1 On a day when the wind blows with a speed v, show that the mass of air hitting the blades of a windmill per second is rAv, where r is the density of air, and A is the area s wept out by the blades.
2 Now show that the maximum possible power produced by the windmill is P = 1 /2 rAv 3 .
3 Calculate the power produced by a windmill of blade length 15 m, when the wind speed is 10 ms -1 . Assume that the density of air is 1.2 kgm -3 , and that the windmill is 60% efficient in its generation of electricity from wind power. Compare this answer with the data in the text.
4 The energy produced over one year depends on the average value of velocity cubed. Explain what the words in italics mean.
