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[Jan Stránský <question690104@xxxxxxxxxxxxxxxxxxxxx>]

Question #690104 on Yade changed:
https://answers.launchpad.net/yade/+question/690104

    Status: Open => Answered

Jan Stránský proposed the following answer:
(1) correct (more or less :-)
you need a vector for cross product, not just a length.
the branch vector = contactPoint - pos
torque = branch.cross(force), which is opposite to force.cross(branch)

(2)
TLTR: have a look at a basic materials for mechanics concerning forces, moments, sums, equlibrium...

I did not get the meaning correctly, but I think I got the confusion.

All this is about force and moment (non)equilibrium.
Consider a rigid body.

Consider a torque applied to the body. From the point of view of force
and moment (non)equilibrium, it does not matter where it acts, you can
shift it to any point with the same effect (the equilibrium / amount of
non-equilibrium remains the same).

Consider a force applied to the body. From the point of view of force
and moment (non)equilibrium, you can shift it only across its action
line to have the same effect. You can shift it to any point, but then
you have to adjust the torque at this point = branch.cross(force) to
preserve the equilibrium / amount of non-equilibrium the same.

You can do this with multiple torques and forces, e.g. to get one sum force and torque with respect to a special point.
You can do this multiple times, i.e. shift the force and then shift the new force-torque couple again.

For the case of one single body, you have multiple interactions, at each contact point you may have torque and force.
For easier manipulation, all these forces and torques are expressed with one force and one torque with respect to b.state.pos.
(this is the value stored in O.forces)

For multiple bodies, you can:
- shift torques and forces w.r.t b.state.pos (from O.forces) to the special point
- shift torques and forces from individual interactions to the special point
the result is the same

I suggest to experiment with a few (e.g. 3) bodies to get some feeling
about it.

cheers
Jan

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