Conceptual Physics (12th Edition)

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

Chapter 9 - Think and Explain: 63


The high-altitude plane is not in free fall. It moves with approximately constant velocity, and passengers experience no net force. If a passenger sat on a scale, it would register approximately his usual weight because the upward support/normal force of the seat cancels the downward force of gravity. The orbiting space shuttle is in a state of free fall. No lift forces keep it up; it orbits the Earth in free fall. Though it falls, it gets no closer to the Earth because it is moving quickly enough that the Earth curves away underneath it at the same rate at which it falls. This is similar to the person in the free-falling elevator in Figure 9.9.
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