Assuming conservation of mass, with the known properties of divergence and gradient we can use the mass continuity equation, which represents the mass per unit volume of a homogenous fluid with respect to space and time (i.e., material derivative D D t ) of any finite volume (V) to represent the change of velocity in fluid media: D m D t = ∭ ∭ V ( D ρ ρ D t + ρ ρ ( ∇ ∇ ⋅ ⋅ u ) ) d V D ρ ρ D t + ρ ρ ( ∇ ∇ ⋅ ⋅ u ) = ∂ ∂ ρ ρ ∂ ∂ t + ( ∇ ∇ ρ ρ ) ⋅ ⋅ u + ρ ρ ( ∇ ∇ ⋅ ⋅ u ) = ∂ ∂ ρ ρ ∂ ∂ t + ∇ ∇ ⋅ ⋅ ( ρ ρ u ) = 0 where