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View allChapter 2: Mechanical Properties of FluidsClass 11 Physics — summary, notes, extra questions & MCQ quiz
Summary
Liquids and gases are fluids because they flow and offer very little resistance to shear stress. Pressure is the normal force per unit area; in a fluid at rest it acts equally in all directions (a scalar) and increases with depth as \(P=P_0+\rho gh\). Pascal's law states that a pressure applied to an enclosed fluid is transmitted undiminished to every point, the basis of the hydraulic lift and brakes. Atmospheric pressure is measured by a barometer. For an ideal fluid in steady streamline flow the equation of continuity, \(Av=\) constant, expresses conservation of mass, and Bernoulli's principle, \(P+\tfrac12\rho v^{2}+\rho gh=\) constant, expresses conservation of energy, explaining the lift on an aerofoil and the working of a venturimeter. Real fluids have viscosity, an internal friction described by the coefficient \(\eta\); Stokes' law gives the viscous drag on a sphere and leads to terminal velocity. Flow is laminar below a critical Reynolds number and turbulent above it. Surface tension arises from intermolecular attraction at a liquid surface, giving rise to excess pressure inside drops and bubbles, capillary rise, and angle of contact, with surface energy equal to surface tension times area.
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