Universal control volume for definition of terms. A universal control volume represented by a shaded region within fluid of density ρ and velocity vector field u. The control surface is shown as dashed line at the boundary of the control volume. Differential volume and surface area elements, dV and dS, respectively, are shown along with corresponding unit outward surface normal vector . Body forces caused by gravity g and acceleration a act on the center of mass of the control volume, while pressure -p and viscous shear stresses τact on the control surface, normal and tangentially, respectively. Reference frame O is a fixed inertial frame with respect to the control volume.