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Homework Help: Tension Problem in a pulley system

  1. Apr 4, 2005 #1
    Tension Problem! Help plz!

    I have a question that relates to a vertical/horizontal pulley system:
    i'll try drawing the diagram, hopefully someone can understand!


    alright, i guess i should explain that ^^^ if it clarifies the diagram a bit. alright, so there's a mass of 2.0kg on a surface attached to a string which attaches to a pulley (shown by "O") on the edge of the surface. This string is also connected to a 5.0kg mass which is shown in the diagram to simply be hanging off the pulley.

    So the question is: A 2.0kg mass, placed on a smooth level table, is attached by a light string passing over the edge of a table. Determine the tension in the string. (the answer given is 14N)

    i've found that the total mass of the two are 7.0kg, and is aware that the tension is the same on both the strings. however, what formula would i use to execute this question. would multiplying each mass by 9.81 assist? if the diagram is difficult to understand, plz respond so i can further clarify it!

    ps) if anyone knows how to add in diagrams, etc. other than using keyboard letters and symbols, that would be great!
    Last edited: Apr 4, 2005
  2. jcsd
  3. Apr 4, 2005 #2


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    Homework Helper

    First, find out all the forces acting on the 2 mass. Which force you think is the force that accelerates the 2 mass?
    Then find out the net acceleration. (the 2 mass have the same acceleration).
    And finally, find out the tension of the string. (Nore that only the tension will accelerate the 2.0 kg mass).
    Viet Dao,
  4. Apr 4, 2005 #3
    i still feel lost. alright, yes, tension due to the 5.0kg mass and gravity creates tension to acclerate the mass. the forces that are working on the masses are obviously tension, force gravity, normal force on the 2.0kg mass. and since the surface is "frictionless", friction is not in this problem.
  5. Apr 5, 2005 #4


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    Homework Helper

    Okay, so there are 2 forces acting on the first mass: P1, and T1. Can you find out its acceleration? Let's call it a1.
    Then find out the acceleration of mass 2. Call it a2.
    Now you have a1 = a2. Remember that:
    [tex]\frac{a}{b} = \frac{c}{d} = \frac{a + c}{b + d}[/tex]
    And you will find out the acceleration of the net. a1 = a2 = a(net). So you now a2, and can you find out the magnitude of the tension force?
    Or you can use the equation:
    [tex]a_{net} = \frac{\sum F_{external}}{\sum m}[/tex]
    There are 3 external forces here: P1, P2, N2. And you have [itex]\vec{P_{2}} + \vec{N_{2}} = \vec{0}[/itex]
    Hope it help,
    Viet Dao,
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