Solve the following :

Volume of 1 hailstorm

$=\frac{4}{3} \pi\left(\frac{10^{-2}}{2}\right)^{3}=\frac{4}{24} \pi \times 10^{-6}$

$=\frac{\pi}{6} \times 10^{-6} \mathrm{~m}^{3}$

Mass $=\rho . V=9000 \times \frac{\pi}{6} \times 10^{-6}$

$=150 \pi \times 10^{-6} \mathrm{~kg}$

Mass of 2000 hailstorms $=2000 \times 150 \pi \times 10^{-6}$

$=0.3 \pi_{\mathrm{kg}}$

average force on $1^{m^{3}}$ roof

$=\frac{\Delta p}{\Delta t} \frac{d p}{d t}=\frac{0.3 \pi \times 20}{1}=18.84 \sim 19$

Average force on $10 \mathrm{~m} \times 10 \mathrm{~m}\left(100^{\mathrm{m}^{2}}\right)$ roof

$=19 \times 100=1900 \mathrm{~N}$