Hand Bent Spoon

[LucyR]

The name of this user has reminded me of an interesting toy called the “rattleback”. It’s the kind of thing that’s probably been discussed on this forum before but I don’t recall seeing it mentioned.

Anyway, take a common or garden metal spoon and bend the handle over the top of the bowl. The clever bit is to bend it so that the handle points somewhat to the right or left of the vertical plane of symmetry of the bowl. The spoon should now balance quite nicely on a point on its bowl that is not quite dead center. Now spin the spoon gently on a surface as close to friction free as you can find. If balanced correctly there should be a preferred direction of spin. In one direction the spoon will rotate happily, in the other it will begin to spin and then change direction! This is pretty cool I think. If you don’t think carefully it looks for all the world like a violation of the conservation of angular momentum.

 

[Hand Bent Spoon]

It finally happened, my own threat title!

That's very interesting!

I've often been told I'm a bit erratic.

Now can anyone here tell us how this spoon can change directions like that (because I know zilch about physics, aside from inertia, and that velocity = mass * speed [and I probably got that wrong ])?

And just for the record, my username refers to Uri Gellar's spoons, and just what bent them . But you knew that.

 

[AK-Dave]

From my limited understanding, the energy from the spin gets transferred to a rocking motion, then back to a spinning motion in the "preferred" direction. There are several toys for sale on the internet that do the same thing.