Work, Energy and Power

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 the windmill
converts 25% of the wind’s energy into electrical energy, and that A = 30 m2
, v = 36
km/h and the density of air is 1.2 kg m–3. What is the electrical power produced ?