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Re: [Phys-l] [ap-physics] The Swinging Bucket Problem




-----Original Message-----
From: Richard Hake [mailto:rrhake@earthlink.net]
Sent: Fri 9/21/2007 6:38 PM
To: AP Physics
Cc: ap-physics@lyris.collegeboard.com;
phys-l@carnot.physics.buffalo.edu;
PHYSHARE@LISTS.PSU.EDU
Subject: Re:[ap-physics] The Swinging Bucket Problem

Dave Van Domelen (2007) in his PhysLrnR post of 21 Sep 2007 titled
"The Swinging Bucket Problem" wrote [bracketed by lines "DDDDD. . .
."; my insert at ". . . .[insert]. . . .":

DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
I'm curious as to whether anyone knows of research that's been done
on. . . . ["The Swinging Bucket Problem"]. . . . (I'll go journal
hunting if no one knows, but it's the sort of thing that I suspect
might be hard to find by name).

When introducing centripetal force, one of the classic examples is
the "how fast do I have to spin a bucket in a vertical circle to
avoid water spilling out?" problem. It's got plenty of subtleties to
help divide those who really understand the situation from the plug
and chuggers (for instance, anyone who draws tension in as a force on
the water is probably not thinking too carefully about the situation).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I am puzzled by the obvious derision being heaped upon someone who would
include the tension in the string in his analysis. The AHA! moment here
is when one realizes that as the speed is lowered the acceleration at
the top is provided less and less by the tension while the gravitational
force (weight) remains the same - eventually becoming the only force
needed to ensure the circular motion. I would suspect that any approach
which uses a free body diagram that does not include tension is simply
guess work on the student's part.

Bob at PC