Comment 6 for bug 493102

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.