In Fig 3.8 a prismatic control volume of very small size, with cross-sectional area 3 and length 3 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