On 10/10/2010 19:03, Tim Streater wrote:

Quite right. If the earth suddenly vanished and left the air behind, a
balloon would no longer rise in the air. [1]

Objects in a gravitational field not in free fall (e.g objects on the
surface of the earth) can be said to be *resisting* gravity, by doing so
they experience weight. Something in free fall, such as a satellite or a
person on the ISS, is not resisting gravity and so experiences no
weight. It still has mass, of course. The word "defy" does not apply in
any relevant context.

[1] Actually this is not quite true. There would be a tiny residual
gravitational field due to the mass of the atmosphere, in a thin shell
(until it dispersed of course). The field would act as if all the mass
were concentrated at the centre of the shell.


Surely if the air stayed still the balloon would still rise, because of
the (gravitationally induced) pressure difference across its height?
And in fact would rise faster, as there would now be no gravity holding
it back?

Of course the pressure difference would vanish as the air scattered
rather rapidly...

Andy.