This is often the "hurdle" for many students. The transition from System (Lagrangian) analysis to Control Volume (Eulerian) analysis is handled with exceptional care. Shames provides a clear derivation of the Reynolds Transport Theorem, the critical link between system laws (like Newton's Second Law) and control volume formulations (like the Bernoulli equation and the Energy Equation).
: The derivation of the governing equations for fluid motion. mechanics of fluids irving h shames pdf