The force required for this action is a measure of the fluid's viscosity. Friction between the fluid and the moving boundaries causes the fluid to shear. Volume viscosity is essential for Acoustics in fluids, see Stokes' law (sound attenuation) Newton's theory File:Laminar shear.png Laminar shear of fluid between two plates. Extensional viscosity is widely used for characterizing polymers. For example, at "room temperature", water has a nominal viscosity of 1.0 × 10 -3 Pa∙s and motor oil has a nominal apparent viscosity of 250 × 10 -3 Pa∙s. Simply put, this quantity is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction, to the change in velocity of the fluid as you move down in the fluid (this is what is referred to as a velocity gradient). That is why they are often referred to as simply viscosity. Shear viscosity and dynamic viscosity are much better known than the others.
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