Often, these forces may be represented as the gradient of some scalar quantity χ, with f = ∇ χ in which case they are called conservative forces. Gravity in the z direction, for example, is the gradient of − ρgz. Because pressure from such gravitation arises only as a gradient, we may include it in the pressure term as a body force h = p − χ. The pressure and force terms on the right-hand side of the Navier–Stokes equation become