# Why are momentum balance equations formulated in terms of net stress rather than net force?

In engineering, a steady state equilibrium force balance is typically expressed as $$\nabla\cdot \sigma=0$$ where $\sigma$ is the stress (force per unit area) applied at a point. It seems more intuitive to me to express the force balance as $$\sum F = 0$$. What does the first equation capture that the second equation doesn't? That is, what advantages does $\nabla\cdot \sigma=0$ have over $\sum F = 0$ to represent this steady state?

-

The stress tensor may be continuous rather than due to a discrete number of forces e.g. in fluid mechanics. For example the Cauchy momentum equation is naturally written using the divergence of the stress tensor.

-