Siphons Suppose a U-shaped piece of pipe is completely submerged in water, filled with water, and then turned upside down under water. As you slowly pull the top of U-shaped piece of pipe out of the water, the water does not run out of the pipe. Air cannot enter the pipe. The pressure here drops to near zero. 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.