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In physics, a fluid is a liquid, gas, or other material that may continuously motion and deform ( flow) under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are Matter which cannot resist any shear force applied to them.
Although the term fluid generally includes both the liquid and gas phases, its definition varies among branches of science. Definitions of solid vary as well, and depending on field, some substances can have both fluid and solid properties. Non-Newtonian fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied. Substances with a very high viscosity such as pitch appear to behave like a solid (see pitch drop experiment) as well. In particle physics, the concept is extended to include fluidic other than liquids or gases.Example (in the title): A fluid in medicine or biology refers to any liquid constituent of the body (body fluid), whereas "liquid" is not used in this sense. Sometimes liquids given for fluid replacement, either by drinking or by injection, are also called fluidsUsage example: (e.g. "drink plenty of fluids"). In hydraulics, Hydraulic fluid is a term which refers to liquids with certain properties, and is broader than (hydraulic) oils.
Solids respond with restoring forces to both shear stresses and to , both compressive and Tensile stress. By contrast, ideal fluids only respond with restoring forces to normal stresses, called pressure: fluids can be subjected both to compressive stress—corresponding to positive pressure—and to tensile stress, corresponding to negative pressure. Solids and liquids both have tensile strengths, which when exceeded in solids creates irreversible deformation and fracture, and in liquids cause the onset of cavitation.
Both solids and liquids have free surfaces, which cost some amount of free energy to form. In the case of solids, the amount of free energy to form a given unit of surface area is called surface energy, whereas for liquids the same quantity is called surface tension. In response to surface tension, the ability of liquids to flow results in behaviour differing from that of solids, though at equilibrium both tend to minimise their surface energy: liquids tend to form rounded droplets, whereas pure solids tend to form crystals. , lacking free surfaces, freely diffuse.
The behavior of fluids can be described by the Navier–Stokes equations—a set of partial differential equations which are based on:
The study of fluids is fluid mechanics, which is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion.
Newtonian fluids follow Newton's law of viscosity and may be called .
Fluids may be classified by their compressibility:
Newtonian and incompressible fluids do not actually exist, but are assumed to be for theoretical settlement. Virtual fluids that completely ignore the effects of viscosity and compressibility are called .
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