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Homework Help: Rate of change of momentum. newton 2nd law

  1. Jul 10, 2012 #1
    rate of change of momentum. newton 2nd law (solved)

    hailstones with an average mass of 4grams fall vertically and strike a flat roof at 12ms-1 .
    In a period of 5 minutes, 6 000 hailstones fall on each square meter of roof and rebound vertically at 3ms-1 .
    Calculate the force on the roof if it has an area of 30m2 .

    the ans is 36 N

    plz help
    Last edited: Jul 10, 2012
  2. jcsd
  3. Jul 10, 2012 #2
    Do you know that Force = change in momentum per second?
    Can you use these figures to calculate the change in momentum in 1 second over tha area of 30m^2?
  4. Jul 10, 2012 #3
    this was what i did so far

    mass for 1 second:
    600 / (5x60)
    = 20 stones s-1

    initial momentum

    final momentum

    rate of change of momentum
    (.96) - (.24)

  5. Jul 10, 2012 #4


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    Remember: momentum is a vector quantity. It has direction! If you take positive as representing upward direction, what would be the sign of the initial momentum?
  6. Jul 10, 2012 #5
    so will it be
    change of momentum
    -.96 - (+.24)
    = - 1.2 N
    now what?
  7. Jul 10, 2012 #6


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    Check the location of that decimal point in your answer. I guess you decided to take downward as the positive direction. That's ok.

    So, you get a force. But is this the force for 30 m^2?
  8. Jul 10, 2012 #7
    i dont understand what to do with the force and the area 30m^2
    can u explain some more
  9. Jul 10, 2012 #8
    You are almost there. What you have calculated is the force on 1m^2
    The area of the roof is 30m^2.......
  10. Jul 10, 2012 #9
    thankkk uu
    36 yayy
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