🚀 go-pugleaf

In article <1992Nov6.170202.17174@husc3.harvard.edu> mcrae@husc8.harvard.edu writes:
>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

If I understood the point of your first post, it was that in the absense of 
pressure differentials in the room, the balloon would still rise due to your
energy argument.  My point is that you cannot have the uniform pressure
with gravity, and with gravity and the resultant forces, the statistical
microscopic effects of which you wrote become paradoxical.

-AIP