1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    mv
    (20)(.004)(12)
    =.96

    final momentum
    20(.004)(3)
    =.24

    rate of change of momentum
    (.96) - (.24)
    =.72
    =F

    stuck
     
  5. Jul 10, 2012 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member

    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

    TSny

    User Avatar
    Homework Helper
    Gold Member

    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
    sorry
    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
    ohh....
    thankkk uu
    1.2x30
    36 yayy
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Rate of change of momentum. newton 2nd law
  1. Newton's 2nd Law. (Replies: 2)

  2. Newton's 2nd Law Lab (Replies: 1)

Loading...