applies on the top plate a force in the direction opposite to its motion, and an equal but opposite force on the bottom plate. An external force is therefore required in order to keep the top plate moving at constant speed.

In many fluids, the flow velocity is observed to vary linearly from zero at the bottom to at the top. Moreover, the magnitude of the force, , acting on the top plate is found to be proportional to the speed and the area of each plate, and inversely proportional to their separation :
: The proportionality factor