Double Pendulum: the Mechanism of Chaos

A pendulum hanging from a pendulum. A single pendulum is perfectly predictable, but a double pendulum is chaotic: two releases that look identical end up moving completely differently.

A memorable demonstration of chaos and sensitivity to initial conditions, the idea behind why weather can only be forecast a few days ahead. Seven printed parts and two skateboard bearings (22 × 8 × 7 mm).

Design by MV on Printables, shared under CC0 1.0 (public domain). The files and photos are the designer's, unchanged.

1. Print all parts in one go; they press together without glue.
2. Fit two 608 (skateboard) bearings.
3. Release it twice from what looks like the same position and film both: when do the paths separate?