The strange motion of the Rattleback
The rattleback is a plastic children’s toy that exhibits some remarkable behaviour: spin it anticlockwise and it will rotate until friction at the pivot point causes it to stop. Spin it clockwise and it rotates, starts pitching up and down and then stops and spins back in the opposite direction !!
If you find this puzzling, you are not alone; the rattleback or celt has been the subject of a great deal of research and only in the 1980’s did full mathematical explanations emerge. Even then, Mont Hubbard, a professor of Mechanical Engineering at UC Davis and one of the first to put forward a rigourous mathematical analysis, still struggled: "It’s only clear through the equations" he said, adding "I don’t intuitively understand it.
What is apparent is that the motion of the rattleback relies on 3 essential ingredients :
- 2 different radii for the base; one long and the other short
- The symmetry axes offeset from the inertial axes
- A different distribution of mass about each axis; i.e. it should be long a thin, like the hull of a ship.
If you would like to know more about how it actually works, then Jearl Walker (he of Halliday, Resnick and Walker fame) wrote an article for Scientific American in 1979 explaining the bahaviour. For that, you’ll have to look in the Library!