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Homework Help: Climber hanging from a cliff

  1. Feb 25, 2010 #1
    1. The problem statement, all variables and given/known data

    A 65 kg climber finds himself dangling over the edge of an ice cliff, as shown in the figure below. Fortunately, he's roped to a 940 kg rock located 51 m from the edge of the cliff. Assume that the coefficient of kinetic friction between rock and ice is 5.7×10−2. What is his acceleration, and how much time does he have before the rock goes over the edge? Neglect the rope's mass.

    2. Relevant equations

    FT - Ff = massrock * a
    N = massrock * g

    FT - massclimber * g = massclimber * a

    3. The attempt at a solution

    I solved for FT for the climber and then stuck this equation into FT in the rock equation. Then solved for a.

    a = (g(massclimber - Mu(k) * massrock)) / (massrock - massclimber)

    Plugging the numbers in the equation, i get a = 0.127904 m/s2 My homework tells me this is wrong and I can't figure out why. any help is appreciated!
  2. jcsd
  3. Feb 25, 2010 #2
    What's the force of friction?

    What is the books answer btw?
  4. Feb 25, 2010 #3
    Ff = Mu(k) * N or Mu(k) * massrock * g

    it's actually homework on the computer and doesn't give me the answer. Just tells me mine is wrong and to try again!
  5. Feb 25, 2010 #4
    figured it out

    FT - massclimber * g = massclimber * a
    should be
    FT - massclimber * g = -massclimber * a
    since the climber is moving downward
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