The experiment shown in Figure 11-34 is useful for measuring viscosity because the only forces making the fluid move are the frictional (viscous) forces that act between parts of the fluid and between the fluid and plates. In most real-life situations, however, there are also forces due to pressure differences acting on the fluid (for example, the pressure difference between the two ends of a garden hose as you water the lawn). To describe the relative importance of forces due to pressure difference and viscous forces in fluid flow, physicists use a dimensionless quantity—that is, a pure number with no units—called the \(\textbf{Reynolds number}\):