27 March 2009
The rattleback (wobblestone) principle
A rattleback or wobblestone is an object with a smooth bottom that can be spun like a top. Unlike conventional tops, however, the rattleback has an asymmetry which gives it a preferred direction of spin. If spun in the opposite direction, it will begin to wobble, and then change the direction of its spin.
It would seem that if a spinning object changes the direction of its spin, it is defying the law of conservation of angular momentum, but apparently it is the rocking/wobbling motion that is converted into counter-rotational motion, not the initial rotation.
I've embedded two videos illustrating the phenomenon. The top one is in a language I don't understand, but the text is unnecessary. The "toy" appears to be hand-crafted and is cleverly made in that the turtle on the "log" can be turned to change a normal top into a rattleback.
In the bottom video the rattleback is more elegantly crafted, and the video performance is more professionally done. There are many more (and perhaps even better) videos at YouTube. These apparently are sold in toy stores.
According to Wiki, some ancient celts recovered by archaeologists from Celtic and Egyptian sites have exhibited this spin-reversal phenomenon.