Date: Tue, 04 Oct 2011 10:31:54
Author: --- "J. Terrence Klopcic"
Subject: Air Track Collision "Discovery"
Please ignore/excuse this "revelation" if it is already well-known.
However, for those who, like me, hadn't known ...
In our elastically-colliding-gliders-on-an-airtrack experiment, we use
repelling magnets to avoid the losses associated with physical contact.
The arrangement was to tape a little button magnet on each colliding
glider with like poles facing outward. Although the collisions appeared
smooth to the eye, we found that the calculated momentum and energy was
not well conserved, especially when a smaller glider was pushed into a
larger one. We surmised - and confirmed with high speed photography -
that the opposing magnetic fields caused the gliders to tip slightly as
one magnet is repelled to move above the other. Tipping, of course,
destroys conservation since any tipping causes a jetting of the
supporting air cushion and any touching of the glider to the track is fatal.
This year, we introduced a remedy that seems to be resulting in far
better momentum and energy conservation. Instead of using button
magnets with opposing poles, we bought some small cylindrical magnets
and mounted them with their poles vertical. Since the magnets on both
gliders have their poles similarly aligned, they still repel, of
course. However, if one considers the shape of the magnetic field from
a bar magnet, one notices that the vertical gradient of the field is
somewhat lower along the side of the bar than at the poles. We believe
that this smaller gradient results in less tipping force, thus lessening
the creation of outside forces on the glider system.
Again, if this is old news to you, please ignore this e-mail. if you
find it potentially useful, enjoy.
P.S. When taping anything to a glider, take care not to cover the air
passage on the top of the "V". Doing so creates a little jet engine. :-[
J. Terrence Klopcic, PhD
Director of Laboratories
Departments of Physics and Mathematics
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