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Homework Help: Two momentum problems

  1. Apr 13, 2007 #1
    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.
     
  2. jcsd
  3. Apr 13, 2007 #2
    OK,first part is clear.They referred to terminal velocity.Hence,the term "average".So,F_ext=0
     
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