A real fluid flowing in a pipe experiences frictional forces. There is friction with the
walls of the pipe, and there is friction within the fluid itself, converting
some of its kinetic energy into heat. The frictional forces that try to prevent
different layers of fluid from sliding past each other are called
**viscous forces**. **Viscosity **is a measure of a fluids resistance to relative motion
within the fluid. We can measure the viscosity of a fluid by measuring the
viscous drag between two plates.

If you measure the force to keep the upper plate moving with constant
velocity **v**_{0}, you find it is proportional to the area of the
plate, and to v_{0}/d, where d is the distance between the plate.

**F**/A = η**v**_{0}/d

The proportional constant η is called the viscosity. The units of η in SI units are Pa-s.

- Viscosity of air (20
^{o}C): 1.83*10^{-5}Pa-s - Viscosity of water (20
^{o}C): 1.0*10^{-3}Pa-s - Viscosity of honey (20
^{o}C): 1000 Pa-s

The work done by viscous forces converts ordered energy into thermal energy. For fluid flowing in a long horizontal pipe, the pressure drops along the pipe in the direction of the flow. The faster the fluid is flowing, the larger is the pressure drop.