The lower plate is stationary and the upper plane is moving to the right at speed . The fluid is sheared, with the fluid moving according to . The fluid velocity matches that of the plates where the fluid touches the plates. The upper plate experiences a drag force where is the viscosity of the fluid and is the area of the plate. The parameter is the velocity of the top plate with respect to the bottom plate and is the separation between the plates.
Viscosity has the dimensions mass per length per time.
The viscosity of water varies from at to at to at . The viscosity of water thus decreases with increasing temperature, which is typical of liquids. In contrast, the viscosity of a gas is independent of the density of the gas and is proportional to the square root of its absolute temperature. The viscosity of a gas thus increases with temperature, in contrast to the viscosity of a liquid. For air at , the viscosity is .