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Kinetic Friction and Ropes and Pulleys

  1. Oct 16, 2008 #1
    The problem statement, all variables and given/known data

    Block A, on a horizontal table, has a weight of 4.91N is attached to a rope, that goes over a pulley, attached to Block B, that has a weight of 2.94N. Once block B is set into downward motion, it descends at a constant speed. Assume that the mass and friction of the pulley are negligible. Need to find the kinetic friction coeffecient for Block A and Block B.

    2. Relevant equations
    Kinetic Friction = (coeffecient of Kf)(Normal Force) in the opposite direction
    Tension - Weight = 0
    Therefore, Tension = Weight
    3. The attempt at a solution
    I have drawn FBDs for both blocks. I understand the tension & normal force of the two, individually. I am having trouble connecting the tension to the normal force to find the coeffecient.

  2. jcsd
  3. Oct 16, 2008 #2
    I don't understand why you need to find the coefficient of friction on Block B unless Block B is sliding along something as it falls.

    As for your question, let's look at Block A. Along the horizontal axis you have Tension pulling the block and friction resisting the block's motion. These two forces will be equal since the blocks are moving at a constant speed. In the vertical axis you have the normal force pushing up on Block A and Fg pushing down. Again these two forces will be equal.
    So we have:
    (for Block A)
    Tension = Ff
    Normal force = Fg

    Once you have Normal force and Ff you can solve for the coefficient of Friction.

    But like I said, if there is friction acting on block B the problem will be a little different. (Specifically the Tension will change)
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