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Effect of distance on jet force

  1. Jan 9, 2015 #1
    1. The problem statement, all variables and given/known data
    Apologies for the vague title but I'm not sure how else I could word it, also this isn't really a homework question per se however I'm not too sure where else to ask it.

    I think it would help if i set up a scenario. Say there was a pipe that released a fluid to a mass scale underneath the nozzle x meters away and got a reading of 5kg. If the distance between the nozzle and the scale was increased would we expect the reading to change? Assume turbulent flow.

    2. Relevant equations
    Mass reading = (density*average flow velocity^2*A)/g

    3. The attempt at a solution
    I would say no but i'm not sure. If the fluid was incompressible then steady flow would be in effect and the mass would remain constant thus the balance should show the same reading but I feel there's more to it. Once the fluid has left the pipe it no longer feels the driving force of the pump so there's like an acceleration gradient in the jet..wouldn't that mess up the steady flow?

    Any help will be appreciated, thank you.
  2. jcsd
  3. Jan 9, 2015 #2


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    Staff: Mentor

    Is it possible that by "mass scale" you are intending a weight scale or force or pressure measuring device of some sort)? In other words, something to determine the average force on a surface due to the fluid impacting it?

    Is the fluid traveling in a vertical direction under the influence of gravity? If so, consider that the change in height will add kinetic energy and hence momentum to every particle (or differential volume unit) of the fluid as it falls through the gravitational field.
  4. Jan 9, 2015 #3
    Yeah that's what I meant. That's an awesome point, can't believe I didn't think of that haha.

    Thank you.
  5. Jan 11, 2015 #4


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    Staff: Mentor

    I'm trying to picture a high speed jet of water directed downwards from a significant height. The velocity will slow down to a terminal velocity, and I expect the stream will break up into myriad globules. This means it must spread out. So I think it will reach the ground over a wide area as a dense shower like raindrops.
    Last edited: Jan 11, 2015
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