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[Chiara Modenese <c.modenese@xxxxxxxxx>]

In Classical Mechanics the convention is compression negative and
tension positive. However, in Contact Mechanics (see Hertzian theory as
well as Johnsons et al.) the convention holds a positive sign for
compression and a negative one for tension (e.g. adhesion normal force
is negative whether presents). I would warmly suggest to follow the
latter convention, hence as positive the force applied by b1 on b2 if
normal is b1->b2.

-- 
NormalShearInteraction::{normalForce,shearForce} has undefined orientation
https://bugs.launchpad.net/bugs/493102
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Status in Yet Another Dynamic Engine: Confirmed

Bug description:
NormalShearInteraction::{normalForce,shearForce} should contain forces on the interaction, which can be used for various purposes by other functions. However, different functors use them with different sign

ef2_Spheres_Elastic_ElasticLaw (a.k.a. ElasticContactLaw):

   applyForceAtContactPoint(-currentContactPhysics->normalForce-shearForce, ... )

Law2_Dem3DofGeom_CpmPhys_Cpm:

   applyForceAtContactPoint(BC->normalForce+BC->shearForce, ...)

This is a convention issue, but must be fixed. Should normalForce and shearForce be oriented as it applies on id1 (first case) or id2 (second case)? 2nd case is nicer WRT applyForceAtContactPoint syntax, but that one is arbitrary as well.