Hanging cable between two poles

MrChips

Joined Oct 2, 2009
24,232
Oh, I know what a catenary is. And I studied those shapes in college, and the elastic properties of cables when they're hung. But the answer to the problem is much simpler than it appears to be.
Well I am stumped. I don't see a simple solution.
 

strantor

Joined Oct 3, 2010
5,646
Well I am stumped. I don't see a simple solution.
Me too. I see graduations along the ground line that appear to be about the same interval as the 10m given for the height of droop above ground, so I would say 80m except the cable is 80m so that can't be right and the scale of the drawing is way off. I don't see the easy answer either.
 

strantor

Joined Oct 3, 2010
5,646
Ok well I've resorted to editing the image to confirm the scale and the dimensions given (both Y-xis values) jive with each other, so as long as they scale equally in X & Y then I'm sticking with my original answer of 80m.
cable scale.png
That means either:
  1. the question is flawed
  2. The scaling in X & Y are not the same
  3. Whoever wrote it thought they were very clever and the "cable" is actually the line at the bottom, not the catenary curve.
  4. There is still something I'm missing
In the case of #3, I don't think it's very clever at all. Intentionally misleading to the point of being actually wrong.
 

MrChips

Joined Oct 2, 2009
24,232
It is a trick question because they drew the drawing to throw heavy thinkers like us into doing the math.

Make the poles 40m tall and you can see that it is a different problem.
Or make the droop 20m above the ground.
Or make the cable longer than 80m, say 100m.
 
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