header bar

Wednesday, September 4, 2013

the rattleback

Few toys have so wrought the attention of physicists as the rattleback. Attempts to pin down the physics behind the curious motion of this device are woven throughout the peer reviewed literature, and the sophistication of the math in some recent articles is on par with serious rocket science (as in this article by Lasse Franti of the University of Helsinki).

Here is a rattleback I crafted from a piece of walnut with some sand paper and a bit of effort:

A video posted by physicsfun (@physicsfun) on

Rattlebacks all have a curious preference for a particular direction of spin− this one's "direction of stability" is counter-clockwise. Note that the reversal has two parts: First the clockwise spin is reduced as the kinetic energy transfers to a wobbling motion, then the energy transfers from this wobble to a spinning motion in the final direction.

The change of direction arises from a complex interplay of friction between the rattleback and the table top, a dance enabled by an instability due to the preferred axis of rotation not being aligned with the geometric axis of symmetry. This is quite like an unbalanced tire; vibrations will occur when spun. Early in the video one can see that the bottom of this kind of rattleback is somewhat propeller shaped. This asymmetry allows the preferred axis of rotation, related to the distribution of mass within the rattleback, to be different than the geometric axis. An alternative design has instead a symmetric bottom, but with added weights to offset the preferred rotation axis (of which I will show an example in a future post).

A shape so simple, yet behavior so complex that the most advanced theoretical techniques are needed to model it. A classic physics toy!

A video posted by physicsfun (@physicsfun) on

These acrylic versions work great and are inexpensive:

From Educational Innovations: BUY NOW Rattlebacks

From Amazon: BUY NOW Rattlebacks

From eBay: BUY NOW Rattleback

No comments: