 When we talk about flowing fluids, there are a couple of ways that we can describe the fluid depending on which parameters or which properties are useful to us. If we consider, say, the flow of fluid through a cylinder, it might be useful for us to describe something like the velocity profile of that fluid. In some circumstances, some properties change as a function of velocity and it might be important that we care about that. In other cases, maybe the velocity profile itself doesn't matter so much. And all we care about is an average velocity. We commonly care about mass flow rates of fluids because that mass flow rate allows us to quantify the energy of that moving fluid in relation to other specific quantities. By recognizing the specific quantities, multiplied by mass, represent the total magnitude of energy, then multiplying by mass flow rate will represent the rate of change of that energy. For steady state analysis, mass flow rate is useful because it brings us to a rate of energy. We can describe that mass flow rate in a couple of different ways, depending on what's useful. The common way that we abbreviate it in terms of volumetric flow rate is by writing it as density times volumetric flow rate. Furthermore, it might be useful to write volumetric flow rate as a function of average velocity. Volumetric flow rate as a function of average velocity would be average velocity times cross-sectional area. As a result of that, we can write the mass flow rate of a fluid as the density of a fluid times the average velocity of that fluid times the cross-sectional area of that fluid. In other cases, it might be more convenient for us to write specific volume instead of density, which you'll remember is going to be the reciprocal of density. So we could write that as specific volume to the negative first power times average velocity times cross-sectional area, which would be equivalent to average velocity times area divided by specific volume.