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Please help with possible test question

  1. Feb 13, 2006 #1
    Johnny jumps off a swing, lands sitting down on a grassy 20 degree slope, and slides 3.5 m down the slope before stopping. The coefficient of kinetic friction between grass and the seat of Johnny's pants is 0.5.What was his initial speed on the grass?

    now i know the answer is 2.96m/s, but i dont know how to get at it...


    heres what i tried:

    Fn= mgsin20

    Ff=.5mgcos20

    mgsin20-Ff= ma

    mgsin20- .5mgcos20 = ma

    (m's cancel out)
    a=2.88 m/s^2

    Vi^2= 0 + 2(-2.88)(3.5)
    Vi= 4.49 m/s

    but this is definately wrong..... how do i get the real answer?
     
    Last edited: Feb 13, 2006
  2. jcsd
  3. Feb 13, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your method is perfect, but redo the calculation for "a".
     
  4. Feb 13, 2006 #3
    thank you, my friend :biggrin:
     
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