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.