Q21 (a) The blades of a windmill sweep out a circle of area 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? 

Answers (1)
D Devendra Khairwa

The volume of wind   =   Av\rhov here  A is the swept circle and v is the velocity. 

Thus the mass of the wind is  : -    \rho Av  ,  \rho   is the density of the air.

Hence mass of wind flowing through windmill in time t is     =\ \rho Avt.