Answer:

Correct Answer: C

Solution:

From Bernoulli’s theorem $ p _1+\frac{1}{2}\rho v_1^{2}=p _2+\frac{1}{2}\rho v_2^{2} $

where, $ p _1,p _2 $ are pressure inside and outside the roof and

$ v _1,v _2 $ are velocities of wind inside and outside the roof. Neglect the width of the roof.

Pressure difference is $ p _1-p _2=\frac{1}{2}\rho (v_2^{2}-v_1^{2}) $ $ =\frac{1}{2}\times 1.2(40^{2}-0)=960N/m^{2} $

Force acting on the roof is given by $ F=(p _1+p _2)A=960\times 250 $ $ =24\times 10^{6}N=24\times 10^{5}N $

As the pressure inside the roof is more than outside to it.

So the force will act in the upward direction. i.e. $ F=2.4\times 10^{5}N $ upward.