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Conductor Tension Due to Birds Landing & Flying Away

  1. Oct 19, 2011 #1

    I have been wondering how a conductor's tension is affected by birds landing on it and then flying away. By itself, the conductor has some tension obviously due to its weight. When a bird lands on the conductor, the tension increases. But what about when the bird flies away? By flapping its wings prior to takeoff, the bird will cause the conductor's tension to increase. But when the bird actually flies away (and lets say its a fat bird or there are a lot of birds) and the conductor sort of bounces up and down, does the conductor tension ever go to zero or become negative??

    I appreciate any help! Thanks!
  2. jcsd
  3. Oct 19, 2011 #2
    Are you talking about the tension on a line between the two poles or some sort of internal tension between the strands?

    For a line between two poles the tension never goes to negative (compression?) because of a perpendicular force on the line.

    The tension should only depend on the displacement from equilibrium.
  4. Oct 21, 2011 #3
    I am referring to the tension between the two poles.

    Does the tension in the line ever go to zero? Because when birds fly away, the line sort of bounces up and down. I am wondering: when the line bounces up, does the tension go to zero? I suppose an analogous situation (?) would be when you toss a ball up and it reaches its apex and momentarily it is suspended in the air.

    My ultimate goal is to be able to imagine some graph of the tension of the conductor over time, as birds land on the line and fly away from the line.
  5. Oct 21, 2011 #4
    you mean decrease.

    And of course the weight of a typical bird is tiny compared with the weight of a typical power or phone wire.

    If the bird flaps first, then ascends, the speed of departure will determine if the wire oscillates up and down or not. A slow asent will allow the wire to return to its equilibrium position smoothly, a rapid ascent may enable the wire to bounce.....oscillate.
  6. Oct 22, 2011 #5
    I doubt very much that the tension ever goes to zero.

    Cables are much heavier than birds...even a bunch of them...in order to get the cable to truly be floating in between the two poles, I think it would need to stretched (like a slingshot) and then released..then it could be floating...otherwise, it is very much suspended at all times.

    ...the cable is not even that elastic and so, what you see as a seemingly floating section of cable is actually the crest of the wave...in other words, when the birds push away, they lower the portion they were standing on and raised the sections on the sides between them and the poles...

    ...when released, the cable will start going to its equilibrium position, the middle trying to go up, the side back down...and you have two symmetric waves, one traveling from the birds former location to the pole on the right and another one from the birds former location to the pole on the left...

    ...when the wave reaches the pole, part of it may refract to the next section after the pole, part of it will reflect back...

    ...until it dissipates.
  7. Oct 22, 2011 #6


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    Thanks for a very cool answer, Gsal.
    The other thing that I wanted to mention is that the question isn't quite specific enough to start with. There are a lot of different weight classes of birds, for one thing. A turkey vulture will probably impart more influence than a titmouse. There are also different launching methods. Some birds merely start flapping and ascend. Others (usually the heavier ones) kick off, which imparts a significant recoil effect to the departure point.
    As Gsal mentioned, though, that is still negligible compared to the tonne or more of conductor stretched between the towers. Also, the towers themselves will flex toward the load to some miniscule extent.
    Bottom line is that you should just ignore the birds. Either that, or sit yourself down with a knife and a fork and a salt shaker, and hope that one of the feathered bastards straddles two lines.
  8. Oct 22, 2011 #7
    Wow, great answers, all of them! I appreciate everyone's time. This situation is much clearer to me now.
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