Re: tether without ball

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 20:31 -0500 11/17/03, John S. Denker wrote:
> Now, when you suddently detach the bob, the formerly
> negligible mass is now not negligible at all. The
> mass of the string is the only interesting mass in
> the problem.
This is an important point for everyone to remember, especially when
they are making up test questions. I remember one several years ago
on the Physics AP-C test (I think--it might have been the physics
Olympiad qualification test) which had a "leaky" pendulum--a
container of "zero" mass filled with sand. One part of the question
was to draw a graph of the period as a function of time, as the sand
leaked out. The authors of the question had forgotten that as the
sand leaked out of the container, the mass of the container became
less and less negligible, and so at some point the period had to come
back to the original period when the container was full. Without
noting that the mass, though small, was not always negligible, the
problem was pretty much a mess.

Hugh
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Hugh Haskell
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<mailto:hhaskell@mindspring.com>

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