🚀 go-pugleaf

In article <1992Nov6.152707.8015@newshost.lanl.gov>, u108502@beta.lanl.gov (Andrew Poutiatine) writes:
> Secondly, although Andrew McRae's example is thought provoking, it is only
> that, since if in a gravitational field you somehow removed the pressure
> gradient, there would be nothing to hold the air in place, and it would
> accelerate constantly, as would the balloon, in the direction of the gravity
> vector.
> You simply cannot hold air in its place without a pressure gradient in a grav-
> itational field.

That's why I wrote:

>>                                         So consider a helium balloon in a
>>closed room with constant (to a close-enough approximation) pressure
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>throughout, so that the balloon's volume is independent of its position.

If you can show that the approximation fails, I'd be fascinated. I only
introduced it to avoid having to worry about the details of the balloon
expanding as it rose -- in fact, I'm sure that replacing it by the
assumption that "no matter where the balloon is in the room, it displaces
air that weighs more than it does" allows the argument to go through
unchanged.

Of course, one should _expect_ that the balloon's behaviour could be
explained both by considering the forces that act and by considering
conservation of energy -- it always works that way! (I just think that in
this case, the energy argument is a damn' sight simpler!)

Andrew.

-- 
Andrew McRae                       Internet: mcrae@husc.harvard.edu
                                     BITnet: mcrae@HUSC