1. The problem statement, all variables and given/known data 2. Relevant equations Navier-Stokes 3. The attempt at a solution Not really trying to solve a problem, trying to understand what is going on in my textbook. So look at the stuff in red first. I see where all that is coming from, it's clear to me. However, the stuff in green indicates that in the example case the right side of the equation should be zero, instead they throw in the stuff in blue out of the blue. What exactly happened here that they totally missed to explain?!