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Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses.
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For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In materials science and engineering, one is often interested in understanding the forces or stresses involved in the deformation of a material. In a general parallel flow, the shear stress is proportional to the gradient of the velocity. A fluid with a high viscosity, such as pitch, may appear to be a solid. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity such fluids are technically said to be viscous or viscid. Zero viscosity is observed only at very low temperatures in superfluids.
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So for a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity.Ī fluid that has no resistance to shear stress is known as an ideal or inviscid fluid. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. In such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity can be conceptualized as quantifying the internal frictional force that arises between adjacent layers of fluid that are in relative motion. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. The viscosity of a fluid is a measure of its resistance to deformation at a given rate.