In Chapter 6, Section 6.1, we developed an expression for the linear momentum of a closed system of particles and showed that the rate of change of linear momentum of the system was equal to the sum of the external forces applied to the system, i.e. F = dp/dt. Since this system was assumed to be closed no mass could, by definition, enter or leave the system across its boundaries. As the system boundary moved the mass moved with it. Although this formulation is very useful for systems built from an assembly of rigid bodies it is not a convenient way of dealing with devices, such as pumps, compressors, turbines and jet engines, which require a flow of fluid into and out of the system.