In article , Roger Chapman wrote:
On 11/10/2010 18:23, Alan Braggins wrote:
In , Roger Chapman wrote:
On 11/10/2010 15:51, Rick Cavallaro wrote:
If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.
[...]
Thanks for the pointer. It is something you either know or don't know as
it is not exactly obvious that the sum of the gravitational forces on a
body within a planetary shell is zero regardless of position, but I take
a crumb of comfort in the fact that the thought that first came to mind
was not of a planetary shell at all but of a man sized sphere with
Dribble weightless within it 'defying gravity'.

A man size sphere isn't going to have significant gravity outside either,
unless it's made of something very exotic. "no gravity inside" means
"no overall gravity inside _due to the shell_", obviously, no-one is claiming
that the shell will block the influence of masses outside that aren't
uniformly distributed. But let's assume the moon vanished with the earth,
so we don't have to worry how its orbit about the sun changes.