• 0

The Classic Moat Problem

Question

Posted · Report post

this is a classic problem so i was surprised that i could not find it in the forum searches but of course that is not to say that it isn't there :thumbsup: .

Problem:
Suppose you have two unit-length boards. What is the widest moat you can cross if you have no means to nail or otherwise attach them together?
0

Share this post


Link to post
Share on other sites

6 answers to this question

  • 0

Posted · Report post

Assuming there's a 90-degree corner, then the widest moat that's possible to cross is slightly smaller than sqrt(5) / 2.



Reasoning:
Lay one board at a 45-degree angle across the outer corner of the moat, and then rest the other board perpendicular to the first such that it lands on the corner on the inside of the moat.

This gives you a triangle with legs 1 and 1/2, which has a hypotenuse (corresponding to the moat width) of sqrt(5) / 2. The actual width has to be a little less than that, since you lose some length of the board in the process of bearing on the ground and on the other board.

I'm not really sure how to post a quick MS Paint image to make it clearer...

0

Share this post


Link to post
Share on other sites
  • 0

Posted · Report post

Assuming there's a 90-degree corner, then the widest moat that's possible to cross is slightly smaller than sqrt(5) / 2.

Reasoning:

Lay one board at a 45-degree angle across the outer corner of the moat, and then rest the other board perpendicular to the first such that it lands on the corner on the inside of the moat.

This gives you a triangle with legs 1 and 1/2, which has a hypotenuse (corresponding to the moat width) of sqrt(5) / 2. The actual width has to be a little less than that, since you lose some length of the board in the process of bearing on the ground and on the other board.

I'm not really sure how to post a quick MS Paint image to make it clearer...

Here's what you're describing, I believe...

post-13141-0-88569700-1374154796_thumb.p

0

Share this post


Link to post
Share on other sites
  • 0

Posted · Report post

With that setup, I don't think you can call the width of the moat equal to the hypotenuse of the triangle formed by half of the first board plus all of the second board. A line from one end of the first board to the end of the second board that's touching the opposite shore would not be perpendicular to the moat. You would need to solve for the length that's perpendicular to the moat in order to find out how wide of a moat you could cross.

0

Share this post


Link to post
Share on other sites
  • 0

Posted · Report post

What shape is the moat? Does it have to be a straight line? Can it vary in width?

If it can be any shape, just choose a moat in the shape of a very pointy arrow and cross over at the point using 1 board.

0

Share this post


Link to post
Share on other sites
  • 0

Posted · Report post

What shape is the moat? Does it have to be a straight line? Can it vary in width?

If it can be any shape, just choose a moat in the shape of a very pointy arrow and cross over at the point using 1 board.

What shape is the moat? Does it have to be a straight line? Can it vary in width?

If it can be any shape, just choose a moat in the shape of a very pointy arrow and cross over at the point using 1 board.

What shape is the moat? Does it have to be a straight line? Can it vary in width?

If it can be any shape, just choose a moat in the shape of a very pointy arrow and cross over at the point using 1 board.

The question isn't whether you CAN cross the moat. The question is, what is the widest the moat can be to make it across.

0

Share this post


Link to post
Share on other sites
  • 0

Posted · Report post

Rookie's classic (with some assumptions not made in the OP).

If the moat has a square corner,

then two planks of length x

will allow you to cross a moat of width 1.5x/sqrt(2).

0

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.