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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:
> why do we need

well, you don't need :-)
you need to use it to get the same result as utils.sumTorques, which is physically the total torque with respect to a given point from individual torques and forces applied to the bodies.

> why not we use ret += r.cross(f)?

because (obviously) you would get different result. As sum(plain
torques), discussed on previous answers, also sum(torques only from
forces) probably may be useful, but it is not what you are asking.

Total torque has two contributions:
- torques directly applied to bodies, i.e. "t"
- forces applied to bodies and their "excentricity", i.e. "r.cross(f)"

To get correct results, you need both (even sometimes one of the
contributions may be negligible compared to the other, it is not good
idea to depend on that fact)

cheers
Jan

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