# Bird's book "Transport Phenomena"

• Classical

## Main Question or Discussion Point

in the book " transport phenomena" (Bird) when obtaining the famous equation of hagen poiseuille
Bird defines a variable that involves the pressure, but when doing an exercise a term magically disappeared, someone could explain to me what happened
why we took into account the pressure in this balance and in the previous exercise (thin film) we did not

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I have a 1960 edition (!) and found HP $$Q={\pi \left ( {\mathfrak P}_0 - {\mathfrak P}_L \right ) R^4\over 8\mu L}\qquad\qquad(2.3-19)$$
${\mathfrak P} = p - \rho gz$ and (2.3-10) is as in your picture.

What exercise are you referring to with • berkeman
I have a 1960 edition (!) and found HP $$Q={\pi \left ( {\mathfrak P}_0 - {\mathfrak P}_L \right ) R^4\over 8\mu L}\qquad\qquad(2.3-19)$$
${\mathfrak P} = p - \rho gz$ and (2.3-10) is as in your picture.

What exercise are you referring to with

View attachment 248629
In chapter 2 ,but in 2nd edition after balancing momentum in a pipe

The term that involves gravity ,what happens with that term?

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