6.67 * 10^(-11) m^3 kg^(-1) s^(-2) * 5.98 * 10^24 kg
(a) a = GM/r2, so a = ------------------------------------------------------
                                      (6.38 * 10^6 m)^2

                    = 9.8 m/s^2.

                       6.67 * 10^(-11) m^3 kg^(-1) s^(-2) * 2.0 * 10^30 kg
(b) a = GM/r2, so a = ------------------------------------------------------
                                      (1.5 * 10^11 m)^2

                    = 0.0059 m/s^2.

The Sun accelerates Earth much less than Earth accelerates an object on its 
surface.  Interesting...