- Systems: fixed qtys. of mass w/ identifiable boundaries - CVs: arbitrary volume in which fluid flows - Bounded by control surfaces (CSs) @ entrance & exit - CVs and CSs can be at rest or motion, even accelerating - Fluids continuously deform under any amount of shear stress - Fluids are a collection of many molecules, each w/ their own KE - Would rather deal w/ averaged qty. than individually testing each molecule, so avg. all discrete particles into continuous field - Fluids treated as continuum if mean free path $\lambda$ is much smaller than some characteristic length $L$ - Knudsen number $K_{n}=\frac{\lambda}{L}$ - Density macroscopically represents mass per unit volume of a substance. There is a critical size at which density does not have a clear definition, $\rho=\lim_{ \delta V \to \delta V'} \frac{\delta m}{\delta V}$ which occurs at $K_{n}\ll{1}$