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I need help on a pulley problem PLEASE

  1. Sep 9, 2007 #1
    a pulley is suspended by a cord with two masses attached. one mass is 1.2 kg. the second mass is 3.2 kg. the two masses are attached on different sides.

    the question: determine the tension in this cord after the masses are released and before one hits the ground. ignore the mass of the pulley and cords.


    thank you very much in advance!
     
  2. jcsd
  3. Sep 9, 2007 #2

    learningphysics

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    How did you approach the problem?
     
  4. Sep 10, 2007 #3
    i am not sure [at all] how to start with the problem...
     
  5. Sep 10, 2007 #4

    learningphysics

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    Hint: The tension above both masses are the same. The acceleration of the lighter mass upward = the acceleration of the heavier mass downward.

    Do a freebody diagram of each of the masses. What are the forces acting on each mass... use:

    [tex]\Sigma\vec{F} = mass*acceleration[/tex]

    for each mass...
     
  6. Sep 10, 2007 #5
    i have solved for the first tension. and tension is 17.1 N
    i am clueless has to how to solve the tension before one mass hits the ground
     
  7. Sep 10, 2007 #6

    learningphysics

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    I think all they meant for that was while the masses are still suspended... before they hit the ground and change the system... 17.1N is the right answer for the rope holding up each of the masses... but what is the tension in the cord holding up the pulley.

    You're almost there. The pulley is massless, so the force on it is ma = 0a = 0. You can also say... the pulley is fixed, so its acceleration is 0 and ma = m0 = 0. Either way, the force on the pulley is 0.

    Use that to calculate the tension in the cord holding the pulley. You've done most of the work, already, this is just the last part.

    Use F = ma for the pulley to calculate the tension in the cord above...
     
    Last edited: Sep 10, 2007
  8. Sep 10, 2007 #7
    i'm confused. if the force is zero, then is the tension the same as it is for the other rope?
     
  9. Sep 10, 2007 #8

    learningphysics

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    17.1N is the tension in the rope above mass 1. 17.1N is also the tension in the rope above mass 2.

    You need to find the tension in the cord above the pulley. So the forces acting on the pulley are the tension above, and the two 17.1N below. net force needs to be 0 on the pulley.
     
  10. Sep 10, 2007 #9
    it just clicked in my head. thank you!
     
  11. Sep 10, 2007 #10

    learningphysics

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    cool. just to make sure, what did you get as your final answer?
     
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