The blades of a windmill sweep out a circle of area A. If the wind flows at a velocity v perpendicular to the circle and its density is ρ, what is the mechanical power received by the windmill?
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Solution
Mass of air hitting the windmill per second, m =Density×Volumepersecond=Density×Velocity×Area =ρvA ∴ Associated kinetic energy per second =12(ρvA)×v2=ρAv32 This is nothing but the power received.