09
Jan
06

### Double Pendulum

The Double Pendulum is a classical example of chaotic motion.
```(* runtime: 0.1 second *) Clear[theta1, theta2]; g = 9.81; soln = NDSolve[{theta1''[t](3 - Cos[2(theta1[t] - theta2[t])]) == -3g Sin[theta1[t]] - g Sin[theta1[t] - 2theta2[t]] - 2Sin[theta1[t] - theta2[t]](theta2'[t]^2 - theta1'[t]^2Cos[theta1[t] - theta2[t]]), theta2''[t](3 - Cos[2(theta1[t] - theta2[t])]) == 2Sin[theta1[t] - theta2[t]](2theta1'[t]^2 + 2g Cos[theta1[t]] + theta2'[t]^2Cos[theta1[t] - theta2[t]]), theta1[0] == 0.999Pi, theta2[0] == 0.999Pi, theta1'[0] == 0, theta2'[0] == 0}, {theta1[t], theta2[t]}, {t,0, 10}][[1]]; theta1[t_] = theta1[t] /. soln; theta2[t_] = theta2[t] /. soln; Do[p1 = -{Sin[theta1[t]], Cos[theta1[t]]}; p2 = p1 - {Sin[theta2[t]], Cos[theta2[t]]}; Show[Graphics[{Line[{{0, 0}, p1, p2}], Disk[p1, 0.1], Disk[p2, 0.1]}], Axes -> True, PlotRange -> 2{{-1, 1}, {-1, 1}}, AspectRatio -> Automatic], {t, 0,10, 0.1}];```

Link: Double Pendulum – Java applet by Erik Neumann

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