# Hanging cable between two poles

#### cmartinez

Joined Jan 17, 2007
8,086

#### AlbertHall

Joined Jun 4, 2014
12,267
Are all the interview questions that easy?

#### cmartinez

Joined Jan 17, 2007
8,086
Are all the interview questions that easy?
It depends... I'd probably feel intimidated by the question if my application was for a job in the HR department...

#### MrChips

Joined Oct 2, 2009
29,187
This ought to be in the Math forum.

#### cmartinez

Joined Jan 17, 2007
8,086
This ought to be in the Math forum.
Feel free to move (or copy) that post to the place you consider more appropriate.

#### cmartinez

Joined Jan 17, 2007
8,086
The shape of the curve is called a catenary and that is a hyperbolic cosine function.

https://en.wikipedia.org/wiki/Catenary
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.

#### MrChips

Joined Oct 2, 2009
29,187
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
6,685
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.

#### cmartinez

Joined Jan 17, 2007
8,086
Let me know when you guys run out of patience...

#### MrChips

Joined Oct 2, 2009
29,187
Ok, I see it now. Trick question.

#### atferrari

Joined Jan 6, 2004
4,698
I managed to remember the answer. So frustratingly simple...!

#### strantor

Joined Oct 3, 2010
6,685
Do you need to know anything about trigonometry or catenary curves to answer it?

#### hrs

Joined Jun 13, 2014
387
edit: nevermind, all wrong.
I see it now ...

Last edited:

#### crutschow

Joined Mar 14, 2008
32,840
The light finally lit.
You just have to think about the given values for a bit.

#### MrChips

Joined Oct 2, 2009
29,187

#### Delta Prime

Joined Nov 15, 2019
1,311
Need a bigger hint!
Pleasssse
Do I need a isometric ellipse protractor?

#### strantor

Joined Oct 3, 2010
6,685
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.

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.

#### AlbertHall

Joined Jun 4, 2014
12,267
Do you need to know anything about trigonometry or catenary curves to answer it?
No.

#### MrChips

Joined Oct 2, 2009
29,187
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.