We just got our fluid mechanics quizzes back today and I was unpleasantly surprised with a 13/20 when I was expecting close to a perfect score. I was counted off a full 6 points (30%) for essentially using the wrong variable because in my rushed state I didn't read the problem carefully enough... Basically the problem had a picture of an intake plenum from what would be on your typical 4 cylinder car. It mentioned that water (that's the problem, I was assuming that AIR would be going through an AIR plenum) was going in at the inlet point 1, and exiting at points 2,3,4,and 5 at rates that decreased by .85 from each outlet forward (ie, mfr3=(.85)mfr2, mfr4=(.85)mfr3, etc.). We were to find the flowrate at port 1 (ie point 2). My problem, is that I wrote the assumption that the cross sectional areas A2=A3=A4=A5, and because I was rushed at this problem, I put V2=V3=V4=V5. The TA circled both of these, and put an x by each one and counted off a total of 6 points for the problem because I solved it by using density (rho) of each port rather than the mass flow rate (m dot). EVEN THOUGH, I got the right answer, he counted off more than people who simply divided the inlet mfr by 4 (ie 12.5 kg/s). They only got 4 counted off. When I went to talk to him he was pretty stern about not changing the grade, because the problem states WATER rather than air, and density wouldn't change (incompressible). My question is this: Is there ever, even in extreme circumstances, any time where density of water could change because of pressure. Because I could then make the argument that velocity COULD be constant, and density WOULD change (based on the fact that mfr is a function of density and velocity). Nevermind the fact that I could have used a smiley face to represent whatever was being calculated because a variable is simply a value holder, and I had the concept right, I think it is a little ridiculous that I get counted off MORE than the people who got the problem completely wrong, but I will need a scenario to essentially prove my way to be right (which it would have if it was AIR going through and AIR plenum, but thats beside the point.) Thanks for any help.