s because both forces act on the same object, the person, and action-reaction pairs ALWAYS act on different objects. Newton's third law says that whenever object A exerts a force on object B, we find that object B exerts an equal and opposite force on object A. The reaction to the support force exerted by the floor on the person is the person pushing down on the floor. The reaction to the weight of the person is the person pulling upward on the Earth.
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