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motion: TOC for Knowledge Concepts, Exercises, and Solutions

In the spirit of Exercise 33 above, let's explain another one of Kepler's 
empirical laws using physical and mathematical theory.

Explain how Kepler's third law is a consequence of Newton's law of gravity applied 
to orbital motion.  Two points are key: first, in an orbit, the gravitational 
acceleration must equal the centripetal acceleration at any point in time; second, 
the period of an orbit is the distance traveled in one orbit divided by the 
orbital velocity.