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Hello, good people - Flagpole rigidity questions

  1. Nov 24, 2012 #1
    Hello, good people -- Flagpole rigidity questions

    Tis about my finals answers :) I just want to know if I'm right or not.

    I was required to determine features of a flag pole that would enable it to withstand strong winds.

    Among them are:
    *String tension of strings attached to the flag pole. I answered high, naturally.

    *Density of the pole. I answered high.

    Reasoning: Higher density would equal more weight, so more downward force. Therefore it would need winds of stronger force to topple it.

    Flexibility of the pole. I answered high.

    Reasoning: If th pole has a high degree of flex, when exposed to strong winds, it will bend at the top, then return to being upright. A rigid pole will lean in its entirety and topple.
     
    Last edited: Nov 24, 2012
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  3. Nov 24, 2012 #2

    berkeman

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    Re: Hello, good people -- Flagpole rigidity questions

    Welcome to the PF.

    Was there a diagram that accompanied this question? What strings? Flagpoles do not normally have "guy wires", so what string tension are they referring to?

    And density alone does not necessarily imply strength. It's more about density distribution. Can you please post the exact question with any supporting diagrams?
     
  4. Nov 24, 2012 #3
    Re: Hello, good people -- Flagpole rigidity questions

    Unfortunately the paper has to be submitted at the end of the exam.

    I can try and draw a similar diagram though :)

    I assume they mean how much force the string can handle due to wind before snapping.

    The question was not specific about density distribution. It just wanted to know whether the density of the pole should be high or low.

    I'm guessing the density should be greater at the bottom then?

    But assuming the whole pole is made up of the same material, technically wouldn't the density be constant in all regions?
     

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  5. Nov 25, 2012 #4

    haruspex

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    Re: Hello, good people -- Flagpole rigidity questions

    Without guys I would have agreed with flexible, but I feel the presence of guys changes that. All the pole is left to do is to cope with compression, and flexing will make that harder.
     
  6. Nov 25, 2012 #5
    Re: Hello, good people -- Flagpole rigidity questions


    Would you mind elaborating, please?
     
  7. Nov 25, 2012 #6

    haruspex

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    Re: Hello, good people -- Flagpole rigidity questions

    A free-standing pole with horizontal load is subject to tension on the windward side and compression on the leeward. Flexing allows it be oriented less square-on to the wind, reducing the load.
    A guyed pole cannot reduce the load that way. Since the guys provide tension, there is increased compression in the pole, maybe compression on all sides now. Flexing will mean buckling, which increases the moment about the flex point. Think of standing on a rubber column. As long as the column stays straight, and just bulges a bit, you may be ok, but once it starts to fold in the middle you're gone.
     
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