This is probably a stupid question - but let's say you have a hollow Sun (M is hollow), with another mass in the center, so M and m share the same center of mass, the distance between the centers is zero. Would GMm/r^2 be infinite in this case? The force at the center of any object - if you define your system to be the center chunk of matter, and how that center chunk of matter sees the rest of the planet? ((o)) sorry... I did not get enough sleep last night, that's my excuse!