Interactive Double Pendulum Playground

12 comments

• Evidlo a day ago ago

  Anyone else feel like the pendulum motion seems off? Maybe the default mass settings are weird, but the movement just does not look physical to me.

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  • magicalhippo a day ago ago

    It's way off. My first guess was that there was something wrong with the physics code, but after carefully checking against this[1] derivation of the Hamiltonian it seemed fine, and once I wrapped my head around the JavaScript the RK4 integration[2] checked out as well.

    So, what else might be wrong I wondered. Well, it seems to move in the wrong direction... so I checked how the pendulum is displayed. And sure enough, I think there's a sign error:

      getUpperBob() {
         const { x0, y0, ang0, l0 } = this;
         const { x, y } = this.calculateBobPosition(x0, y0, ang0, l0);
         return { x, y };
       }
    
       getLowerBob() {
         const upperBobPos = this.getUpperBob();
         const { ang1, l1 } = this;
         const { x, y } = this.calculateBobPosition(
        upperBobPos.x,
        upperBobPos.y,
        -ang1,
        l1
         );
         return { x, y };
       }
    

    Note how the upper bob uses ang0 while the lower one has -ang1. Meanwhile the physics derivation assumes both angles are against the vertical, so have same sign.

    Changing -ang1 to ang1 does indeed make the pendulum move in a natural way, except now dragging it is flipped. Ie you drag it left and it moves right. Another sign error in setLowerBobPos. Fixing that as well it now works as I'd expect.

    [1]: https://dassencio.org/46

    [2]: https://lpsa.swarthmore.edu/NumInt/NumIntFourth.html##sectio...

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    • Evidlo 16 hours ago ago

      Just made a PR

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      • magicalhippo 4 hours ago ago

        I considered it, but had to go to work and I figured since the last commit was over 4 years ago it might just sit there idly, so I skipped it.

        But no, PR was merged in short order. Lesson learned.

  • navtoj a day ago ago

    There's a "How To Use?" section on the GitHub repo page.

    https://github.com/theabbie/DoublePendulum?tab=readme-ov-fil...

  • mihau a day ago ago

    Yep, it's just bad. Physics feels totally off.

  • WithinReason a day ago ago

    Yes I think the physics is wrong

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    • kookybakker a day ago ago

      When you bring up the lower mass and let it go, it seems to push the upper mass away, which should never happen. This whole site smells off vibe coded jank.

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      • Evidlo 16 hours ago ago

        I don't think that's fair. Also the project is 4 years old so the timeline doesn't really work out for being vibe-coded.

      • probabletrain a day ago ago

        > This whole site smells off vibe coded jank.

        A vibe-coded double pendulum sim should produce a much better result than the physics on this page. Claude Code made this just now off one prompt, the physics are much better: https://keir.is/swinging

• isoprophlex a day ago ago

  This is hilariously bad.

  If you want to see what a real physically sensible double pendulum sim looks like:

  https://www.myphysicslab.com/pendulum/double-pendulum-en.htm...

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  • gus_massa 8 hours ago ago

    I don't see the difference. The motion in the version you linked is slower but I guess it can be fixed changing g. What are the visible errors in the main post?

    EDIT: I just noticed someone found an error and apparently it's fixed now.