Nobody has yet mentioned the *quantitative* demonstration that the 
gravity of an uniform hollow sphere is cancelled inside.

Consider a particle within the sphere and a pair of equal and opposite 
narrow cones whose apex is that particle.  Each cone meets the sphere in 
an ellipse.  (The ellipses have similar shape because the axis of the 
cones meets the sphere at the same angle on both sides.)  The area of 
the ellipse, and thus the amount of mass pulling the particle in that 
direction, is proportional to the square of the distance from that part 
of the sphere to the particle.  But to get the amount of force you must 
then divide by, guess what, the square of that same distance.  Thus the 
forces on the particle from the two opposite cones are equal and opposite.

-- 
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine

Post a reply to this message