There are 2 flagpoles that are each 100 foot high.  A rope that is 150 feet long is strung between the tops of the flagpoles.  At its lowest point the rope sags 25 feet about the ground (see schematic diagram below).  How far apart are the flagpoles?

As ever, please do NOT post your answers, but do say if you think you have solved the puzzle and how long it took.  Solution on Monday.

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

120 comments

  1. A more interesting question would be “what’s the relationship which describes the distance between the poles and the height of the rope above the ground?”

  2. 30 seconds. And BTW, it’s probably also worth mentioning as a part of the question that the curve is a Catenary and not a Parabola ;-). B. ‘Nary’ Narayanaswamy / New Delhi / India

  3. Wait…
    So I spend about five minutes looking at quadratic equations, getting nowhere, then look at the comments for a hint and see that people are getting it very quickly.
    Then I read the question again.
    That’s… rather silly, actually. One day I’ll learn that the more mathematical the Friday puzzle looks, the less it probably is.

  4. Tricky…

    I was thinking about my maths:
    – The shape of a hanging rope is that of a hyperbolic cosine.
    – Then derive the length out of it.

    Aaarghh…

    Then read again and thought… Right. Not that hard.

  5. Solved it, 5 min 10s. 5 minutes doing calculus, 10 seconds going, oh wait… and 10 minutes hitting my head against the desk.

  6. Read the puzzle. Got scared away by all the math to do. Came back 20mins later. Some scribbling. Still afraid, turn away. Third attempt, 10 seconds.

  7. The schematic threw me at first. I sort of got it straight away but it took me a minute or so before I realised I had the actual answer and that the puzzle wasn’t misstated.

  8. Hmmm, normally get these quite quickly. I do have a solution involving ratios, however I’m not sure that’s actually correct, especially as people are saying it requires no arithmetic. Hmmm.

  9. Even though there is a logical answer not requiring advanced maths, wouldn’t this answer assume that the fundamental laws of physics are violated?

  10. I lol’d after thinking the same as everyone else, but read through the comments going “I don’t get it… I don’t get it… I don’t – Oh.”

  11. I heard the puzzle on Car Talk, an American radio program, several months ago. It took me a couple of minutes while I was listening to the rest of the show and walking to work to get it.

    So… either a couple of minutes while busy with other things, negative several months, or as long as it took me to read enough of the question to verify I’d heard it before and all the particulars matched, say 10 seconds. Take your pick.

  12. Sneaky sneaky! Took me a minute of conceptual figuring on how to set up the math problem. Then facepalmed as I actually looked at the numbers.

  13. Not long – after the first reading, thought to myself “Hell, it’s been 20 years since I did a catenary curve problem”. Read the problem again and had the inevitable facepalm.

    Wonder how long it’ll be NOW before I even think the word “catenary”?

  14. This was pretty easy, and lack of mathematical skill was actually a benefit to me, I think. Instead of starting with some preconceived notion of how to solve the geometry, I just started assembling everything I knew and could easily discern from the information available, and the answer immediately became obvious. Then I spent some time trying to figure out how I must be wrong.

    1. You do kind of have to use mathematics, it’s just that the mathematics involved are facepalmingly simple once you realise!

  15. I started out thinking about how to do an integral in calculus and then started thinking about how I could use a Newtonian approximation before thinking about how I could just do a linear approximation and then the solution became very very obvious. I might have overthought this one.

    1. Actually, the solution to the generalized problem (where the shortcut is no longer applicable) can be written in algebraically closed form (no approximation or iteration required).

  16. Like several others I started out trying to work out how to work out the solution. At first I tried using an approximation to get some rough numbers. That’s when I realized my rough answer was the real answer. Total time about 5 minutes.

  17. Read the question, thought about picking up my old textbook in maths, thought about how hard (impossible?) this would be for people who have not done maths at this level. Read the question again trying to find an answer wich did not require any skills at math. Got it in about 15s total, while beeing slightly intoxicated. (guess I fail at life for spending 5 min writing a message online on a friday night while intoxicated)

  18. About a minute, but I too had to double back after the first routine approach, as so many else here.

    Alas, same story as last week, sigh. The intent is clear, but there’s trouble if you take the words and the picture at face value. I’m on the fence if this makes the puzzle more engaging or tedious.

  19. Another facepalm here!

    Ten minutes of digging out catenary equations.

    45 seconds of approximating trigonometric distance from top of pole to bottom of curve.

    A couple of seconds of revelation! :~O

    1. If you’re getting that picky, you’d also need to know its bending modulus, the temperature, and the radii of the poles!

      There’s a “right” (as in, “close enough”, conceptual), SIMPLE answer that doesn’t require that level of sophistication.

    2. … You’d also need to know the precise manner of fastening employed at the top of the pole, not to mention the radius of the planet you’re on.

    3. Not really, you know: while in a homogenous gravitational field all catenaries are similar to each other, I doubt that’s still the case in a radial field. Further, we need to know the azimuth of the line connecting the flagpoles, their latitudes, and the day length, all to account for the influence of centripetal forces. Shame that this of course makes symmetry fly out the window (in a strictly Newtonian manner I’m sure).

    4. Well, you raise an interesting point: things are already complicated enough when considering just the Newtonian effects, but what about relativistic ones? We’d need to know not only the precise planetary gravity gradient, but also the distance from the primary and its characteristics, as well as those of any other large masses in the system. Then there are quantum effects, and don’t forget Brownian motion of the atmosphere!

      Well, folks, that does it: the problem’s CLEARLY unsolvable.

      🙂

  20. I got the answer after 1/8640 days, but needed an extra 1/17280 days for converting “feet” into metric units.
    Best regards from the continent.

  21. I started reading the comments before I thought clearly about the problem, and was happy to read that I didn’t need to use catenaries coz I’ve never heard of them.
    I think I’ve got the answer without facepalming too, though I did a bit of face slapping before I got it.

  22. Thought it would involve parabola! Just did some maths, then thought parabola could also be ellipse. Solved for 5-10 minutes. Read comments, people reported 15 seconds solved! I admit to doing something complex. And there was the answer – 5 seconds.

  23. Thanks for the puzzle Danil. I also agree that as a math student, every neuron was firing off, making the problem more difficult and quickly I wanted to dismiss degenerate answers.

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