AngMom - 3 slow spin
Rods 10 × Geomag rod icon
Spheres 7 × Geomag ball icon
Author PolyClare

Use this very simple construction to demonstrate the principle of Conservation of Angular Momentum. The Geomag Manual describes this as "DANCING BALLERINA EFFECT”: the principle of energy conservation"; the original demonstrations described here follow those in the Geomag Manual. But when you construct them, be sure to use Peter Jepsen's better modification version below, for best effect and more fun!

My friends ask me what's so cool about Geomags, and I've showed them several things. Amazingly, this one is the one that gets the biggest "Oooooooooooh!"!

Building Instructions Edit

  1. Build a line of 10 Gomag rods.
  2. While keeping all the Geomag rods aligned the same way as they were in the original line of 10, arrange them as shown below. There should be two balls between the top purple bit and the pink spinning part, and two balls beneath the pink spinning bit connected to the lower purple bit. Arrange the three 'arms' of the pink spinny part so that they're spread out equally. Set the angles adjacent to the 'orbiting' balls to be as small as the Geomags will allow.
  3. Give the spinny pink bit a good spin. Grasp the bottom part too, to steady the spin.
  4. Now pull down gently but firmly on the lower purple bit. The arms of the pink spinny bit will be forced closer to the axis of the spin. And there it is: CONSERVATION OF ANGULAR MOMENTUM causes the spinny bit to SPIN FASTER!

Cooler Model Edit

The first demo is cool, but you can't slow it back down because you can't push on those joints (well, I couldn't do it easily). However there is a way to widen those arms after it's spinning.

  1. Re-make the pink spinny part, with two geomag bars above and below the orbiting elbow balls. The first step picture shows it spinning slowly.
  2. If you start off the double-length spinny bit at a reasonably high speed, then when you pull on the bottom purple bit you'll feel the momentum pull back. You can actually see the arms widen if you let up on the downward pressure on the bottom. It'll even take some pushing back, since you're pushing on a longer lever arm toward those further-away elbow joints.

A Better Modification Edit

An even better modification is suggested by Peter Jepsen. The idea is to reverse all polarities from the middle of the model on down. That creates repulsion in the flexing elbow joints, easing them up, while maintaining the same strength in the rest of the model.

I've tested this modification on both models listed above, and they work like a charm! You can speed up the spinny part, and then slow it back down, even repeating this several times. Way cool!

Related Links Edit

Geomag Manual Conservation of Angular Momentum.

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