Balance laws Let be a physical quantity that is flowing through the body. Let be sources on the surface of the body and let be sources inside the body. Let be the outward unit normal to the surface . Let 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 is moving be (in the direction ).
Balance laws Let be a physical quantity that is flowing through the body Let be sources on the surface of the body and let be sources inside the body Let be the outward unit normal to the surface  Let 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 is moving be (in the direction )