By using this equation and the Vlasov Equation, the momentum equation for each fluid becomes m n D D t u = − − ∇ ∇ ⋅ ⋅ P + q n E + q n u × × B , where P is the pressure tensor. The material derivative is D D t = ∂ ∂ ∂ ∂ t + u ⋅ ⋅ ∇ ∇ .