I am stuck on the questions, 5.3.10 and 5.3.11 which are part of a chapter on dimensional analysis in fluid mechanics by Streeter. Question 5.3.10 (see also the attachment): It asks me for the given fluid dynamic problems, where is the inertial force not important. It lists several fluid dynamics situations including spillway crest, open channel transition, waves against a sea wall, capillary tube and flow through a half opened valve. Solution? I generally understand the inertial force as the resultant force acting on a specific mass, due to applied forces such as pressure, gravity and viscosity. So I don't really understand how the resultant force can ever be "unimportant". The book also mentions that the Froude number is important for free surface flows, but this is not clear to me. I don't understand why is gravity important for free surface flows? Question 5.3.11 (see also the attachment): A similar question, asking me to identify which forces are important for laminar flow between closely spaced parallel plates. The forces are inertial, viscous, pressure and gravity. Solution? For this I suspect viscous force are important, because of the small, but very large gradients which may be created due to a boundary layer on the edge of each plate, not sure what else though? There aren't any solutions provided for these questions.