Anton Sherwood wrote:
> 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.

Thanks everyone for this great discussion. I've been following along, 
and sticking my two cents in when I can, but by the time I got as far as 
Anton's post (above) I realised I was getting out of my depth. So I 
released the tension this had caused with a quick render!

Thanks again! (and don't stop!)
Rick Measham

(P.S. Anton: please don't take offense as none is intended)

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