In order to conceptualize and perform balances, one must choose a representative "region" to perform the balance on. This "region" may be:
so that regions to "anchor" a balance can be grouped as either:
A System is a collection of fluid matter of fixed identity (i.e. always the same fluid particle(s)) that will move, flow, and interact with its surroundings. (Lagrangian concept)
A Control Volume is a geometrically defined volume in space through which fluid particles may flow (in or out of the control volume). Hence a CV in a flowing fluid will contain different fluid matter (fluid particles) at different points in time. (Eulerian concept)
Define systems and control volumes and identify when each is a useful frame of reference
Give an example of a balance that might be simpler using a CV? a System?