4 ms·

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.

by Evidlo 1y ago

WithinReason 1y ago
Yes I think the physics is wrong
- kookybakker 1y 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.
  - probabletrain 1y 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 https://keir.is/swinging
  - Evidlo 1y 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.
navtoj 1y ago
There's a "How To Use?" section on the GitHub repo page. https://github.com/theabbie/DoublePendulum?tab=readme-ov-file#how-to-use https://github.com/theabbie/DoublePendulum?tab=readme-ov-fil...
mihau 1y ago
Yep, it's just bad. Physics feels totally off.
magicalhippo 1y 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 https://dassencio.org/46 [2]: https://lpsa.swarthmore.edu/NumInt/NumIntFourth.html##section3 https://lpsa.swarthmore.edu/NumInt/NumIntFourth.html##sectio...
- Evidlo 1y ago
  Just made a PR
  - magicalhippo 1y 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.
  - cabirum 1y ago
    Hn shows this story is 3 hours old, yet your GitHub PR was made 2 days ago, linking here?
    - pansa2 1y ago
deleted 1y ago
[deleted]

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.

by Evidlo 1y 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.

WithinReason 1y ago

Yes I think the physics is wrong

kookybakker 1y 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.
- probabletrain 1y 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 https://keir.is/swinging
- Evidlo 1y 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.

navtoj 1y ago

There's a "How To Use?" section on the GitHub repo page. https://github.com/theabbie/DoublePendulum?tab=readme-ov-file#how-to-use https://github.com/theabbie/DoublePendulum?tab=readme-ov-fil...

mihau 1y ago

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

magicalhippo 1y 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 https://dassencio.org/46 [2]: https://lpsa.swarthmore.edu/NumInt/NumIntFourth.html##section3 https://lpsa.swarthmore.edu/NumInt/NumIntFourth.html##sectio...

Evidlo 1y ago
Just made a PR
- magicalhippo 1y 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.
- cabirum 1y ago
  Hn shows this story is 3 hours old, yet your GitHub PR was made 2 days ago, linking here?
  - pansa2 1y ago

deleted 1y ago

[deleted]