Ask HN: Toolbox or library for simulating classical mechanics problems?

I want to for example see the oscillations of a usual double pendulum problem with ability to plot angles and forces at arbitrary positions. Or physical parameters of “box on a cart with friction” problem or real-time animation of a N-body problem. 1- In summary, suppose someone wants to go through Landau-Lifshitz or Goldstein books and simulate the problems on his computer.

Source: news.ycombinator.com