Have you ever used your thumb to control the water flowing from the end of a hose? If so, you probably have seen that the water's velocity increases when your thumb reduces the cross-sectional area of the hose opening. This kind of fluid behavior is described by the equation of continuity , which expresses the simple idea that the mass of fluid entering one end of a pipe must leave at the other end. In other words, the mass flow rate (in kilograms per second) must be the same when entering and leaving a pipe. The mass flow rate at the beginning of a pipe (or position 1) can be written as the following : = change in mass (kg) = change in time (seconds) = density of fluid () = cross-sectional area of pipe () = velocity of fluid (m/s) Whereas the mass flow rate when the fluid comes out of a pipe (or position 2) is written as : The mass flow rate must be the same at the beginning and at the end. It is written in an equation as the following : Suppose that you use your thumb to cover the end of a pipe. The velocity of the fluid will increase and the mass flow rate will still be the same.