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.
http://www.youtube.com/watch?v=puaif3OTJL4&feature=related
http://www.youtube.com/watch?v=PydoEA5Jx5s
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.
My take on the "mysterious" 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 that tries to keep it in motion, friction that tries to slow it down, an interplay of gravity and inertia during the rocking phase, and the principle of least action that constantly seeks to balance these various forces, always taking the most easy path which eventually leads to reversal of spin.
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 tumbling motion.
4) soon we have all the energy that remains from the initially supplied push to the rattleback transformed from rotating motion 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...
6) just as the imbalance of the rattleback, in conjunction with the originally supplied spin transformed that spin into an accelerating rocking motion, during the deceleration phase, the imbalance will transform that rocking motion into rotating motion, again with the help of friction, which works to complete the 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.
I can model the forces pretty much in my mind, but it is a bit difficult to describe with only words what exactly is happening.
It is a bit as if you took a spinning ball and changed its axis of spin by 180 degrees. It would still be spinning, only now in the opposite direction. In the rattleback, the spin during the reversal phase is transformed into rocking motion and then back into spin, only 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 ...
http://www.planet-scicast.com/view_clip.cfm?cit_id=2685
or
http://www.exo.net/~pauld/activities/sweden/spoonrattleback.html
By Neil Hambleton on October 10, 2009 4:19 PM
I've lost a lot of sleep over this curious toy (which is new to me) but I think it's epicyclic. When a tilted cup rolls round a circle larger than its base with the axis of spin preceeding clockwise the body of the cup, as indicated by the handle, rotates anticlockwise. If the cup body is given a clockwise spin to start with it skids (rattles) round the epicycle, losing spin due to friction as you describe, Sepp, until it stops and then uses whatever inertia is left in the system to briefly revert to anticlockwise as before. In order not to tumble over the shape must be shallow, like a spoon or boat. The 'rocking of the boat' is due to the precession of the axis of spin being exaggerated across the narrow part of the hull.
By Neil Hambleton on October 11, 2009 1:34 PM
Now for the practical. Roll back the carpet and grab a plate. Some work better than others and some just won't work at all. One that works every time for me is of thin glass 215mm in diameter with no annulus on the base which has a 150mm diameter flat before rolling up to form a shallow rim. Spin the plate with a flick of the wrist onto a hard floor. It skids, then wobbles (i.e. precesses as explained yesterday) and finally turns briefly in the opposite direction. There's no mystery. It's a simple epicyclic process most easily understood by repeating it with the plate turned upside down. It doesn't work so well as before but the mechanism is now obvious.
A symmetrical plate can't be made to rotate by vertical pressure on its rim. This animation shows how this works for some asymmetrical rattlebacks:
http://www.technologystudent.com/cams/swash1.htm