# Momentum for a control volume (Fluid Mechanics)

1. Sep 26, 2013

### PhyIsOhSoHard

1. The problem statement, all variables and given/known data
http://i988.photobucket.com/albums/af10/postop/equationsmomentum_zpse8c8d44a.png

2. Relevant equations

3. The attempt at a solution
Can somebody explain to me in the two equations above what is the "u" and what is the "v"?
In the question, it says that the water at the nozzle has a speed of 25 m/s and the vane maintains the speed at 5 m/s.

It just doesn't make any sense because my textbook writes the following when calculating the reaction force for the x-axis:

But with the reaction force for the y-axis it writes:

And they argue that it is because $v_1=0$
Why is there no A1 segment for the y-axis???
I just don't understand, what is "v" and "u" in these equations and how did $v_1=0$ happen?