Liquids are nearly incompressible. At standard temperature and pressure, the speed of sound is approximately 5000 ft/s for many common liquids, whereas it is in the vicinity of 1000 ft/s for most common gases.
Viscosity The viscosity of a fluid is a measure of its thickness, or how easily it flows. The viscosity, ?, of a substance is defined as the shear stress per unit area divided by the velocity gradient. Fluids for which the viscosity is independent of shear stress are called Newtonian fluids and this class includes gases and most common liquids. Polymer solutions, inks, coatings and paints are often non-Newtonian-that is, their thickness and flow characterizes change with shear stress. Viscosity has dimensions of mass per length per time. A common viscosity unit is the poise, defined as one gram per centimeter per second. The kinematic viscosity is defined as the ratio of viscosity to density. A common unit for kinematic viscosity is the stoke, defined as cm²/s. For pure liquids and pure gases at low pressure (near one atm) the viscosity is a function of temperature but is insensitive to changes in pressure.
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