In fluid mechanics, the force density has the physical dimensions of force per unit volume. Force density is a vector field representing the flux density of the hydrostatic force within the bulk of a fluid. Force density is represented by the symbol f, and given by the following equation, where P is the pressure:

$\backslash mathbf\{f\}\; =\; \backslash nabla\; P$.

The net force on a differental volume element dV of the fluid is:

$d\backslash mathbf\{F\}\; =\; \backslash mathbf\{f\}dV$

Thus, the force density at a point in a fluid, divided by the density, is the acceleration of the fluid at that point.