I posted this picture of someone on a zipline on Facebook.

![enter image description here][1]

One of my friends saw it and asked [this question](http://physics.stackexchange.com/q/63893/24233), so he could try to calculate the speed at which someone on the zipline would be going when they hit the water.

The only answer, which was accepted, includes the disclaimer, "*Assuming the pulley being used to slide to be friction less.Though not possible.Also the rope is assumed to be in-extensible and straight.*"

I used to have a zipline of about the same length in my back yard as a kid and even when I was young, I noticed that we could never straighten the line completely, even when it was slack, we could not make it completely straight.  And, naturally, once weight was added, there was a curve where the weight pulled the line down.

One of the comments from the member providing the answer is "*Well i can show you why the string cannot be ever straight.*"  I know that from experience.  We could never make it completely straight with no sagging.  I asked the reason for this and was directed to a book on Amazon.  Having just spend $50 on a number of books for summer reading, my book budget is gone for a while.

So can someone answer that?  Why will the line never be straight when it's set up (and when there is no load on it)?

  [1]: https://i.sstatic.net/uazFd.jpg