in the couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed formula_1 (see illustration to the right). if the speed of the top plate is low enough (to avoid turbulence), then in steady state the fluid particles move parallel to it, and their speed varies from formula_2 at the bottom to formula_1 at the top. each layer of fluid moves faster than the one just below it, and friction between them gives rise to a force resisting their relative motion. in particular, the fluid 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.