Andy Champ wrote:

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,

Agreed.

because of
the (gravitationally induced) pressure difference across its height?

What pressure difference? If you're saying the air pressure outside
the balloon is lower near the top of the balloon than near the bottom
of the balloon, isn't this compensated for by the gas pressure inside
the balloon also being higher at the bottom than the top? The motivating
force would continue to be entirely due to density difference between gas
and air.

And in fact would rise faster, as there would now be no gravity holding
it back?

Eh? Gravity never did hold it back, it was what *caused* it to go up!
It would rise slower because there would be less gravity.

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

The pressure of both air and gas would decrease so much, that the balloon
would soon burst. But if the balloon were strong enough not to burst, the
tension in its material would at a certain size prevent the gas pressure
inside from dropping further. In due course the gas's density would exceed
that of the air outside, which is still expanding, and then the balloon
would "sink" to the middle of the air cloud.