Date: Fri, 12 Jul 2002 14:51:55 -0700 (PDT)

Author: William Beaty

Subject: Re: buoyancy/fluids labs?


On Fri, 12 Jul 2002, john power wrote:

> Hi Eric,
> Well, this will probably fall on deaf ears, but I would like urge you to
> keep the lab the way it is. You imply that this lab is boring, but I'd
> point out that while it may be boring to a veteran physics instructor,
> it
> would probably be viewed as "understandable" by a student. The
> current lab you describe has none of the modern gee whiz nonsense
> that may impress casual students but probably discourages the serious
> ones.

I agree. It answers personal questions I myself had as a student.

About the only possible change I could suggest is to measure the increased
force which comes about from inflating a submerged balloon (but then it's
harder to measure the balloon volume.)

Static forces are "force pairs," so whenever we inflate a helium balloon
in the everyday environment, where is the other end of that lifting force?
(Doesn't the Earth's entire atmosphere become a bit deeper as we inflate
the balloon, and the weight of the balloon is therefore distributed
uniformly over the entire Earth?) Use a submerged balloon with the water
tank representing Earth's atmosphere.

Hang some weights on your submerged balloon so it hovers at constant
altitude rather than accelerating vertically. Must those weights not
"push" somehow upon the Earth? Even without the weights, doesn't the F=MA
force which accelerates the balloon upwards require another equal and
opposite which somehow pushes the Earth downwards?

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