I did the following two questions of the same category.But the answer is not matching.Please see if I went wrong. 1. The problem statement, all variables and given/known data 1.During rainfall,2000 hailstones per square metre (radius 0.5 cm) falls onto a 10mx10x roof with velocity 20m/s.FInd the average force exerted onto the roof without rebound.Density of hailstones 900kg/m3 2.A ball of mass m is dropped onto the ground from a certain height.Colliding elastcally,it again rises to that height.Again it falls.What is the force exerted onto the grond in a long interval of time? 2. Relevant equations I think in both cases we may use this standard equation: P=F_ext+u(dM/dt) where u is the relative velocity 3. The attempt at a solution (1) In problem (1),the total mass is 30*pi.You may check it. So,total force exerted on the roof F=F_floor+F_gravity Taking magnitudes--- =u(dM/dt)+Mg =20*30*pi+30*pi*g (where g is the acceleration due to gravty). ~2808N whereas the book says it is 1900 N I noted that if we neglect the gravity term,the answer (1885N) is close to 1900N (2)Here what I got is F=F_floor+F_gravity Taking magnitudes,- =u(dm/dt)+mg =d(mu/dt)+mg since the u has not been giveen,I could not proceed more. However,they have provided the answer mg.