I did the following two questions of the same category.But the answer is not matching.Please see if I went wrong.(adsbygoogle = window.adsbygoogle || []).push({});

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) whereuis 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.

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# Homework Help: Two momentum problems

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