Dureing a heavy rain hailstones of average size 1.0 cm in diameter fall with an average speed of 20 m/s. Suppose 2000 hailstones stgrike every square meter of a `10mxx10m` roof perpendicularly in one second and assume that the hailstones do not rebound. Calculate the average force exerted by the falling hailstones n the roof. Density of a hailstone is `900 km/s^3`