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Fluid mechanics question

  1. Aug 11, 2016 #1
    Screen Shot 2016-08-12 at 11.31.38.png 1. The problem statement, all variables and given/known data
    See image.

    2. Relevant equations
    I thought that Fx=p*u1*A1(u1+u2)+(p1*a1)+(p2*a2) would be the correct formula however I cannot seem to get the stated answer of 1.8kN.

    3. The attempt at a solution
    Using the above formula, p1v1=p2v2 to find u2 and p1a1=p2a2 to find p2
    gives 105
  2. jcsd
  3. Aug 12, 2016 #2


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    Science Advisor

    Could you show all your working please.
    It's a little unclear why our how you think you found P2, you don't need to find it, you're told it's atmospheric which is 0 Pa relative to P1 (presumably 'gate pressure' is a typo for gauge pressure).
    Also not sure how you found the outlet velocity, conservation of mass/the continuity equation ( vA = constant) is the standard approach.
  4. Aug 12, 2016 #3
    You have applied the conservation of momentum equation incorrectly. The first and third terms are incorrect.

    Including the unit vectors, what are the forces acting on the fluid contained within the control volume?
    Including the unit vector, what is the rate of momentum exiting the control volume?
    Incuding the unit vector, what is the rate of momentum entering the contol volume?
    Last edited: Aug 12, 2016
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