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The Ladder & the Box

  1. May 5, 2006 #1
    A classic problem - one of the highest ever on

    diffuculty solving / describing simple problem


    lets see how good you are :

    A ladder 4m long , is leaning against a wall in such a way that it just touches a box, 1m by 1m.

    How high is the top of the ladder above the floor ?
  2. jcsd
  3. May 5, 2006 #2


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    It's not difficult at all, just tedious. You get a quartic that can be solved using one of the methods detailed in the tutorial I linked : https://www.physicsforums.com/showthread.php?t=119284
  4. May 5, 2006 #3
    Maybe tedious, but it's still intriguing !

    Let's see the detailed solution...
  5. May 5, 2006 #4


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    I'm sorry, but I'm not a masochist. :biggrin: I know it can be done, I know how to do it algebraically, and I know how to estimate it numerically. That's good enough for me.

    If you'd like to edify others, you could work the thing out in full. :smile:
  6. May 5, 2006 #5


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    There is not enough information given to complete the solution.

    At least, there isn't unless we make some huge assumptions - ones that you really need to state in the problem.

    See attached diagram for arrangements that meet the criteria as specified in the problem.
    Last edited: Jul 20, 2007
  7. May 5, 2006 #6


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    On the other hand, the problem also didn't state that it had a unique solution.

    Thus, my answer:

    The top of the ladder above the floor can range between 0m and 5m.

    Where's my prize!
    Last edited: May 5, 2006
  8. May 5, 2006 #7
    i'd prefer to see the "brute-force" solution

    i've only got the lateral-thinking "similar triangles" solution in my book :

    i want to see "brute-force" !
  9. May 5, 2006 #8
    listen boyz

    it has a unique solution :

    ~ 3.76m

    now show us this algebraically !!!
  10. May 5, 2006 #9


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    Not unless you restate the problem...
  11. May 5, 2006 #10


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    Similar triangles gives a quartic in one of the dimensions. The form of the quartic depends on which length you take.

    The quartic is reducible to a quadratic with some manipulation.

    There is no unique answer because it's not stipulated whether the ladder is higher up the wall or farther up the wall (mutually exclusive conditions). Therefore the answer can be 1.362 meters OR 3.761 meters. The exact answer is [tex]h = \frac{4}{2 \pm \sqrt{5 - \sqrt{17}}}[/tex].

    (1.362^2 + 3.761^2 = 4^2, this is the obvious symmetry).
    Last edited: May 6, 2006
  12. May 6, 2006 #11

    matt grime

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    Please can I ask that you stop posting these 'classical' puzzles. I don't think that this is the correct arena for doing that. At least use the math Q and A game thread that was set up for this very purpose.
  13. May 6, 2006 #12
    if you are the moderator of this forum, i humbly apologise & will do so

    if you are not the moderator, then i suggest you mind your own business
  14. May 6, 2006 #13

    matt grime

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    No, I am not a moderator, it was just a polite request. (Prefaced by the word 'please'.) And I did point you in the direction of a stickied thread that would almost serve you perfectly before you start a new post every well known puzzle from a Martin Gardner book, or whereever these are from. You're more than welcome to ask a moderator to berate me for overstepping the mark (they're the ones indicated as moderators on the forum list page next to the forum name). There really ought to be a puzzles section or something for this kind of thing.
    Last edited: May 6, 2006
  15. May 6, 2006 #14
    then i suggest you do mind your own business

    when you become a moderator, then tell me what to do

    this is not from a gardner book ( he is NOT the the only puzzle setter in the world )

    the only puzzles i post are the ones that i found extremely stimulating & intellectualy challenging
  16. May 6, 2006 #15


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    Dude, why all the hostility ? It's not good for the heart, and you should know, being a specialist in that organ. :smile:

    Matt isn't saying don't post puzzles, he's just saying this is not the best place for it. And he's right - there's a complete section devoted to Brain Teasers and another for specific Math challenges (which is the thread he referred you to). This forum is more for "genuine" problems that are unrelated to homework - in the sense that the poster does not know the answer or needs some help or someone wants to discuss an important open question. Not for well-worn puzzles.
  17. May 6, 2006 #16
    point taken - & i will do so in future

    back to the puzzle - i'm not sure if you can have 2 solutions - the answer in the book is 2.76m : the alternative answer of 1.36m looks intuitively difficult as the box is already 1m high & for the "small" angle involved between ladder & box, it woud seem that the ladder woud have to be a lot longer than 4m to satisfy the criteria ( i.e. touch the ground ) ?
  18. May 6, 2006 #17


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    Sure it's possible, just rotate the picture 90 degrees. Either answer is possible as long as there's enough friction between the ladder and the floor to hold that position.
  19. May 6, 2006 #18


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    When the top of the ladder is 3.76... high on the wall, how far from the wall is the base of the ladder?
  20. May 6, 2006 #19


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    i got this system of equations:
    where h is the height, x is the horizontal distance from the wall, and y is part of length of the ladder.
  21. Dec 28, 2008 #20
    3.76090563295442m (1.36219999266324m from base of the wall) or 1.36219999266324m (3.76090563295442m from base of the wall) :)

    I wrote some software to calculate it, all variables and angles (Attached), the software errors if you give it impossible numbers (Only very basic error handlers).

    If anyone is interested in a mind-numbingly simple explaination, I'm willing to elaborate :)

    Attached Files:

    Last edited: Dec 28, 2008
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