Let f ( x , t ) be a physical quantity that is flowing through the body. Let g ( x , t ) be sources on the surface of the body and let h ( x , t ) be sources inside the body. Let n ( x , t ) be the outward unit normal to the surface ∂ ∂ Ω Ω . Let v ( x , t ) be the flow velocity of the physical particles that carry the physical quantity that is flowing. Also, let the speed at which the bounding surface ∂ ∂ Ω Ω is moving be u n (in the direction n ).