Comment 8 for bug 493102

> OK, my list of weird people has 2 items now:
>
> * gomechanicians
> * contact law freaks
>
>
Haha!
I fell like a contact law freak more than a geomechanician. The contact
law freak asks Vaclav : what the hell continuum mechanics as to do with
interactions in multibody systems? ;)

There are two different logics and I think both of them are right.
Vaclav convinced me that there was a consistent reason for his sign
convention. However, he didn't persuaded me that his logic was more
intuitive (read post #4).
The alternative is :
1. We want the same signs as in continuum mechanics for
tension/compression. As a consequence, we need to define the shear force
as applied by b2 on b1 (right?).
2. We use the convention that an interaction is "from 1 to 2", which
apply for the definition of the normal as well as for contact forces,
which represent forces applied by b1 on b2, defined in the global
coordinate system.

In logic 2, there is no inward or outward normal. The sign of fn in
traction is the (unfortunate) consequence, not the objective. I agree
with Chiara on the fact that logic 2 is at the same time more common,
and more intutive (for the majority, not for Vaclav).

Perhaps make a vote in yade users to evaluate what is the most popular
convention?