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Homework Help: Torque on a swing

  1. Dec 23, 2016 #1
    1. The problem statement, all variables and given/known data
    A 355.0N boy and a 300.0.0N girl sit on a 50.5N porch swing that is 1.5m long. The swing is supported by a chain on each end. What is the tension in each chain when the boy sits .35m from one end and the girl sits .45m from the other? ( ans: boys side = 390 N, girls side = 320 N)

    2. Relevant equations

    3. The attempt at a solution
    I don't have much practice with torque questions especially not ones that are hanging.
    Στ= 0 = (T2)(1.5m) - (355N)(0.35m) - (50.5N)(0.75m) - (300N)(1.05m)
    Στ = 0 = (T2)(1.5m) - 124.25Nm - 37.875Nm - 135Nm
    (T2)(1.5m) = 477.125Nm → divide both sides by 1.5m
    318N = T2
    I'm trying to figure out how to get the tension on T1 now.
    Last edited: Dec 23, 2016
  2. jcsd
  3. Dec 23, 2016 #2
    It looks like you have positioned the girl sitting somewhat on the boy.
  4. Dec 23, 2016 #3
    Do you see the mistake?
  5. Dec 23, 2016 #4
    Awesome! Didn't notice that, thanks! I can't figure out the tension on the boys side though.
  6. Dec 23, 2016 #5
    Well, if you figured out one tension, then you know all of the forces except one - the other tension. And since the swing is in equilibrium (not accelerating), you know that the sum of the forces have to equal 0, true?
  7. Dec 23, 2016 #6
    Yes. The T1=T2. 320N = 320N? If I added his extra 55N it would only equal 375N
  8. Dec 23, 2016 #7
    I haven't worked this problem yet so I don't know the answer. But, you can't just say that T1 = T2. Neither can you add his weight to one tension to get the other tension. The sum of ALL of the forces have to equal 0. Or, how I prefer to think about it: The upward forces have to equal the downward forces. But you have to make sure that you include all of them.
  9. Dec 23, 2016 #8
    Ah I see! Thanks a lot! I've gotten my answer again!
  10. Dec 23, 2016 #9


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    Huh? They can't be equal.

    Have you drawn a force diagram? What are the 5 (4 known and 1 unknown) vertical forces acting on the swing? What must they add up to?

    EDIT: Sorry cross posted
  11. Dec 23, 2016 #10
    Yes I drew a force diagram and I realize that statement is wrong. I added up all my weights going down and minused the tension I found in T2 to get my last tension.
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