# [✓] Size step is effectively zero?

Posted 10 months ago
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 Hello, I am trying to plot a double pendulum. But when I try to plot the results, this error occurs: "NDSolve::ndsz: At t == 0.5517583468854775, step size is effectively zero; singularity or stiff system suspected." Any idea what I've got wrong? Lagrangian = (m1 + m2)/2*l1^2*\[Theta]1'[t]^2 + m2/2*l2^2*\[Theta]2[t]^2 + m2*l1*l2*\[Theta]1'[t]*\[Theta]2'[t]* Cos[\[Theta]1[t] + \[Theta]2[t]] + (m1 + m2)*g*l1* Cos[\[Theta]1[t]] + m2*l2*g*Cos[\[Theta]2[t]]; g = 9.81; l1 = 1; l2 = 1; m1 = 1; m2 = 1; eq1 = D[D[Lagrangian, \[Theta]1'[t]], t] - D[Lagrangian, \[Theta]1[t]]; eq2 = D[D[Lagrangian, \[Theta]2'[t]], t] - D[Lagrangian, \[Theta]2[t]]; sol = NDSolve[{eq1 == 0, eq2 == 0, \[Theta]1[0] == Pi/10, \[Theta]1'[0] == 0, \[Theta]2[0] == 0, \[Theta]2'[0] == 0}, {\[Theta]1, \[Theta]2}, {t, 0, 10}]; Plot[{Evaluate[\[Theta]1[t] /. sol], Evaluate[\[Theta]2[t] /. sol]}, {t, 0, 10}, PlotRange -> A] 
 Is it possible that second Lagrangian term was supposed to be m2/2*l2^2*\[Theta]2'[t]^2` (note derivative of theta_2)? With that change the integration does not go singular and the plot seems reasonable.