A double pendulum, demonstrating the three-body problem.

The three-body problem comes from physics and mathematics, specifically dynamical systems.

Double pendulums are an easy way of demonstrating the three-body problem.

In a nutshell:

• A pendulum is 100% deterministic. A pendulum's future positions are predictable once you know certain facts about it.
• A double-pendulum (which is nothing but adding another pendulum to the first one) on the other hand, behaves chaotic. Mind you, it is still deterministic. But long-term predictions where the pendulums will be in the (far) future are almost impossible.

• DoublePendulum is just a double pendulum you can drag to give it momentum
• DoublePendulumSliders has got sliders to adjust below parameters

This animation is interactive:

You can grab the middle-pendulum with your mouse and give it a push for more acceleration.

You can also adjust the pendulum-size via a slider:

Finally, you can change the gravity, too, via a slider: