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Friction/KE problem

  1. Oct 12, 2009 #1
    1. Two blocks, A and B (with mass 50 kg and 100 kg, respectively), are connected by a string, as shown in the figure below. The pulley is frictionless and of negligible mass. The coefficient of kinetic friction between block A and the incline is µk = 0.21. Determine the change in the kinetic energy of block A as it moves from C to D, a distance of 25 m up the incline if the system starts from rest.

    http://img26.imageshack.us/img26/5803/p564.gif [Broken]




    I used
    a1=-a2
    T-m1g*sin37=m1a1
    T-m2g=m2a2 -T+m2g=m2a1
    m2g-m2a1-m1g*sin37=m1a1




    3.

    Solved for a (1.142 m/s^2), solved for Vf (.7556 m/s) and used the equation KE=.5mv^2. I did something wrong, but I'm not sure what. Please help


     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 12, 2009 #2

    rl.bhat

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    Hi madcat1090, welcome to PF.
    In your attempt you have not taken into account the frictional force, which acts in the opposite direction of motion of A
     
  4. Oct 12, 2009 #3
    So should i use T-m1g*sin37-ukn=m1a1?
     
  5. Oct 12, 2009 #4

    rl.bhat

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    Yes.
    What is the normal force?
     
  6. Oct 13, 2009 #5
    Ummm, should be mg*cos37 right?
     
  7. Oct 13, 2009 #6
    You can also solve the problem by using energy.
     
  8. Oct 13, 2009 #7

    rl.bhat

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    Right.
     
  9. Oct 13, 2009 #8
    Awesome. I think I can solve it now. Thanks for the help!
     
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