Rattleback

For the fictional / theorized future animal, see Rattleback (rodent).

Also known as a "wobblestone", "celtic stone" or simply "celt", the rattleback is a semi-ellipsoidal object that reverses its spin direction, when spun one way, but behaves normally if spun the other. Also if held up by one end then let go it begins to spin. The rattleback seems at first sight, to violate conservation of angular momentum.

The reversal of the spin, as can be seen on the movie available 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).

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.

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".

"Torque of the Devil" http://www.physics.brown.edu/physics/demopages/Demo/solids/demos/torque.html http://www.4physics.com:8080/phy_demo/rattleback.htm

Physics demo instructions http://www.sfu.ca/physics/ugrad/courses/teaching_resources/demoindex/mechanics/mech1q/celt.html http://www.tam.uiuc.edu/toys/celt/

Sources of Rattlebacks:

http://www.physlink.com//estore/cart/BizzaroSwirls.cfm

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