Bird's book "Transport Phenomena"

[Est120]

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

watch this

 

[BvU]

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

 

[Est120]

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

 

[Est120]

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

 

[BvU]

we took into account the pressure in this balance and in the previous exercise (thin film) we did not

Didn't find a thin film exercise -- but it will likely be vertical flow. Perhaps yours is horizontal ? Still don't know what exercise you are referring to. Can you post the complete problem statement for the example you are asking about or do you want me to keep sleuthing ?