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Question:
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?



Replies:
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...!

topper


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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