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Homework Help: Test Review help

  1. Feb 2, 2008 #1
    Test Review help!!!!

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

    Two children whose masses are 35kg and 55kg are on a playground "teeet totter" which is 3.0m long. Where must the fulcrum be placed so that the children will balance?
    (35kg on left, 55kg on right)
    2. Relevant equations

    sum of T=0

    3. The attempt at a solution
    I started with T=0=F(left)L(left)-F(right)L(right)
    The two lever arms must add to be 3.0m so L(left)=3.0m-L(right). Through some algebra i got the equation into terms of L(right):
    I get the right lever to be .95m, but is supposed to be between 1-1.5

    Where am i going wrong??
  2. jcsd
  3. Feb 2, 2008 #2
  4. Feb 2, 2008 #3


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    Homework Helper

    Start simple.

    Let the distance from one end be x metres, then the distance from the fulcrum to the other end is (3-x)metres.

    For it to be balances the resultant torque about the fulcrum must be zero.
    Can you set up the equation now?
  5. Feb 2, 2008 #4
    This is all assuming the teeter totter is massless.
    Last edited: Feb 2, 2008
  6. Feb 2, 2008 #5
    teacher said you can ignore mass of teeter totter.
    And to rock.freak667, thats exactly how i started. I got 0=(x-3)(Fleft)-(X)(Fright).....
  7. Feb 3, 2008 #6


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    Homework Helper

    Clockwise moment about pivot=(33gx) Nm
    Anti-clockwise moment about pivot=55g(3-x) Nm

    For balance


    Are you sure you did the algebra correctly?

  8. Feb 3, 2008 #7
    r*m = r*m
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