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Chapter 13 - ANGULAR MOMENTUM

Published online by Cambridge University Press:  05 August 2013

Steven C. Frautschi ,
Richard P. Olenick ,
Tom M. Apostol  and
David L. Goodstein
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Summary

Whereupon I computed what would be the Orb described by the Planets. … I found now that whatsoever was the law of the forces which kept the Planets in their Orbs, the areas described by a Radius drawn from them to the Sun would be proportional to the time in which they were described.

Isaac Newton

ROTARY MOTION

The world is full of things that exhibit rotary motion. They range in size from galaxies to electrons orbiting around atoms, and they include such familiar objects as orbiting planets, amusement park rides, flywheels, and bathtub vortices. What underlying principle explains the persistence of such motions? Can we describe them all in a unified way?

In Chapter 7 we studied two partial answers to such questions. One was that the moon circles the earth (or a planet circles the sun) by falling toward it continuously. Another was that an object can undergo uniform circular motion if a centripetal acceleration of magnitude v2/r is supplied. But the first answer is restricted to gravitational attraction, the second is restricted to the special case of uniform circular motion, and each addresses the persistence of such motion rather obliquely.

The concept that unifies the description of all rotary motion is angular momentum and the persistence of such motions is most directly described in terms of the conservation of angular momentum.

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Chapter
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The Mechanical Universe
Mechanics and Heat, Advanced Edition
 , pp. 335 - 362
Publisher: Cambridge University Press
Print publication year: 1986

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