sorry to disturbe all
but
if the equation is (F = GMm/d^2) how come an object at the center of a planet has a gravitional force of 0 (x/0 doesn't equal 0)??? what is D in this equations, i mean i know it is ditance but distance from where to where ???
and about the hollow sphere, an object not in the middle of the hollow sphere will be closed to one part of the sphere then another, if D is the distance between the center of object and center of shpere, how come it has no gravitional force ????
BTW, X/0 is really undefined, but english is not my native language, so i didn't know what it is called in english, so i thought infinite will be the closest i can think of
thanks in advance