# How is flow work calculated?

I am reading thermodynamics from Cengel and Boles. I am currently studying flow work. The following image contains text of the book:

The work done is calculated by considering the pressure of upstream fluid but the downstream fluid also applies pressure on it,but that is not considered why? According to me,the work done should be (P2-P1)*V where P2 is pressure of fluid upstream and P1 is pressure downstream at the boundary of control volume.

• Are you sure Cengel and Boles don't get around to taking the downstream pressure into account if you read on for another page or two? Either way, remember that a given parcel of fluid is not guaranteed to have the same volume at exit that it had at entrance, so it's $P_2V_2-P_1V_1$, not $\left(P_2-P_1\right)V$. May 4, 2021 at 16:07