🚀 go-pugleaf

In article <1992Nov5.130316.28438@kth.se> e92_anh@vaxkab.lne.kth.se () writes:
>This will probably sound like a silly question to a lot of you, but WHY
>does a helium-filled balloon rise upward? Is it because of the
>difference in air-pressure at the balloon's top and bottom, causing the
>resulting force to press it upward?

Yes, amazingly enough.

>
>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.  (The two effects are equal if the balloon has the same
density as air, which is why for Helium the air pressure dominates.)
Furthermore, density alone does not determine pressure; your statement
is true if the temperature is the same above and below the balloon.

If you try to construct a naive equilibrium model of the atmosphere,
where pressure gradients exactly compensate gravity, I seem to remember
that you find temperature decreasing monotonically as altitude increases,
and reaching absolute zero at an altitude on the order of 50 miles.
Amusing since, though the model fails to describe at all the actually
quite complicated dynamics of the atmosphere, 50 miles is the figure
typically taken as an arbitrary "end of the atmosphere, beginning of
space" estimate.  (I guess before the shuttle that was as high as anything
resembling an airplane had flown, too.)

-- 
Aephraim M. Steinberg                    | "WHY must I treat the measuring
UCB Physics                              | device classically??  What will
aephraim@physics.berkeley.edu            | happen to me if I don't??"
                                         |       -- Eugene Wigner