What is this 'trick' and how does it work?

[Lucy_Four]

Hi Dopers,

It’s not a magic trick that I want revealing, just a cool something someone gave to me and I can’t find anything about it on the internet and wondered how it works.

Please bear with me if this is a tortuous description - it is a brass … umm… thing that is about the size of a standard pen but shaped a little like a propellor and a little fatter (maybe 1cm across the middle).

If you spin the ‘thing’ anti clockwise on the desk, it will spin quite happily, but if you spin it clockwise it appears to become unbalanced almost immediately and rattles to a halt.

Rotational speed appears to have little or no impact on how quickly it becomes unbalanced so I’m guessing it is not aerodynamic factors that cause this effect. Since it appears to be made of brass, I’m also guessing it is not a magnetic effect.

Lastly, when spun in the ‘wrong’ direction at the end of the spin it eventually ends up spinning in the opposite direction slightly (maybe quarter of a rotation on my desk here).

I was wondering what the correct name for this ‘thing’ is as well as how/why it works.

Thanks in advance!

[ftg]

It’s a rattleback.

There’s a slight unbalance to it. Rotational energy is converted to rocking motion which is then converted back to rotational energy in the other direction.

[Lucy_Four]

ftg - thank you very much!

Much appreciated - my google-fu was weak and it was driving me insane!

Cheers!

[Keeve]

Cool!!!

[engineer_comp_geek]

You can make one out of a plastic spoon (break off the handle) and some push-pins. I’ve got one like that on my desk at work that I made. It baffles people the first time they see it.

[CalMeacham]

Jearl Walker called these “The Rebellious Celts” in the first edition of his book The Flying Circus of Physics (although I notice that the current edition calls them “rattlebacks”, too). If you follow his references to articles in physics journals discussing the motion, you’ll find that they are immensely complicated. If you study the construction, you find that they’re not quite symmetric, which explains to some degree the preference to rotate in one direction. But a real understanding of the motion is surprisingly complex – much more than a common symmetrical top.
http://www.flyingcircusofphysics.com/News/NewsDetail.aspx?NewsID=50

Descriptions of the odd behavior and the name celt made their way into 19th century books about rotation but attention to them disappeared until I wrote about this curious physics in the first edition of The Flying Circus of Physics (1975). Later, A. D. Moore of the University of Michigan sent me several of the hundreds of celts that he had fashioned from dental stone. I included their description and his name for them — rattlebacks— in my Amateur Scientist article for the October 1979 issue of Scientific American. Some of Moore’s rattlebacks exhibited a second, weaker reversal. (These too resemble people I have known: “I insist on this way. No, no. I mean this other way.” Argh!)
My old article has provoked at least eight mathematically-sophisticated articles about rattlebacks. Although simple explanations are (well, er) simple, explaining the details of how the energy is shuffled from rotation to rocking and then back to rotation by instabilities and wobbling is mathematically challenging.

[Half_Man_Half_Wit]

Another toy with similar curious rotation properties is the Tippe top, which, when spun, will gradually shift its axis of rotation until it completely inverts, and rotates on the stem. Here’s a good description; scroll down to about the middle for a lovely picture of Wolfgang Pauli and Niels Bohr playing with one.