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Pulley HELP

  1. May 31, 2008 #1
    1. The problem statement, all variables and given/known data
    a box weighing 77N rest on a table. a rope tied to the box runs vertically upward over a pulley and a weight is hung from the other end. determine the force that the table everts on the box if the weight hanging on the other side of the pulley weighs 30N.

    (box in on the left n weight on the right hand side of the pulley)


    2. Relevant equations
    box
    F=mg-T
    ma=mg-T
    weight
    F=T-mg
    ma=T-mg

    3. The attempt at a solution
    i let

    T of box= T of weight
    mg-ma=ma-mg
    2ma=30N + 77N
    ma=107N / 2
    F=53.5N

    (i think i am missing something in the equation)
     
  2. jcsd
  3. May 31, 2008 #2
    If the weight hanging opposite the box was 77N, what force would the table exert?
     
  4. May 31, 2008 #3
    i am not sure but i will be guessing its a normal force.....
     
  5. May 31, 2008 #4
    The table only exerts a force equal to what is not supported by tension in the rope.

    As one of your formulas you have T of box= T of weight. You know that since the system is at rest and the weight is suspended the tension has to be equal to the weight. The tension in the rope is pulling up on the box with a force equal to the weight. The table supports the difference between the weight of the box and the weight of the weight.
     
  6. May 31, 2008 #5
    alright i think i get wat u mean..... if i go wrong i will let u know
     
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