Hi everyone and thank you in advance. My first problem is: if v1=constan in a section S1, it should not be ∑ F = 0 ? Or maybe Σ M = 0 ? Because all forces that i recognize are gravity and pressure, but i never found the equation mg - PS = 0 or something similar. I was thinking about something like F = d/dt (p) = (dm/dt * v + dv/dt*m) = (dm/dt * v + 0 ) = dm/dt * v but i don't know. Can someone help me please? I'm going crazy.