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...
There are several videos on YouTube that show the spin reversal we see in a rattleback. You can start with this one and see below or check the "related videos" on the YouTube page for more.
Wikipedia about the rattleback: "This spin-reversal motion seems, at first sight, to violate the angular-momentum conservation law of physics. Moreover, for most rattlebacks, the motion will happen when the rattleback is spun in one direction, but not when spun in the other. Some exceptional rattlebacks will reverse when spun in either direction. This makes the rattleback a physical curiosity that has excited human imagination since prehistorical times."
I was first made aware of the existence of this curiosity by David Tombe, who mentions rattlebacks in some of his article and tries to explain their spin reversal with reference to the coriolis force. In the latest revision of one article titled The Cause of Coriolis Force (http://www.cartesio-episteme.net/ep8/vorticity--new.pdf) Tombe's treatment of the rattleback starts out like this:
The rattleback is the most mysterious of all the mechanical devices. A spinning rattleback undergoes a complete 180 degree reversal of its angular momentum without any apparent source of reversal torque. The situation is further complicated by the fact that some rattlebacks can work in both directions. Modern physicists cannot explain the rattleback mystery because they deny all three of the vital ingredients that are necessary for its full understanding. We first need centrifugal charge followed by axial Coriolis force. We also need the electric sea in order to induce these convective effects, and also to give the reversal torque something to kick off against, as per Newton's third law of motion. The rattleback is very obviously tangled up in a very subtle elastic medium, but this medium along with the centrifugal force and the axial Coriolis force which are the basis of the interaction with this medium, are denied in modern physics.
In a March 2011 email, David Tombe sent a link to a new, much simplified article explaining the rattleback. The new article, titled "The Coriolis Force and the Rattleback" can be found here: http://www.cartesio-episteme.net/ep8/reversaltorque.pdf
My own take on the reversal of spin of the rattleback is that it is due to the interplay of known forces - inertia, gravity and friction. How exactly? If you check on the net, several people say they understand how the rattleback works, but I have yet to see a clear and simple description of the mechanics and the forces involved.
So here is my attempt to give a point for point explanation of the process of torque reversal in the rattleback ...
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My view on the operation of the rattleback
No unknown forces are needed to engineer the reversal of spin. All we need are the initial input that sets the rattleback in motion, inertia which tries to keep it in motion and friction which slows down the rotation and ends up giving way to a rocking phase. There is an interplay of gravity and inertia during the rocking phase, which combined with an imbalance of weight distribution induces a circular, instead of a simple up-and-down rocking motion. The principle of least action that constantly seeks to balance these various forces, always taking the most easy path, eventually leads to a reversal of the original spin that set the rattleback in motion...
1) acceleration is supplied by the initial push that sets the rattleback in spinning motion. Inertia tends to perpetuate that motion.
2) friction counters the motion, supplying a decelerating force that over time would force the rattleback to come to a standstill, if it weren't for an imbalance in its weight distribution...
3) before the rattleback can come to a standstill, the unbalanced weight distribution of the rattleback works towards evading the friction by inducing a rocking motion.
4) soon we have all the energy that remains from the initially supplied push to the rattleback transformed from rotation to rocking motion. This kind of motion is an interplay between inertia and gravity, where gravity and inertia will continually overcompensate. The tendency is to just keep rocking, friction is reduced to almost nothing.
5) this rocking motion would in time exhaust itself, leaving the rattleback motionless, if it weren't for the imbalance in its weight distribution giving a circular component to the rocking motion...
6) the circular component of the rocking motion induced by the imbalance of weight distribution will now transform that rocking motion back into a rotating motion - again with the help of friction - which works to induce a reversal of spin.
7) We now have a spinning motion again, much slower than the original, and in the reverse direction. It was friction and imbalance that worked together to engineer this reversal of motion, with the help of inertia.
It is a bit as if you took a spinning ball and changed its axis of spin by 180 degrees. It would be continuing to spin, only now it appears to spin in the opposite direction.
Similarly, in the rattleback, the originally supplied spin is transformed into rocking motion and then back into spin, now in the reverse direction.
How to make your own rattleback
If you want to experiment with this, you can do so simply by using ...
Update July 2014:
Here is a description by Brian Lewis, who analysed the movement of a rattleback and shared it with us...