[" Question "21/" The blades of a windmill sweep out a circle of area "A.],[" (a) If the wind flows at a velocity "v" perpendicular to the circle,what is "],[" the mass of the air passing through it in time "t" ? "],[" (b) What is the kinetic energy of the air? "],[" (c) Assume that windmill converts "25%" of the wind's energy into "],[" electrical energy and that "A=30m^(2),v=36km/h" and the density of "],[" colution."]