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Need help finding tension in cables

  1. Mar 27, 2009 #1
    1. The problem statement, all variables and given/known data

    pysicssign.jpg
    2. Relevant equations



    3. The attempt at a solution
    I really have no ides where to start I assume you have to use moments to find the tension in each cable but im really not sure how to go at it.
    I think the 20 N force hanging from the sign would only act through the on cable and wouldnt effect the other cable because it is the same distance from the end as the support cable.
    1. The problem statement, all variables and given/known data

    Thanks for the help I have been trying to figure this out for a while
     
    Last edited: Mar 27, 2009
  2. jcsd
  3. Mar 27, 2009 #2

    PhanthomJay

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    Are you sure of those dimensions? That sign doesn't look stable.
     
  4. Mar 27, 2009 #3
    Yes those are the dimensions...its just really out of wack for the question obviously no one would build a sign like that
     
  5. Mar 27, 2009 #4

    PhanthomJay

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    But the scale is way off; the first support rod is way off to the right, almost right next to the other support rod. You don't show it that way. And both rods would not be in tension, with the dimensions shown.
     
  6. Mar 27, 2009 #5
    Sorry i didnt really mention that...It is out of a text book and it is not scale..dont know why so just ignore the scale
     
  7. Mar 27, 2009 #6

    PhanthomJay

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    Something then is amiss, because the question asks for the tension in each rod. The second rod at the right will not be in tension. I think the problem means to say
    a = 0.1 m and b = 1.0 m. You are correct in that the 20 N weight just gets carried by the right rod only. But the force value in that rod from the sign weight will far outnumber the 20 N force in that rod, and put it into compression. Try summing moments about the first rod, applying the sign's weight at the center of the sign.
     
  8. Mar 27, 2009 #7
    I know the question is right...If anyone can explain how to do it I would really appreciate it
     
  9. Mar 27, 2009 #8

    PhanthomJay

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    Unless I'm misreading your a and b dimensions, the question is improperly worded, or the dimensions given are wrong.
     
  10. Mar 27, 2009 #9
    This question is right out of a physics textbook
     
  11. Mar 27, 2009 #10

    PhanthomJay

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    And Physics books once in awhile make errors; their authors are human, you know.
     
  12. Apr 4, 2009 #11
    I had my teacher explain this question to me and I dont know why phantomJay thinks the second rod isn't under tension when it clearly is.. to find the tension you just have to select a point and find the moments around that point which will give the tension on on rod, you than subtract that from the total weight since the tension will equal the total weight and that gives you the tension on the second rod.

    So just to clarify it was not a typo
     
  13. Apr 4, 2009 #12

    nvn

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    Nanart: The right-hand, upper support rod is in compression. Perhaps if you show your work, we could locate your mistake.
     
  14. Apr 5, 2009 #13
    Nanart,

    If you are a practising engineer or physicist, you will find that it does help to sketch your problem reasonably to scale. Many mistakes (including those from textbooks) are due to improperly drawn and hence misleading sketches.

    You can sit there and insist that the question is straight from the textbook, but it's not going to help you. Textbooks make mistakes, we all know it.

    I suggest you make a sketch reasonably to scale, re-do your moment calculations and post the results, as nvn suggested. When you are convinced, show your teacher the sketch and the calculations, then you'd be getting somewhere.
     
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