Conceptual Physics (12th Edition)

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

Chapter 18 - Think and Discuss: 92

Answer

The distinction is between atoms and planets in "perpetual motion", and "perpetual motion machines". A perpetual motion machine would have to output more energy than is put into it, in order to overcome the continual energy-draining effect of friction, and the law of conservation of energy forbids that. Incidentally, even planets and stars are not in perpetual motion. They bump into interstellar hydrogen molecules, transferring a bit of kinetic energy each time. This process, slow as it is, will eventually drain away the large object's kinetic energy if given an eternity.
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