∂ ∂ u ∂ ∂ t ⏟ ⏟ Variation + ( u ⋅ ⋅ ∇ ∇ ) u ⏟ ⏟ Divergence ⏞ ⏞ Inertia (per volume) = ∂ ∂ ∂ ∂ − − ∇ ∇ w ⏟ ⏟ Internal source + ν ν ∇ ∇ 2 u ⏟ ⏟ Diffusion ⏞ ⏞ Divergence of stress + g ⏟ ⏟ External source .