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Homework Help: Calculating Work done by Friction

  1. Dec 3, 2008 #1
    1. The problem statement, all variables and given/known data

    Alrighty, I have most of this set but I'm getting the wrong answer and I'm not sure why.

    A block of mass 18.0 kg is sliding down an 6.0 metre long ramp inclined at 56.0 deg. to the horizontal. If the coefficient of kinetic friction between the ramp and the block is 0.48, how much work is done by friction as the block moves from the top to the bottom of the ramp ?

    m = 18kg
    d = 6 m
    Theta = 56 degrees
    mu = .48

    2. Relevant equations

    W = Fd
    F = ma
    mu = fk / fn

    3. The attempt at a solution

    First I calculated work done by gravity. . .
    W(g) = Fd = (18)(9.8)(6)(cos 56) = 591.85 J

    Then I attempted to calculate the work done by friction. . .
    W(f) = (.48*18*9.8*6*sin56) = 421.178 J

    I tried that as a negative number as well, since the distance is negative.

    So, clearly I'm missing something.

    Any help would be greatly appreciated. Thanks!
  2. jcsd
  3. Dec 3, 2008 #2


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    Homework Helper

    fk = mu*fn
    And fn = mg*cos56
  4. Dec 3, 2008 #3
    Oh, should I be using cosine then??
  5. Dec 3, 2008 #4
    I think your solution seems right. By definition the work done by friction is W = f (dot) d and f = muN = mu mgcos(theta). So W = fdcos(180deg). Its 180 degrees because f and d are in opposite directions. Then like you have it W = - fd = mu d mgcos(theta). It should be negative I think.

    The only thing left to check that i can think of is if the work energy theorem is satisfied you can check that Wnet = the change in kinetic energy. I think you can do that by finding the final velocity of the object through kinematics. Wnet = Wg + Wf. If that checks then you can make sure that your answer is correct.

    I hope this helps a little or makes it more clear.....
    Last edited: Dec 3, 2008
  6. Dec 3, 2008 #5
    Hmm, I don't get it.

    Oh, well I just used my last guess. (It's online, we get 5 tries per question to get credit)

    Thanks for the responses though!
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