This is a double double pendulum - that is, two double pendulums on one pivot. If you set both pendulums going from roughly the same position you'll find that they behave very differently. This is an example of chaotic behaviour, which is also seen in other physical systems such as the weather. Chaotic behaviour refers to the situation where a tiny change in the starting conditions (in this case the position of the pendulums when you let go of them) leads to significant changes in their behaviour. You can see that even if you try and start the two pendulums in exactly the same place, they will end up behaving very differently.

I use this model as an example of chaotic behaviour in science talks I give at schools and public science groups.

You will need some bearings that are 8 x 22 x 7mm, for example the 608ZZ series. I bought mine from Amazon, but they're easily available from various sources. There are even some models to print your own here on MakerWorld - just search for 608ZZ - but I can't guarantee how well they will work.

To make the model:

1. Print all parts
2. Insert bearings into each of the large holes - they should push fit, but you can use a little bit of glue to hold them if necessary. If they don't fit then scale the pieces fractionally larger and reprint.
3. Push the bearings at the top of the larger pendulum onto the pivot at the top of the stand. Again, use glue if needed - and keep the two pendulums as far apart from each other as possible to stop them hitting each other
4. Enjoy!