What is the Origin of this Equation?

[ricky_fusion]

Hi,
Anyone have a literature that describe this equation:
[PLAIN]http://i588.photobucket.com/albums/ss326/ricky_fusion/P_3.jpg[/PLAIN]

I need to know how where is it coming from..
Thanks a lot by your helpfull.

 

[ricky_fusion]

Btw,

I want to give more information about the problem,
It's a pipe that position in horizontal direction

I don't have any idea about how it come from..
I am sorry if i put the question in wrong criteria, I just guest it's same as physics.
I am waiting any information or literature about this formula.
Thanks a lot for your helpfull

 

[Cyrus]

Try R.C. Hibbeler Mechanics of materials or statics.