# Homework Help: Poiseuille's law - Need help determining pressure difference

1. Jul 23, 2012

### Insidium

1. The problem statement, all variables and given/known data

The problem is if i have a barrel of water, and at the bottom there is a pipe sticking out going horizontally (leading to an open end), how do i determine the pressure difference?. I think im right in saying the pressure at the end of the pipe would be pressure from the atmosphere, but its the other end that confuses me

2. Relevant equations

f=(πPr⁡^4)/(8ηL)

3. The attempt at a solution

I think it could be found via finding the mass of the water in the barrel, then with the area of the pipe use p=f/A

2. Jul 23, 2012

### LawrenceC

The pressure difference between what two points?

By the way, the pressure at the bottom of the barrel does not depend on how much liquid is in the barrel. It only depends on the depth of the liquid.

3. Jul 23, 2012

### Staff: Mentor

It is reasonable to assume that the inlet pressure to the pipe is equal to the pressure at the bottom of the barrel. There is an entry effect from the barrel to the pipe, but this will be negligible if the pipe is long enough. As LawrenceC implied, the pressure at the bottom of the barrel is equal to the atmospheric pressure plus the static head ρgh, where h is the depth of liquid in the barrel.