Mechanical Properties of Fluids — Class 11 Physics

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 . 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, constant, expresses conservation of mass, and Bernoulli's principle, 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 ; 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.