Consider a body that is acted on by a variable external force density $\vec{f}(\vec{r})$.
I want to know what the pressure and shear stress would be within the body as a result of these external forces.
However, the problem as stated seems to be under determined to actually solve for a stress or pressure tensor according to the equation:
$f_i=-\nabla^j\sigma_{ij}$
Furthermore, intuitively I would expect the internal pressures to somehow depend on the variation and therefore derivative of the external force, since if it was uniform the entire body would accelerate in unison (assuming uniform density) without any part having to pull another part.
Are there additional material constraints we must assume (isotropy?) to find the stress tensor, and is my intuition correct and somehow accounted for?