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[Jan Stránský <question689153@xxxxxxxxxxxxxxxxxxxxx>]

Question #689153 on Yade changed:
https://answers.launchpad.net/yade/+question/689153

    Status: Open => Answered

Jan Stránský proposed the following answer:
> in GUI

I don't use Yade GUI, so I cannot help here.. just have tried and indeed
it behaves strangely.. You can open an issue [2] if you like

> If only the force is zero, the unbalanced force should be zero, but
not NaN. The infinite value may come from the zero divisor, i.e. the
number of the interactions.

unbalancedForce() is defined as bodyForce / interactionForce. If interactionForce is zero, then I see no reason why the result should be zero.. NaN makes total sense to me, even in the case 0/0.
You can consider NaN as a special case - zero interaction force, no interactions (pointed by you if I got it correctly) etc.
Of course, you can make up different definitions of unbalanced force more suitable for a specific situation.

> Basically, in fact since there are already some overlaps between the
spheres in the above model, the force should also not be zero.

Some models are designed to specific situation. I have experience with CpmMat being used to model a continuum, where initial "overlaps" (in that case rather gaps) are considered as stress-free / force-free state.
I guess JCFpmMat does the same.

> When I change the O.engines to be the second group as above, both the
force and number of interactions are nonzero.

because Frict* model just takes penetration depth and compute forces.
Now they are nonzero

cheers
Jan

[2] https://gitlab.com/yade-dev/trunk/issues

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