Momentum Conservation

Surface forces have no bearing on the conservation of momentum principle. Surface forces only affect the pressure drop through the conduit, in the case of a conduit, and the drag in the case of foil surfaces placed in the fluid in an open channel situation.

Body forces result when a fluid changes direction and when the conduit changes shape or the force on the foil is the desired result, such as an aeroplane wing developing lift. The body reaction force is the first derivative of the momentum change. Thus, if more fluid turns left per second, one must expect the reaction force to the right to increase in proportion. That is why buried push-fit pipes often have thrust blocks at bends, for example, to spread the force over a greater mass of soil.

The derivations are contained in the textbooks and those here are very, very dusty....