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(John Flanagan) wrote:

On Mon, 21 Jul 2003 02:05:39 GMT, Jeff Wisnia
wrote:

So what *is* the pressure in a typical rubber party balloon. IIRC, it gets
*lower* as the balloon expands. I think it's the same with soap bubbles.

I suppose the pressure in one of those mylar party balloons must be pretty
close to one atmosphere.

If the sides are not rigid it's always *exactly* atmospheric pressure.

This is not true unless there's a hole in the balloon to allow the
inside and outside pressures to equalize. As long as the balloon is
sealed, the internal pressure can be anywhere from ambient atmospheric
pressure up to the maximum (gauge) pressure the balloon can withstand.
The internal pressure is balanced not only by the external pressure,
but also by stresses in the material of the balloon. The same is true
for basketballs and pneumatic tires.

If it expands either atmospheric pressure has dropped or the
temperature of the balloon has gone up.

This is generally true, but it doesn't mean that the final pressure is
equal to the initial pressure, or that either one is equal to the
ambient atmospheric pressure. The expansion will relieve some of the
increased pressure differential, but not necessarily all of it. The
amount will depend on the stress/strain characteristics of the balloon
material.

Here's a question for everyone. Explain *exactly* the mechanics of
how and why a balloon floats :^)? Not as easy as one might think.

The forces due to gravitational acceleration acting on the mass of air
in the atmosphere results in a vertical pressure gradient (lower
pressure at higher altitudes), which means the pressure acting on the
top part of the balloon is lower than the pressure acting on the
bottom part. Integrating those pressures over the surface of the
balloon gives a net upward force (aka buoyant force). If the weight of
the balloon is less than the buoyant force, the balloon will rise; if
the weight is more than the buoyant force, it will sink; if the weight
is exactly equal to the buoyant force, the balloon will stay where it
is (but it's an unstable equilibrium).

And a second one is why when you have a helium balloon in your car and
put the brakes on to stop, the balloon will float to the back of the
car instead of flying to the front like everything else. Again, what
are the mechanics that make it do so?

The same as above, except that now we're talking about an acceleration
due to braking instead of due to gravity, resulting in a horizontal
pressure gradient instead of a vertical gradient. The balloon's mass
multiplied by this acceleration presumably results in a horizontal
"weight" which is less than the horizontal "buoyant" force, so the
balloon "rises" (i.e., moves in the direction of the acceleration).

Bert