Comment by throwanem

Very neat! But possibly afflicted somewhat with rounding errors; I spent a few minutes reading about Lagrangian mechanics, and came back to find the pendulum whirling around its origin point like a thrown bolas. It looks like you've put the proper amount of effort into scope hygiene and the debugger doesn't seem to want to work, so I can't give the values of any of your code's variables, but I'm using Firefox 45 ESR, if that helps at all.