Conceptual Physics (12th Edition)

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

Chapter 8 - Think and Explain: 95


Building skyscrapers slows down the Earth's rotation, lengthening the day by a tiny, tiny amount. Leaves falling have the opposite effect. Both effects are entirely negligible.

Work Step by Step

This situation is similar to the one shown in Figure 8.52, but in reverse. The skyscraper materials that are mined and erected end up farther from the Earth’s rotational axis. Moving mass away from the axis of rotation increases the rotational inertia of the system. No external torques act on the system (Earth + materials), so angular momentum is conserved. The product of rotational inertia and rotational speed stays constant. Since the first went up, the second must go down. The rotational speed slows down, making the day longer. The opposite effect occurs for falling leaves, since the process decreases their distance from the Earth’s rotational axis. Arguably, the falling leaves are just undoing the process that occurred when the tree lifted some of the leaf's raw materials out of the soil.
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.