Qn:
During a thunderstorm, hail of diametre 1.0 cm falls directly downwards at a speed of 25 m/s.
There are estimated to be 125 hailstones per cubic metre of air.

a) What is the mass of each of hailstone (density = 0.92 g/cm^3)?

[ i got this one, don't know if its needed for the 2nd part ]

V = (4/3)(pi)(0.5 cm)^3 = (0.524 cm^3)(0.92 g/cm^3) = 0.48 g

b) Assuming that the hail does not bounce, find the magnitude of the average force on a flat roof measuring 10 m x 20 m due to the impact of the hail.
( Hint: During impact, the force on a hailstone from the roof is approximately equal to the net force on the hailstone, because the gravitational force on it is small. )
.. this is where I'm stuck
any help on how to do this part of the question would be appreciated = )

Thank You !
- mandy : )