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Homework Help: See Saw and Pivot Location

  1. Nov 6, 2008 #1


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    1. Harry, who weighs 360 N, and 170 N Gretchen are about to play on a 8.00 m long seesaw. He sits at one end and she at the other. Where should the pivot be located if they are to be balanced?

    2. F1R1 = F2 R2; F1=360N F2 = 170 N. The problem gives the total length, and I know I need to find out the distance of Gretchen first, but I can't seem to figure it out. I have also used the formula (M1*g)R1 = (M2*g)R2, but again I'm not sure how to figure out the distance of Gretchen.

    3. I tried to use this formula to figure out Gretchen's distance. To = r (mass).

    530 = 170 (8) = 3.12, but that answer is incorrect. Please help.
  2. jcsd
  3. Nov 6, 2008 #2


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    I forgot to include:

    They are looking for the distance in meters from Harry.

  4. Nov 6, 2008 #3


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    Homework Helper

    Let L be the length from Harry. Then if it balances doesn't

    360N * L = 170N * (8 - L) ?
  5. Nov 6, 2008 #4


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    In order to figure out the distance of Harry, don't we need to figure out the distance of Gretchen? I'm not sure what valut to put in for L where you have 8-L.
  6. Nov 6, 2008 #5


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    Gold Member

    Uno: You have two unknowns R1, R2, so you require two equations for a solution:
    Balance the torque moments:
    F1*R1 = F2*R2,

    Then solve for R1 or R2:
    R1=(F2/(F1+F2)) * 8, the pivot is 2.57M from the larger force. Note the three significant figures in the answer. You were given 3 sig figs in the problem, be sure to include 3 in your answer or a tough TA/Prof might ding you.
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