(a) The distance from the Moon to the closest edge of Earth's surface is

    384,000,000 - 6,378,000 = 377,622,000 m

                            6.67 * 10^(-11) m^3 kg^(-1) s^(-2) * 7.36 * 10^22 kg
So acceleration a = GM/r2 = -----------------------------------------------------
                                           (377,622,000 m)^2
   = 0.000035 m/s^2.


(b) The distance from the Moon to the farthest edge of Earth's surface is

    384,000,000 + 6,378,000 = 390,378,000 m

                            6.67 * 10^(-11) m^3 kg^(-1) s^(-2) * 7.36 * 10^22 kg
So acceleration a = GM/r2 = -----------------------------------------------------
                                           (390,378,000 m)^2
   = 0.000033 m/s^2.


(c) The difference between these two accelerations is 0.000002 m/s^2, or 2 
millionths of a meter per second squared.  Since Earth's gravitational 
acceleration is about 10 m/s^2, this "tidal differential" is 1/5,000,000 the
strength of Earth's gravity at Earth's surface.

This "tidal differential," as you know from the text, is responsible for our ocean 
tides here on Earth. The difference in acceleration seems tiny - but it works over 
the entire surface of Earth as our planet rotates; so the amount of total mass 
that's moved is tremendous.  As a result, ocean tides carry a lot of power 
compared to typical human consumption levels.  Can we harness that tidal power for 
use in our homes and businesses?  At this time, the technology is promising.