1. The problem statement, all variables and given/known data I am reading an introductory book on fluid mechanics (freely available on the WWW) and I came across this on page 10: I don't understand the underlined part. We have a force-vector πx, which we decompose to find the overlap with some surface element dS. Shouldn't the force entering dS be directed along dS, and not πx as the author claims? Thanks in advance.