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Yes I think the physics is wrong
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.
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...
EDIT: I just noticed someone found an error and apparently it's fixed now.
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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…
Just made a PR
But no, PR was merged in short order. Lesson learned.
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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…
Just made a PR
For the demonstration, I'd have it under a blanket in the front of the room when students came in. I'd turn the lights out, only turn the outer LED on, and set it in motion. Then I'd cover it up, turn the lights on, and ask students to sketch what they thought was under the blanket. Then I'd turn the lights out again, turn both LEDs on, and set it in motion again.
Most people were able to sketch something pretty close to a double pendulum after that second demonstration. I also set up a camera and did some time lapses, and got pictures that look just like this online demo.
Super fun project, and students loved it! :)
Not sure if this got a second lease on life from the mods after clearing the old comments and resetting the submission date to today, or how did it make the front page again.
[0] https://news.ycombinator.com/from?site=theabbie.github.io
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Just made a PR
Hn shows this story is 3 hours old, yet your GitHub PR was made 2 days ago, linking here?
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.
The motion is still off, as it was 2 days ago [0] in the previous submission. There's something that just doesn't look right, and some particular setups trigger very unrealistic behaviors. Not sure if this got a second lease on life from the mods after clearing the old comments and resetting the submission date to today, or how did it make the front page again. [0] https://news.ycombinator.com/from?site=theabbie.gith…
If not, describe what feels off, as it feels fine to my untrained eyes after the sign fix.