# Need help with spring constraint

This topic is 1762 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

Hi. I'm trying to implement a spring/elastic constraint into a game and am having trouble getting a certain part to work. Right now, 'Part B' (Pb) is attached to 'Part A' (Pa), and it works fine. The problem is that when Part B is moved, Part A does not move. I've tried applying an equal and opposite force on to Part A but that causes them both to fly off and go crazy at a high speed, so I guessed that was wrong.

Here's the code I have so far: (It's in lua, but is self-explanatory)

k = 64
b = 0.1
length = 10

dist = (Pa.Position - Pb.Position).Magnitude
unit = (Pa.Position - Pb.Position).Unit
force = (dist - length) * k
resist = Pb.Velocity * b * Pb:GetMass()
vector = (force * unit) - resist

applyForce(Pb, vector)


Don't know if this is the right place to post this, but Idk where else I could. Thanks

##### Share on other sites

The code strikes me as strangely asymmetric. The `resist' term should probably depend on the difference of the velocities of Pb and Pa, not just the velocity of Pb. Also, I would either not mention the mass of Pb or I would use something like the sum of the masses of Pb and Pa. And you should definitely apply the opposite force to Pa.

##### Share on other sites

Thank you. I made those changes and it works.

1. 1
Rutin
44
2. 2
3. 3
4. 4
5. 5

• 13
• 10
• 12
• 10
• 13
• ### Forum Statistics

• Total Topics
632997
• Total Posts
3009797
• ### Who's Online (See full list)

There are no registered users currently online

×