LinkNotes: The Rattleback

2009-10-07

The Rattleback

Clipped from: The Amazing Rattleback!

Try to spin it clockwise. Watch it spin, shake, and rattle, and slow down.
Then, watch it spin in the opposite direction!

In Wales, it is called a rebellious celt. To us, it's the rattleback. "It" is a long, thin plastic toy with a base shaped like the hull of a boat. When you spin it one way, it turns a few times before the ends start to rattle up and down. The more it wobbles, the slower it rotates - until it stops spinning altogether. Finally, it starts to spin in the opposite direction. What could possibly cause this?

Clipped from: Sepp Hasslberger: Rattleback 'mystery' explained

Rattleback 'mystery' explained

A rattleback, also called a celt or wobblestone, is an interesting little piece of material with a roundish bottom and an unbalanced distribution of weight. When spun, it will soon start to rock and then, as the rocking motion subsides, it will settle down to spinning in the reverse direction. Rattlebacks do not work well on a slippery surface, which means that a certain amount of friction is a necessary condition for it to work...

Clipped from: YouTube - russian rattleback

Clipped from: Rattleback - Wikipedia, the free encyclopedia

Physics

The spin-reversal motion follows from the growth of instabilities on the other rotation axes, that are rolling (on the main axis) and pitching (on the crosswise axis).

When there is an asymmetry in the mass distribution with respect to the plane formed by the pitching and the vertical axes, a coupling of these two instabilities arises; one can imagine how the asymmetry in mass will deviate the rattleback when pitching, which will create some rolling.

The amplified mode will differ depending on the spin direction, which explains the rattleback asymmetrical behavior. Depending on whether it is rather a pitching or rolling instability that dominates, the growth rate will be very high or quite low.

This explains why, due to friction, most rattlebacks exhibit spin-reversal motion only when spun in the pitching-unstable direction, while they slow down and stop spinning before the rolling instability arises when spun in the "stable" direction. Also, after stopping, the spin in the "stable" direction is considerably slower than the original spin speed. Some rattlebacks, however, exhibit "unstable behavior" when spun in either direction, and incur several successive spin reversals per spin.^[3]

Other ways to add motion to a rattleback include tapping by pressing down momentarily on either of its ends, and rocking by pressing down repeatedly on either of its ends.

Clipped from: YouTube - Rattleback