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Mathematics Aptitude Survey

  1. Feb 4, 2009 #1
    I'm collecting some data for school and need data from as many people as possible. Thanks!

    Who can solve this problem in one try without looking at the answer?

    A fool wants to tie a rope around the earth. So he buys a rope of 40,000 KM and ties it around the world. His neighbour, also a fool, wants to do the same only he wants the rope on sticks 1 meter above the ground.

    How much more rope does he need?

    And how much more rope do you need when you use a tennis ball instead of the earth?

    Be honest! And be sure to leave a post saying whether or not you get it on the first try!

    The answer can be found at http://mindcipher.com/puzzle/42-don-t-hang-yourself

    Thanks for your help!
  2. jcsd
  3. Feb 4, 2009 #2
    If the circumference of the earth is 40,000 then the radius is [itex]\frac{20000}{\pi}[/itex]. So the circumference in the second instance is [itex]2 \pi \left(\frac{20000}{\pi} + \frac{1}{1000}\right) = 40000 + \frac{\pi}{500}[/itex]. Obviously a tennis ball would be the same thing. That was my first try.
  4. Feb 5, 2009 #3
    He needs an extra [itex]2\pi[/itex] metres of rope, regardless of the radius of the object it was wrapped around.

    First try, although I have seen this problem before. (I got it the first try then as well.)

    I believe that even if you wrap the rope around a non-circular object (with reasonable restrictions; using a convex object will suffice), you would still have to add [itex]2\pi[/itex] metres to make each point on the rope 1 metre from the object.
  5. Feb 5, 2009 #4


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    We actually had a discussion on this after a math society meeting (I've seen the question before and answered it on my first try for the record). 9 out of 10 university students were able to get the answer right on the first try, the 10th took 10 minutes of explaining because "your notation is stupid"
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