Double Pendulum

Two rods, two weights, one pivot — and no closed-form answer. This solves the real equations of motion step by step (RK4, 240 steps a second) and traces where the lower weight has been. A second pendulum runs alongside it in amber, started just 0.001 radians away. Same rules, same maths, same clock. Watch how long they stay together.

An animated double pendulum. Your browser does not support canvas.
drag either weight

space play/pause · ←→ rotate arm · 12 pick arm · f fork · r reset

Readout

—Jenergy
—°arm 1
—°arm 2
t = 0.00 s · 240 Hz · RK4
starting

The twin

—gap
—spread
—split at
tip gap over time · log scale10 s

The twin starts 0.001 rad from this one — about the width of a hair at arm's length.

Physics

Earth. Slide down for slow, floaty swings; up for a frantic one.
Off. Friction at the joints — leave it at zero and the energy stays put.