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[Jan Stránský <question673130@xxxxxxxxxxxxxxxxxxxxx>]

Question #673130 on Yade changed:
https://answers.launchpad.net/yade/+question/673130

    Status: Open => Answered

Jan Stránský proposed the following answer:
> using the 'ForceRecorder' function
> are these forces related to the center of gravity of the surface or to other point?
> how is it calculated? is it the sum of all the forces acting on the single facet which built the surface?

Yes, ForceRecorder just sums forces acting on individual bodies [1].
Forces are considered to act on b.state.pos (not necessarily center of gravity), but in the case of forces themselves it is not that important (however, it is important for torques).

> I tryied to compare the recorder engine results with the lybrary : O.forces.f() (plot on graph). What i obtained is that the force acting on the 'clumped body' is always zero.
> why is this difference present? (even if i write from terminal O.forces.f(NumberOfOneClumpedFacet), it returns (0,0,0), or O.forces.f(clumpID).norm(), it still returns (0,0,0))

You have different cases here, therefore this difference :-) I guess if you put [clumpId] to ForceRecorder, you would get zero, too.
It is just implementation choice, the force acting on clump itself is computed when needed from clump members [2,3], but is not stored..

cheers
Jan

[1] https://github.com/yade/trunk/blob/master/pkg/dem/ForceTorqueRecorder.cpp#L6
[2] https://github.com/yade/trunk/blob/master/pkg/dem/NewtonIntegrator.cpp#L134
[3] https://github.com/yade/trunk/blob/master/pkg/dem/Shop_01.cpp#L134

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