When looking at small elements in continuum mechanics, why do we draw that the stress/force the way shown in diagrams? How do we know the "pairs" of forces on opposite faces always opposite, and which direction are the shear forces.
Can't forces on opposite faces point in the same direction?
In continuum mechanics, the stress tensor is a point function, and all 9 components exist at each point. When you do a force balance in a fluid parcel, the stress vector is evaluated locally at each point over the outer surface of the parcel using the Cauchy Stress Relationship,and integrated over the outer surface.
