Question

A strong wind of speed 33 m s1 blows against a wall. The density of the air is 1.2 kg m3. The wall has an area of 12 m2 at right angles to the wind velocity. The air has its speed reduced to zero when it hits the wall.

Answer 1

Now consider a column of length 33 meters at right angle to the wall so, in 1 second all the air in this column will strike the wall (as velocity is 33m/s)  

now calculate the momentum of this column of air i.e. P=m*v  

P = (volume*density)*v = [(33*12)*1.2]*33 (i took length as 33 and area as 12m2 as what happens to remaining air doesn't matter)  

thus on calculating P = 15681.6 kg.m/s  

now force applied = 15681.6/1 N (as in 1 sec all this air will collide and come to a stop)  

therefore F =15681.6 N.