Rattleback: Difference between revisions

Revision as of 16:34, 24 July 2006

Revision as of 09:58, 25 July 2006

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:''For the fictional / theorized future animal, see [[Rattleback (rodent)]].''

Also known as a "wobblestone", "celtic stone" or simply "celt", the '''rattleback''' is a semi-[[ellipsoid]]al object that reversescan itsbe spinspun direction,about whenits spunaxis onelike waya [[top]], but behavesafter normallya ifwhile spunreverses theits otherspinning direction. AlsoThis ifseems, heldat upfirst bysight, oneto endviolate then[[conservation letof goangular itmomentum]]. beginsMoreover, tofor spin.most Therattlebacks, the spinning reversal will happen when the rattleback seemsis atspun firstin sightone direction, tobut violatenot [[conservationwhen ofspun angularthe momentum]]other. These two peculiarities make the rattleback a physical curiosity that has excited human imagination since prehistorical times.

==History==

Rattlebacks were found by archeologists investigating ancient Celtic and Egyptian sites, and some old Celtic axes demonstrate properties similar to rattlebacks. Note however that the term "celt" is not related to Celtic people, but to a Latin word for ancient chisel-shaped tools and weapons. It is pronounced with a "c" as in "ceiling".

==Physics of the rattleback==

The reversal of the spin, as can be seen on the movie available [http://www.tam.uiuc.edu/toys/celt/ here], follows from the growth of an instability about the other axes of rotation of the rattleback, that is, rolling (about the main axis) and pitching (about the crosswise axis).

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[[Image:Rolling-pitching.png|thumb|Rolling and pitching movements]]

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 asymmetrical behaviour of the rattleback. 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 only exhibit spin reversal when spun in the pitching-unstable direction, while they slow down and stop spinning before the rolling instability arises when spun in the other direction. Rattlebacks made of glass, however, were reported to be able to reverse spinning in both directions, and even to incur up to four or five successive reversals during a single experiment.

which explains the asymmetrical behaviour of the rattleback.

==References==

Rattlebacks were found by archeologists investigating ancient Celtic and Egyptian sites, and some old Celtic axes demonstrate properties similar to rattlebacks. Note however that the term "celt" is not related to Celtic people, and is pronounced with a "c" as in "ceiling".

H. K. Moffat, ''Talk for the 50th anniversary of the Journal of Fluid Mechanics'', Cambridge, 21 July 2006.

A. B. Pippard, How to make a celt or rattleback, ''Eur. J. Phys.'' 11:1, 63-64, 1990. (The rattleback is made with a part of a glass wine bottle and a metal bar fixed at a small angle to the rolling axis)

==External links==

"Torque of the Devil" http://www.physics.brown.edu/physics/demopages/Demo/solids/demos/torque.html

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http://www.grand-illusions.com/toyshop/russian_rattleback/

{{classicalmechanics-stub}}

[[pl:Kamień celtycki]]