Rigid Body Physics - accuracy question

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1

I try to use Blender for a (scientific) physics experiment, not for a cool looking animation. I see some effect that does not follow rules of mechanics and I am not sure if I do something wrong or if Blender´s
Rigid Body Physics is not accurate:
I accelerate several sticks with the same force to make them spin and move along one axis. About 50% of sticks do what mechanics (Newton physics) would predict, the other 50% of sticks change their position also in the other dimensions or move to quickly or slowly! Rigid|445x271 . The picture shows for clarity reasons only two of the misbehaving sticks.
I modified all rigid body and rigid world parameters that I am aware of, but it is never the case that all sticks behave correctly. Do I do something wrong or is my wanted degree of accuracy simply not possible in the current Blender versions? StickFlaw.blend (886.7 KB)

2

One minute…
First of all, don’t you think your “sticks” should have a actual body? They are just a line of vertices. There’s no volume there.

I am not sure if that could be the reason, but since an object don’t have a real body, a closed shape, I don’t really see how Blender should calculate rigid body physics correctly. I have no idea if you are going to achieve your objective with solids, but I think it’s the right way to start with.

3

First I tried to do this with 3 dimensional sticks (round and rectangular ones) and they behaved worse

4

Did you adjust the sensitivity? Well, I started to have some results here. Give me some minutes more.

5

Look at this:
StickFlaw1.blend (928.5 KB)

Now I will have a look at your file again. In this I sent I realized that the origin point was messing a bit with my model.

With your original material I could minimize the problem, but not solve it. Still testing though.

Yes, could not solve it. I think that meshes should really be closed to calculate those physics.

6

It seems like a floating point precision issue, you are using the mesh collision shape, and it might not be behaving, it almost never behaves, in any engine I have tried.

I changed it to a box collision shape and now it seems to work.
StickFlaw-box_collision.blend (970.9 KB)

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7

Well, it must be something else, cause I downloaded your model and changed back to geometry and it works exactly the same.

I also opened again the original model and just changed to BOX as you said and it doesn’t work as it does in your model.

So, i deleted the rigid body system and started again from scratch and it works. It looks like Blender prefers that you start again instead of changing something that is not working.

I also think that the problem with the mesh collision shape was that his mesh was not closed, cause I made some changes in the mesh to have some thickness and created the rigid body from scratch too, but using the same settings and it works. The only problem it has is that the sticks spin at different speeds.

9

Thanks Jeacom for the new file, it works for me too. The key seems to be the shape of the accelerator plane. It works correctly in your file when set to “Box” and it works correctly in my original file when set to “Convex Hull”.
I do not understand this, but thanks to both of you, Calandro and Jeacom for helping me out!

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