# Math Help - An oldie

1. ## An oldie

Here's an oldie you might enjoy . . . or not.

Two 20-foot poles are erected perpendicular to the ground.

The ends of a 30-foot rope is attached to the tops of the poles.

The rope hangs naturally and its lowest point is 5 feet from the ground.

How far apart are the poles?
Code:
      *               *
|               |
| *           * |
|   *       *   |
20 |       *       | 20
|       :       |
|       :5      |
|       :       |
--*-------+-------*--
: - - - ? - - - :

NOTE: Ropes, cables, etc. do NOT hang in a parabolic shape.
. . . . . The shape is a catenary.

.

2. Originally Posted by Soroban
Here's an oldie you might enjoy . . . or not.

Two 20-foot poles are erected perpendicular to the ground.

The ends of a 30-foot rope is attached to the tops of the poles.

The rope hangs naturally and its lowest point is 5 feet from the ground.

How far apart are the poles?
Code:
      *               *
|               |
| *           * |
|   *       *   |
20 |       *       | 20
|       :       |
|       :5      |
|       :       |
--*-------+-------*--
: - - - ? - - - :
NOTE: Ropes, cables, etc. do NOT hang in a parabolic shape.
. . . . . The shape is a catenary.

.
Well I didn't know the answer but it took only ~5 seconds to deduce it.

RonL

3. Got it in two seconds before I've even finished reading the ques However, I wonder if the diameter of the poles should be taken into consideration :P

4. I think, in white -> 0

5. I didn't get it after about 4 minutes and I gave up! I leave a comment in white : I was thinking on the sketch... that is probably why I didn't get it. I could have sketch it on a sheet though. Nice one!

6. ## Re :

So what is the correct answer ?

So what is the correct answer ?
i believe Obsidantion has the right answer (post #4)

8. ## Re :

gotk it , the diagram brings confucion . haha , nice oldie

gotk it , the diagram brings confucion . haha , nice oldie
i agree. haha, i've tricked a lot of students with this since i saw it, because they all try to use the diagram to figure stuff out. and that whole explanation about catenaries (which i extended, of course) doesn't make things any easier.

10. Originally Posted by Soroban
Here's an oldie you might enjoy . . . or not.

Two 20-foot poles are erected perpendicular to the ground.

The ends of a 30-foot rope is attached to the tops of the poles.

The rope hangs naturally and its lowest point is 5 feet from the ground.

How far apart are the poles?
Code:
      *               *
|               |
| *           * |
|   *       *   |
20 |       *       | 20
|       :       |
|       :5      |
|       :       |
--*-------+-------*--
: - - - ? - - - :

NOTE: Ropes, cables, etc. do NOT hang in a parabolic shape.
. . . . . The shape is a catenary.