The atmospheric pressure on the surface of the water in the bucket pushes the water into the U-shaped pipe. Atmospheric pressure helps to push the liquid over the barrier. Fluids in motion Fluid 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. The proportional constant h is called the viscosity. The units of h in SI units are Pa-s. Under all circumstances where it has been experimentally checked, the velocity of a real fluid goes to zero at the surface of a solid object. A thin layer of fluid next to the walls of the pipe does not move at all.