Why does overhead cables look saging at a long distance???

Answer 1

It's called foreshortening. Get a piece of paper, draw a very slightly curved line, and then pick up the paper and look at the curved line at an angle...it will look like a "u".

Answer 2

The cables must sag in order to support their own weight without breaking. The angle of the sag is what supplies the vertical force to counter the weight. The smaller you try to make that angle, the more tension you need in the cable, until, finally, you exceed the strength of the cable.

Answer 3

All extended objects sag under the force of gravity. Even objects that seem solid like wooden poles, the bending is just less pronounced. The shape they take is a well known mathematical curve called a catenary (essentially a hyperbolic cosine). Here's the wikipedia article:

http://en.wikipedia.org/wiki/Catenary