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Homework Help: Here's a toughy about pulleys and friction

  1. Sep 27, 2005 #1
    This is my problem:

    A 28 kg. block is connected to an empty 1.00 kg. bucket by a cord running over a frictionless pulley. The coefficient of static friction between the table and the block is 0.45 and the coefficient of kinetic friction between the table and the block is 0.32. Sand is gradually added to the bucket until the system just begins to move. (a) Calculate the mass of sand added to bucket. (b) Calculate the acceleration of the system.

    Now, I found this problem pretty vague. I'm assuming the the heavier block is on the table, which makes the most sense. I'm not sure why they gave me both the static and kinetic friction. My problem is I'm not sure how to decide the exact amount of sand to add.
  2. jcsd
  3. Sep 27, 2005 #2
    I don't follow this at all. How can the question say:
    "The coefficient of static friction between the table and the block..."
    when a table has not been previously mentioned?

    Is there a diagram?
  4. Sep 27, 2005 #3
    Nope, this is all the problem says. It's meant to "make us think", but I'm just getting a headache. From what I can gather, the heavier block is on a table, their is a frictionless pulley on the corner of the table, and an empty bucket is hanging on the rope tying the two masses together. Any ideas?
  5. Sep 28, 2005 #4


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    Homework Helper

    The block is placed on a horizontal table and the bucket and the send in it creating tension in the string which acts on the block to pull it towards the pulley, gives rise to the friction between the block and the table. You have to find the mass of the sand for limiting equilibrium of the block.

    Ones the block start slipping, the friction is reduces to kinetic friction and hence the net force is resultant of tension and the kinetic friction is accelerating the block and the bucket.
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