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Pulley System Kinetic Energy

  1. Oct 14, 2007 #1
    1. The problem statement, all variables and given/known data
    Force of gravity = 10
    [​IMG]
    http://img80.imageshack.us/img80/1156/kineticec6.png
    Pulley System
    Two unequal weights, A = 6KG and B = 4KG
    What is the kinetic energy of the movement after 4 seconds?

    I think my calculations are so far correct... please point me in the right direction.

    2. Relevant equations

    KE = 1/2mv^2

    3. The attempt at a solution

    Acceleration of the two weights

    a = g (A-B)
    -------
    (A+B)

    a = 10 (6-4)
    ---------
    (6+4)

    a = 2ms

    Force of gravity acting on weights

    Weight 'A' (6KG)

    F = MA
    F = 6 * 10

    Weight 'B' (4KG)

    F = MA
    F = 4 * 10

    Ive also made an attempt to calculate velocity:

    Velocity of masses

    V = Sqr 2gh (6-4)
    ------
    (6+4)

    = (Sqr Rt)100


    = 10


    However I made another calculation and found the acceleration of the both weights to be 7.3, and by using SUVAT that the velocity is 29.3 with that figure of 7.3 for acceleration. To be honest im completely lost, please help.


    1. The problem statement, all variables and given/known data
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Oct 14, 2007
  2. jcsd
  3. Oct 14, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You found the acceleration. Assuming it starts from rest, what must be the speed after 4 seconds?

    Sounds like you found the acceleration using two methods and got different answers. What was the second method that you used?

    As far as the first method goes, did you derive that equation yourself or are you using a formula given in your text?
     
  4. Oct 14, 2007 #3
    My tutor gave a homework question with that diagram, he said. Find the kinetic energy of the movement after a period of 4 seconds. Nothing was provided to us, the formulas I have used come from this website http://www.sparknotes.com/testprep/books/sat2/physics/chapter8section2.rhtml . I read through it and tried to apply the given formulas.
     
    Last edited: Oct 14, 2007
  5. Oct 14, 2007 #4

    Doc Al

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    Staff: Mentor

    It's generally better to derive your own equations from basic principles (Newton's 2nd law). But the formula and answer for the acceleration that you gave is correct. Use kinematics to find the speed after 4 seconds.
     
  6. Oct 14, 2007 #5
    S 25
    U 0
    V ?
    A 2
    T 4

    V = U + A * T
    V = 0 + 2 * 4
    V = 8

    So where do I go from here? I now know the Velocity, what about the masses.
     
  7. Oct 14, 2007 #6

    Doc Al

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    Staff: Mentor

    You have the velocity, now calculate the kinetic energy.
     
  8. Oct 14, 2007 #7
    Kinetic Energy = 1/2 M V^2

    1/2 10 * 8^2

    = 320
     
  9. Oct 14, 2007 #8
    So 320 is the final answer?
     
  10. Oct 14, 2007 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Yes. Be sure to attach proper units to your answer.
     
  11. Oct 14, 2007 #10
    Thanks very much for your time, much aprechiated :D
     
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