The momentum equation is applied in complement to the energy equation, which is useful only for a control volume. However, depending upon the fluid flow problem, one may apply the differential form of...

The momentum equation is applied in complement to the energy equation, which is useful only for a control volume. However, depending upon the fluid flow problem, one may apply the differential form of the momentum equation (for a fluid system) or the integral form of the momentum equation (for a control volume). Explain this in detail.