Conceptual Physics (12th Edition)

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

Chapter 16 - Think and Discuss: 90

Answer

In the atmosphere, species of gas molecules are mixed, which means they have the same temperature. This means they have the same average translational kinetic energy per molecule. $KE = \frac{1}{2} mv^{2}$. Helium molecules must have a higher average speed than heavier molecules (e.g., oxygen, carbon dioxide) to have the same average kinetic energy per molecule. Their average speed exceeds Earth's escape speed, and they eventually fly off into space.
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