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Homework Help: Friction up a slope

  1. Feb 18, 2009 #1
    1. The problem statement, all variables and given/known data
    Lindsey's car is driving up a hill when her book bag, which she forgot on the roof, falls off. The bag has a mass of 19kg and slides up the hill 60 meters from its initial speed of 20m/s to rest. If the hill climbs at an angle of 30 degrees then what was the coefficient of friction?

    2. Relevant equations
    Fn = mgcosx
    Ff = [tex]\mu[/tex]cosx
    F = ma
    Vf2 = Vi2 + 2ad
    Fp = mgsinx (?)

    3. The attempt at a solution

    Fn = 19*9.8cos30 = 161N

    Vf2 = Vi2 + 2ad
    0 = 202 + 2a(60)
    -400 = 120a
    a = -3.33 m/s2

    F = ma
    F = 19 * -3.33 = 63.27N

    Ff = [tex]\mu[/tex]mgcosx
    63.27 = [tex]\mu[/tex]19*9.8*cos30
    [tex]\mu[/tex] = .393

    Is this correct? Is the parallel component required in this case? In what cases is it required?
  2. jcsd
  3. Feb 18, 2009 #2
    you mean it slides down a hill? o_O
  4. Feb 19, 2009 #3


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    Homework Helper

    I would draw an FBD (Free Body Diagram) before attempting this one

    Looks close...
    Acceleration & net force on the book look correct...

    however the book also has a gravity component along the slope, which i don't think has been taken into account - this will also act to slow down the book
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