An idealized treatment of a fluid between two large parallel plates (to permit ignoring edge effects) of area , separated by a distance . If one plate moves relative to the other with a constant velocity , requiring a force acting in the direction of movement, and the density, pressure, and viscosity throughout the fluid are constant, the Newtonian equation can be coupled with the equations of motion and of continuity to show that the velocity gradient in the fluid is constant (= ), and that . This idealized case (simple shear) is sometimes used to define shear viscosity.