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Tension in a cable with a traffic light -- Torque question

  1. Nov 15, 2015 #1
    1. The problem statement, all variables and given/known data

    An 11 kg traffic light hangs fro ma structure as shown.
    The aluminium pole AB is 7.5 m long, and weighs 8 kg. Determine the tension in cord CD

    2. Relevant equations

    T = rf



    3. The attempt at a solution

    trafficlight-jpg.45825.jpg

    Ignore the AC height of 3.8 m, and replace it with an AD length of 7.5 m.

    Would the pivot point be A or D?

    If D, how would I find the r for D to B?

    For the equation ΣT = r1f1 + r2f2 + r3f3, with r1f1 being the pole, r2f2 being traffic light, what would the cale be as?

    I understand how to do the calculations, but looking at this I'm not entirely sure on how to get the F for the cable.

    I know that it'll be either sin or cos 37 to take into account the angle, but I'm lost.
     

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  2. jcsd
  3. Nov 15, 2015 #2

    BvU

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    [edit] hint: draw a few forces such as ##\ m_{\rm \, pole}g## , ##\ m_{\rm \, light}g## , cable tension in your diagram...
     
  4. Nov 15, 2015 #3

    SteamKing

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    Why is the distance AC being ignored?

    Ask yourself, what member is in equilibrium here? What would you show in a free body diagram for this problem?

    Looks like you're stuck if you picked D. And ignored AC.

    It's not clear what you are doing here. What is the reference for writing ΣT?

    There are two equations of equilibrium you must write: Σ F = 0 and Σ M = 0

    Why not tan 37? How did you eliminate that one?

    You should understand your trig better than this by now. Here's a reminder of what is what:

    http://demo.activemath.org/ActiveMath2/LeAM_calculusPics/TrigFunctionsTriangle.png?lang=en [Broken] ​
     
    Last edited by a moderator: May 7, 2017
  5. Nov 15, 2015 #4
    Sorry, I forgot to add tan.

    Also; ignoring AC because that's not on the question we were given, our teacher changed it up a little before handing it to us.

    The photo I posted was just the closest approximation to the one he's given us.
     
  6. Nov 15, 2015 #5

    BvU

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    So be clear and tell us what exercise you are working on...
     
  7. Nov 15, 2015 #6
    The image in the attachment in my original post is the exact question I'm doing.

    The information I I gave in my original type up of the question is what we're given.
     
  8. Nov 15, 2015 #7
    So, which has a length of 7.5 m, the aluminum pole AB or the length AD?

    When you say that ##r_1F_1## is the pole, what do you mean?

    Do you think the change your teacher made was to make the points D and B coincide?
     
  9. Nov 15, 2015 #8
    AB is the 7.5 m.

    And r1(f1) is the pole being (8)(9.81)cos37 (8 is mass of pole) and r is the distance from the center of mass to the pivot.
     
  10. Nov 15, 2015 #9

    SteamKing

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    I'm not sure you can ignore the distance AC and still solve this problem, at least not without knowing where point D is located.
     
  11. Nov 15, 2015 #10

    haruspex

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    There are no lengths in the attached image. In the OP you said to take AD as 7.5m, so is AB still 7.5m or is it more? If it's more, we need some way to find how much more.
     
  12. Nov 15, 2015 #11
    Sorry; I didn't proof read before I posted.

    AB is 7.5 m.

    We are given the weight of the pole, the stoplight, the angle of 37 degrees, the length of AB and that is all.
     
  13. Nov 15, 2015 #12
    And AD is also 7.5 m?
     
  14. Nov 15, 2015 #13
    We don't know AD.
     
  15. Nov 15, 2015 #14

    haruspex

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    Then there is still not enough information. You need to know both.
     
  16. Nov 15, 2015 #15
    The exact question is: An 11kg traffic light hangs from a structure as shown. The aluminium pole AB is 7.5m long and weighs 8 kg. Determine the tension in cable CD.

    He told us the angle CDA is 37.
     
  17. Nov 15, 2015 #16
    Are we still ignoring the AC height of 3.80 m? If so, then you have misunderstood what your teacher told you to do. I suggest you start over with just a statement of the problem you want us to help you solve, along with a single diagram that contains the correct information.
     
  18. Nov 15, 2015 #17

    haruspex

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    You say 'as shown', but the diagram attached looks like it might have been something you drew. Does it accurately represent the diagram given to you?
    My suspicion is that B and D are now the same point.
     
  19. Nov 15, 2015 #18
    "An 11 kg traffic light hangs from a structure as shown. The uniform aluminium pole AB is 7.5 m long and weighs 8kg. Determine the tension in the cable CD"
     

    Attached Files:

  20. Nov 15, 2015 #19
    And here; a photo of the question
     

    Attached Files:

  21. Nov 15, 2015 #20
    Can't be solved as stated. I suggest you make a new drawing with the points B and D coinciding. Then try to solve that.
     
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