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Thread: Hanging Cable Test

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    Default Hanging Cable Test

    A cable is hanging from the top of two poles that are both 50 m above the ground.
    What is the distance (X) between the two poles, to one decimal place, if the centre of the cable is 10 m above the ground?

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    What is the length of the cable??

    Obviously it can't be 80m because that would make the distance between the poles zero (actually they would have to overlap by the width of the pole, assuming the cables are attached at the centre of each pole).

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    32m by my reasoning : poles minus bottom of cable = 40m: 40x40 =1600sqroot =16m x2=32m
    Last edited by hinekadon; 09-08-18 at 12:03 AM. Reason: more

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    What's the tension on the cable? (Max tension determined by cable construction)
    Factors like load affecting cable temp rise above ambient (I^2.R), wind speed (cooling factor) all come into it as well when estimating the worst case sag (hottest day with max loading and no wind will be the worst case).

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    Quote Originally Posted by Skepticist View Post
    What's the tension on the cable? (Max tension determined by cable construction)
    Factors like load affecting cable temp rise above ambient (I^2.R), wind speed (cooling factor) all come into it as well when estimating the worst case sag (hottest day with max loading and no wind will be the worst case).
    Yup, just ask Powercor on those 40 degree days.
    Regional distribution lines sag heaps on a hot day!

    But as sag has been defined.
    The answer can be worked out
    Last edited by ol' boy; 09-08-18 at 09:26 AM.
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    its two right angle triangles the cable has no bearing on shape the points are ten m line : is bottom of pole of the ground 10 meters higher than given height slope to center is half the distance between poled the shape of the cable dont matter as that point has to be in the middle : the hypotenuse therefore by pythagarus the sum of the square etc : if you dont know the rest and are over 70 you are forgiven : if younger you had a miss spent youth lolo therefore add the two baselines together = total distance between poles

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    Due to my total lack of understanding of algebra and geometry, I would use the markings on the cable to show how much is used between the poles.
    As for the distance above the ground, I need to go think about that.

    Talk about sag, one warm summers day I watched the 13mm (1/2 inch) solid copper centenary wire used for by the Railway to supply power via the Photograph for an electric train, wobble about like a lump of soggy celery......
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    @hinekadon are you assuming that the picture is to scale or something, I don't see that mentioned anywhere?

    Yes you have a pair of right-angle triangles, but you only know one angle (90°) and the length of one side (40m), how are you determining the other two angles / sides??

    The cable could be 100m long...



    ...or it could be 10km long...

    Last edited by Bigfella237; 09-08-18 at 01:49 PM.

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    P.S. The square root of 1600 is 40 not 16... you actually squared 40 to get 1600, you can't root it again and get a different number??

    Quote Originally Posted by hinekadon View Post
    32m by my reasoning : poles minus bottom of cable = 40m: 40x40 =1600sqroot =16m x2=32m
    I just noticed the OP has "Thanked" my first reply, but not answered my question, are we to take it that the answer is supposed to be zero?

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    The cable is 80m in total length.

    The hanging cable on each side is 40M, so if it's hanging from the top of a 50m pole, it will be 10 metres above the ground when dropped vertically.

    Given this happens equally on both sides, the 80m cable is exactly folded in half, meaning the distance between the poles must be zero, or as bigfella said, the poles must overlap a bit, given the mounting point of the cables is in the centre of the pole.

    The diameter of the poles is not stated, but the simplified answer would have to be zero distance between the poles.

    The diagram is misleading, no doubt intentionally, as there's no way a 40m length of cable suspended from a 50m pole could be moved horizontally away from the vertical and still end up being 10m above the ground.

    I guess that's the illusion.... the diagram you are seeing is different to the figures quoted, as those figures can't possibly equate to anything that resembles that pic. (Eg: the poles can't be separated)

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    yep i can be forgiven as im over seventy ? glad to see there are still some thinkers on the forum and not just gassers lol. thought i might arouse some thoughts :if you go up 40m then come down 40m you end up at the same place thanks tvguy i enjoyed that while it lasted hehehe

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    If this is one of those weird Maths questions asking for the hypotenuses of a three legged duck flying South on a Tuesday, then ignore me but years ago I dug 2x 750 mm holes in the yard some 20 metres apart and strung a line between them.
    Each pole was about 3.5 metres so after taking away the 750mm in the ground, each pole was approximately 2.9 metres tall.
    The clothes line was attached to the pole some 100 mm from the very top now making the line 2.8m above the ground.

    The line was tensioned as it was to hang clothes on so now to ask, how much dip would occur in a 4 strand Galvanised wire line at 30C on a full sunny day after 4 hours at that temperature?

    I am trying to avoid my Dhoti dangling in the dirt as there's nothing worse than a dirty Dhoti.
    Last edited by gordon_s1942; 09-08-18 at 04:07 PM.
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    hang up your burka first and see if the wife gets dirty an then you will have your answer

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    I had a quick look earlier and thought the 40 each side must be referring to the 40 metres between the vertex of the parabola and the top of the poles. If this was the case, of course, a little algebra would be needed to see how far apart the poles needed to be. But I didn't think it through any further than that. When I checked in a short while ago and saw Bigfella's first post and looked at the diagram again I thought okay, I was on the right track but no cigar, the answer is indeed zero. My next thought was that this must come from somewhere. And it did. And the answer is indeed zero. @Bigfella. Well done mate!

    Last edited by DB44; 09-08-18 at 05:08 PM.

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    Just picked up on this and the first thought is to look at the question correctly.

    Assuming that it is a maths question rather than an engineering question. And a trick maths question at that, then the problem should be solvable with some logical reasoning and basic maths without a calculator.

    If we are to assume the cable length is correctly labeled and that it is 40 metres from each end to the center and the center of the cable hangs 10m above the ground strung from the top of 50m tall towers, then the distance between the towers is zero.
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    Well here's me thinking the 40m was the sag and dreading getting into the numbers - been a long time since I did any sub-transmission long span designs and back then we did it all without 'puters too.

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    If we're playing maths problems.... I will torture you all with this puzzle.
    Be sure to show you're working and not your cut and past abilities

    Last edited by trash; 10-08-18 at 12:18 AM.
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    Pie x D (15) / 3
    But the circles borders dont overlap.

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    Quote Originally Posted by gordon_s1942 View Post
    If this is one of those weird Maths questions asking for the hypotenuses of a three legged duck flying South on a Tuesday, then ignore me but years ago I dug 2x 750 mm holes in the yard some 20 metres apart and strung a line between them.
    Each pole was about 3.5 metres so after taking away the 750mm in the ground, each pole was approximately 2.9 metres tall.
    The clothes line was attached to the pole some 100 mm from the very top now making the line 2.8m above the ground.

    The line was tensioned as it was to hang clothes on so now to ask, how much dip would occur in a 4 strand Galvanised wire line at 30C on a full sunny day after 4 hours at that temperature?

    I am trying to avoid my Dhoti dangling in the dirt as there's nothing worse than a dirty Dhoti.
    Most people wouldnt know what a "clothes line" was these days. Dip was decided by how long and how far the stick in the middle was set ?

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    circular proportioning into 15 we have two circles therefore two at seven and a half therefore there can only be the next step left so the answer has to be 5 meters which is a third of total diameter

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