Double Pendulum

Two linked pendulums, one chaotic dance. A tiny nudge to the start sends the motion somewhere completely different.

Double Pendulum StudioLive

chaotic dynamics · RK4

Controls

Speed1×

Tiny changes in the start explode into totally different motion — deterministic chaos. Drag either bob to set a new starting position; it pauses playback automatically.

Presets

Lower bob mass

Gravity g

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Data Inspector

IntegratorRK4

SystemChaotic

DOF2

Lower bob mass10

Gravity g9.8

Governing equation

\ddot{θ}_{1} \ddot{θ}_{2} = \frac{- g ( 2 m _{1} + m _{2} ) sin θ _{1} - m _{2} g sin ( θ _{1} - 2 θ _{2} ) - 2 m _{2} sin ( θ _{1} - θ _{2} ) ( θ ˙ _{2}^{2} L _{2} + θ ˙ _{1}^{2} L _{1} cos ( θ _{1} - θ _{2} ) )}{L _{1} ( 2 m _{1} + m _{2} - m _{2} cos ( 2 θ _{1} - 2 θ _{2} ) )} = \frac{2 sin ( θ _{1} - θ _{2} ) ( θ ˙ _{1}^{2} L _{1} ( m _{1} + m _{2} ) + g ( m _{1} + m _{2} ) cos θ _{1} + θ ˙ _{2}^{2} L _{2} m _{2} cos ( θ _{1} - θ _{2} ) )}{L _{2} ( 2 m _{1} + m _{2} - m _{2} cos ( 2 θ _{1} - 2 θ _{2} ) )} m_{1} = 10, m_{2} = 10, L_{1} = L_{2} = 1.2, g = 9.8

Reading this result: Starting both arms at 90° (π/2) with lower-bob mass 10 and g 9.8 puts the pendulum firmly in its chaotic regime. Unlike a small-angle swing, which would be near-periodic, here trajectories that differ by 0.02 rad diverge exponentially — a positive Lyapunov exponent.

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How it works

The double pendulum's exact nonlinear equations of motion are integrated with a fourth-order Runge–Kutta scheme. It is the classic demonstration of deterministic chaos: fully determined by its equations, yet practically unpredictable because infinitesimal differences in initial conditions grow exponentially.

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