🚀 go-pugleaf

In article <1992Nov5.154520.17143@husc3.harvard.edu> mcrae@husc8.harvard.edu writes:
>I've been out of physics since 1987, so don't be at all surprised if I
>make some dumb mistakes in this post. But I'm sure _someone's_ got it
>wrong, so here goes: 
>
>>>Thus, consider this: If the air surrounding the balloon had exactly the
>>>same density on all sides, then the balloon would not move (I think).
>> 
>> No, actually it would sink, I think, because there is still the effect
>> of gravity.
>
>Surely, surely not! We're talking about a standard helium-filled balloon,
>aren't we, which is less dense than air. 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 the balloon rises, then some air falls to replace it, and the
>gravitational potential energy of the system has decreased (because the
>air is more massive than the balloon). Similarly, for the balloon to fall,
>the GPE of the system would have to increase. So given that when a balloon
>rises the released GPE is dissipated as heat in the system, the balloon's
>ascent is thermodynamically irreversible (and can be driven e.g by thermal
>motion of air molecules). Therefore, it seems to me that in the case of
>uniform pressure, the gravitational field will still cause the balloon to
>rise. 
>
>The real-life case where there's a pressure gradient is trickier, because
>one has to justify the claim that the air that falls is more massive than
>the balloon allowing for them both changing volume during the process. But
>consider a rigid hollow ball, with vacuum inside. This could function as a
>balloon, being less massive than the air it displaces, and it won't change
>volume -- the above account seems to work again, suggesting that it'll
>work for everyday balloons (which don't seem to change volume much at all
>in going from, say, ground level to 100ft). 
>
>In sum, I'd have thought that the explanation for why balloons rise is the
>same as that for why submarines rise when they blow their ballast tanks --
>they're less dense than the surrounding medium, so when they rise the
>medium sinks and gravitational potential energy is lost as heat.
>
>Of course, if I've got it all wrong please don't hesitate to tell me so.
>
>Andrew.
>-- 
>Andrew McRae                       Internet: mcrae@husc.harvard.edu
>                                     BITnet: mcrae@HUSC


First off, it is not the density changes in the air that cause the balloon to 
float, it is the pressure gradient.  After all, imagine a room in a zero-g
field with air that has a one-d density gradient set up, but no pressure
gradient.  The balloon will not float or sink (neglecting diffusional effects).

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.

What would be the consequences of such a setup that Andrew described?  Well
first rework all of classical and modern physics, and then let's decide. %^)

-AIP