Conceptual Physics (12th Edition)

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

Chapter 18 - Think and Discuss: 92


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.
Update this answer!

You can help us out by revising, improving and updating this answer.

Update this answer

After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.