Physics

Problem

5.21 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 the windmill converts

25 \%

of the wind's energy into electrical energy, and that

A=30 \mathrm{~m}^{2}, v=36 \mathrm{km} / \mathrm{h}

and the density of air is

1.2 \mathrm{~kg} \mathrm{~m}^{-3}

. What is the electrical power produced?