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Homework Help: Quick question on Pulley Force Problems

  1. Oct 28, 2013 #1
    Alright, so I'm doing this homework problem: "A 32·kg block is connected to an empty 1·kg bucket by a cord running over a frictionless pulley. The coefficient of static friction between the table and the block is 0.41 and the coefficient of kinetic friction between the table and the block is 0.34." And I need to find out the acceleration if 6.1kg of sand is put in the bucket, which I figured out is 0. Now I'm trying to find the acceleration if 7.3kg MORE is put in the bucket.

    I have my Fnet as +24.696. When I put it in the equation Fnet=ma, to solve for the a, what masses do I use. Do I have it set up as 24.696=(32+13.4+1)a? Because I have that right now and it's giving me .53 which isn't the right answer. Where did I go wrong in that calculation?
  2. jcsd
  3. Oct 28, 2013 #2


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    Looks like your net force calculation is wrong, maybe you have made a math error. Please show how you arrived at it.
    It is often not wise to look at the combined system of masses in one fell swoop . You instead should look at FBDs of each part of the system separately, getting 2 equations with 2 unknowns you can solve.
  4. Oct 28, 2013 #3
    Once the additional 7.3 kg are added and you find that the gravitational force is enough to overcome the static frictional force, you have to use the coefficient of kinetic friction to calculate that force. That should correct the problem.
  5. Oct 29, 2013 #4
    I actually just figured out what I did wrong. I forgot to incorporate the 1kg of the empty bucket into it. Thank you!
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