The blades of a windmill sweep out a circle of area A. If wind flows with velocity v perpendicular to blades of wind mill and its density is p, then the mechanical power delivered by the wind mill is
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https://www.meritnation.com/ask-answer/question/the-blades-of-a-windmill-sweep-out-a-circle-of-area-a-a/work-energy-and-power/6299569