Double Pendulum

Drag a bob to set the starting angle, then release to let it swing. The motion is a genuine RK4 integration of the double-pendulum equations of motion, so it is chaotic — tweak gravity, rod lengths, masses or damping and watch it change character. Turn on Divergence to launch three pendulums whose start angles differ by ~0.001 rad: they track together for a moment, then split apart completely — sensitive dependence on initial conditions. Works with touch, too.