Help Getting the force acting on a Rod

[josemendez]

Hello, pretty simple question I was hoping for some help with.

I have a rod; I want to know how much force is acting on one of its elements/particles. That is, how much total force are the constraints exerting (how deformed the rod is).

I don't know how to approach this, outside of doing a clunky measurement of the particle's position vs its rest position. I imagine it involves something to do with reading the constraint batch data in the solver?

Hi,

You'll need to read the "lambda" array from each constraint batch. The constraints you're interested in for a rod are StretchShear and BendTwist.

These "lambdas" are lagrange multipliers, you can convert them to forces (expressed in newtons) by dividing by the step time squared. There's an example of this in ObiRope.cs, ApplyTearing() method: it iterates over all distance constraints, calculating the force they're exerting and then selecting the ones with largest force to tear.

Each individual constraint calculates and applies its own lagrange multiplier, ao you'll have to sum these up.

how much total force are the constraints exerting (how deformed the rod is).

Note these are two completely different things. Stress is the force, strain is the deformation caused by it. The relationship between them is often non-linear.