Gizmo: Newsletter Challenge (08/08/06)

I've heard of a toy like that, which the shape of the bottom is curved in such a way that it spins freely in one direction, but when reversed it tends to oscillate oblique to the direction of the plane that it spins in.

I think, and maybe someone else knows the physics here, that the kinetic energy of the spin is translated into the oscillation (rocking up and down) and then back into a spin in the other direction.

I don't know exactly what the underlying physics is that causes this nor the details on how to shape the bottom curve so that it does this, but you can manufacture these so that you can preset the direction that it freely spins.

I am looking forward to hear from someone that knows more on this.

Scientific American ran an article on celts - [Walker, Jearl , "Rattlebacks and Tippe Tops" (from the Amateur Scientist, Scientific American Oct 1979, 172)] IIRC the sord is from Latin "celtis", ="chisel" because some stone chisels exhibiti this preferred spin. I first made one based on a magazine article: The Mysterious Celt (wobblestone, rattleback, tates). A wooden "top" which will spin in only one direction. Started wrong, it will wobble, rattle, and then reverse its direction. FINE WOODWORKING #53 Jul-Aug 1985 pg. 68. "Tates" is from a pun - it seems that people attempted to use the device as a compass (failing, of course); we still say, "He who has a tates is lost." I've made them in clockwise and counterclockwise versions, and one with a shpe like a worn soap bar that has both right- and left-handed surfaces. There are videos of these in action, but the ones I knew of are dead links. Try http://www.sfu.ca/physics/ugrad/courses/teaching_r esources/demoindex/mechanics/mech1q/celt.html .

http://www.physics.brown.edu/physics/demopages/Dem o/solids/demos/torque.html explains it all

Use this link to view the page Waidesworld is talking about:

http://www.physics.brown.edu/physics/demopages/Dem o/solids/demos/torque.html

Well, you beat me to it. I remember that article from SA. Interesting enough that the name stuck but I don't remember the physics.

it seems to me that the friciton is the real key here. instead of just rocking back and forth like a symmetrical piece would, it rocks and rolls at the same time, and the friction on the bottom actually causes the torque to make the toy turn. this is just my guess, the above link didn't get into any specific detail. does this sound right or am i way off?

So let'd work this backwards, spin the gizmo against the preferred direction, and a 'wobble' is initiated...it starts to rock...air pressure might do this, much like an uneven wing aerofoil. Th point of ballance must be smooth and relatively friction free. A rocking motion must impart a preferred directional spin.....It's time to soak that old bucket of modelling clay!

I still have my GlobalSpec.com rattleback toy, sent to me when I joined the mailing list about two years ago and started entering the Challenge - the first question being this one here.

It confused the h*ll out of several other engineers here.