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Homework Help: Find the coefficient of kinetic friction

  1. May 9, 2012 #1
    1. The problem statement, all variables and given/known data

    A skier weighing 90kg starts from rest down a hill inclined at 17 degree. He skis down the hill and then coast for 70 m along level snow until he stops. Find the coefficient of kinetic friction between the skis and the snow. What velocity does the skier have at the bottom of the hill?

    2. Relevant equations
    θ=17degree, sA=100m, sB=70m, m=90kg

    3. The attempt at a solution
    A-motion skis down the hill
    B-motion along level snow

    mg sin θ - Ff = maA ------ 1 ;Ff is the frictional force
    N - mg cos θ = 0
    → N = mg cos θ -------- 2

    Ff = μN ----------------- 3 ; μ is coefficient of kinetic friction

    mg sin θ - μN = maA ------- 4

    mg sin θ - μmg cos θ = maA
    aA = g(sin θ - μcos θ) ---- 5

    -Ff = maB ---------------- 6
    down the hill
    N - mg = 0 → N = mg ---- 7

    -μN = maB --------------- 8

    -μmg = maB
    aB = -μg

    using v2=u2+2as ;
    u=0 since it starts from rest,


    vA2 = vB2
    2aAsA = 2aBsB
    aA = -(μgsB)/sA ; aB = -μg

    g(sin θ - μcos θ)= -(μgsB)/sA
    μ(cos θ - sA/sB) = sin θ
    μ = (sin θ)/(cos θ - sA/sB)

    Substituting θ=17degree, sA=100m, sB=70m into the equation,
    μ = (sin 17)/(cos 17 – 70/100)
    = 1.14

    The value of μ should be between 0 and 1. Can anyone tell me where I went wrong in solving this question? Thank you.
  2. jcsd
  3. May 9, 2012 #2
    Apply work-energy principle.
    Last edited: May 9, 2012
  4. May 9, 2012 #3

    Doc Al

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    Staff: Mentor

    Your work looks OK to me.
    I think you have a typo here. That ratio should be sB/sA, not sA/sB.

    But it looks like you plugged in the correct ratios.

    I don't think you went wrong. The data is just unrealistic.

    Edit: I found the mistake; see my post #7. (And please look at azizlwl's alternative solution. It's always good to solve things multiple ways.)
    Last edited: May 10, 2012
  5. May 9, 2012 #4
    length on slope=100m
    length on level =70m

    Mg(100sin17°)=μMgCos17°(100) +μMg(70)
    μ=100sin17°/(100Cos17° +70)
  6. May 9, 2012 #5
    Deleting my comment on this.
    Last edited: May 10, 2012
  7. May 10, 2012 #6

    Doc Al

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    Staff: Mentor

    Looks good to me.

    I just redid the problem using the kinematic method used in post # 1 and got the same answer.

    I'll have to look to see where the error in post #1 is. Oops!

    (I see the mistake... a sign error!)
  8. May 10, 2012 #7

    Doc Al

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    Staff: Mentor

    Careful here. For part A, the speed goes from 0 to V; but for part B the speed goes from V to 0.

    So: 0 - VB2 = 2aBsB.

    You have a sign error in applying the kinematic formula to the horizontal motion. Fix that and you'll get a sensible answer.

    Sorry for not spotting that earlier. :redface:

    Thanks to azizlwl!
  9. May 10, 2012 #8
    Thank Doc Al, I'm too looking for Yanase's error which lead to wrong answer and your advice of using multiple methods.
  10. May 12, 2012 #9
    Thank you very much to both of you. I really appreciate you help in solving my question. :D
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