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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Hi,

You may be familiar with this puzzle.

Consider a regular polygon with n sides, and side length 'a'.

There's an ant at each vertex. Every ant moves in one direction(all left or all right) towards the nearest ant in that direction, all at the same,constant speed. They move until they meet. What is the distance traveled by each ant?

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense" - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 95,983

Hi gAr;

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Hi bobbym,

Thanks for the link.

I knew of the formula, and but cannot recall the site from where I read. That site too only cited the formula.

I tried hard to derive the formula, but couldn't. Then I posted it here!

I'm hoping someone may work it out here

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense" - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 95,983

It is a pursuit curve. But I am having some trouble with it.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Ok, thanks for letting me know the name.

I'll search for some materials related to it. It was difficult without knowing the keywords!

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense" - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Hi,

There is a book dedicated to this topic.

"Chases and escapes: the mathematics of pursuit and evasion"

By Paul J. Nahin

You may check few pages in google books

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 95,983

Hi gAr;

I believe I have the book in my closet under a ton of others. I only went through a little of it.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Wow! You seem to be having a good collection of books.

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 95,983

Best in the city! Now getting through them is another matter.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Do you have a library at home or do you live in a library?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 95,983

Sometimes it seems like both.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**123ronnie321****Member**- Registered: 2010-09-28
- Posts: 128

Hi gAr,

Consider three ants at the end of an equilateral triangle of side length = s moving towards each other at constant speed = v.

As the ants move towards each other, points joining the three ants will be an equilateral triangle of a smaller side.(by symmetry)

As time passes the side of this equilateral triangle decreases.

If you choose any of the ant as your frame of reference, the component of velocity of the other ants towards the fixed (frame) ant

will be v + v/2. where v is the speed of the ant. [because velocity of A wrt. B is equal to velocity of A wrt. F + velocity of F wrt. B].

This means that the fixed ant will see the others coming closer to it at speed = 3/2v.

And in the ant frame the other ants travel towards the fixed one in a straight line. Therefore distance traveled(in ant frame) = s.

Time for which the ants move is = T = dist/speed = s/(3v/2).

In the ground frame they move with speed v for a time = T. Therefore distance traveled by the ants = vT = (2/3)s

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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Hi 123ronnie321,

Thanks for the explanation.

Did you check: http://mathworld.wolfram.com/MiceProblem.html

Actually, I wanted to know whether there's any easy derivation for a regular polygon with n sides.

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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**123ronnie321****Member**- Registered: 2010-09-28
- Posts: 128

That same logic can be applied to an n sided polygon. I have not seen any shorter method for this puzzle.

The above solution is similar to that given in the book 'Concepts of Physics' by H.C. Verma. Have you seen that book?

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**gAr****Member**- Registered: 2011-01-09
- Posts: 3,479

Hi,

Okay.

No, I haven't seen that book. I'll check.

Thank you.

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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