spinning toy-- the arc
A fellow teacher recently bought a toy that leads to
questions. It is a piece of wood, elliptical in shape,
flat on top and curved on the bottom. If you spin it
one direction, it spins for quite a while. If you spin
it the opposite direction, it spins briefly, stops,
then the spinning reverses! We are stumped as to how
this could happen. We thought about coreolis effect,
but she has two of these toys, and their preferred
spin is in opposite directions!
What could cause this?
The toy, sometimes called a "rattleback,"
is said by highfalutin types to exhibit
"vibration-rotation coupling." The curvature of
the surface is indeed the critical factor that
causes rotational motion to be transferred into
out-of-plane vibrations (rattling) which in turn
couples back into rotational motion - but now
in the opposite direction.
It is a heck of a gizmo, and some of my molecular spectroscopy
friends and I have argued long into the night about some
of the details. I would really be interested to hear what
timo and the other NEWTON scientists can tell us about some
of the details...!
The rattleback has been analyzed in great detail. For a general
description in layman's terms, refer to The Amature Scientist
(published by Scientific American), It has been discussed there
a few times and these write-ups are contained in an Amature Scientist
compendium about the physics of rotation. This book also has crude
analyses of tops, including the flipping tippe top, boomerangs,
martial arts, and other popular topics that usually interest those
intrigued by the rattleback. I do not have the exact reference info
handy at the moment. If anyone cannot find it, or would like references
for more accurate, mathematical analyses of the rattleback, just drop
me an e-mail and I will provide them.
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Update: June 2012