A fluid is a substance that deforms continuously under the application of a shear stress (no matter how small). Unlike solids, fluids cannot resist shear; they flow. Fluids are divided into:
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Viscosity is perhaps the most critical property in fluid dynamics. It is the "friction" of the fluid world. It explains why a car needs oil to lubricate its engine and why syrup pours slowly onto pancakes. Without viscosity, there would be no drag on cars, and pipes would never suffer from friction losses. A fluid is a substance that deforms continuously