In Fig 3.8 a prismatic control volume of very small size, with cross-sectional area ll and length ll is selected. Fluid velocity is along the streamlines. By assuming that the viscosity is zero, or that the flow is frictionless, the only forces acting on the control volume in the X direction are the end forces and the gravity force. The momentum equation Eq 3.3.8 is applied to the control volume for the s component