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Homework Help: Physics Problem that I need help with.

  1. Jul 11, 2004 #1
    I really just need help getting started...

    What additional force is necessary to hold a water hose stationary after the water flow is turned on, if the discharge rate is .6 kg/sec with a speed of 25 m/s?

    Thanks in advance..
  2. jcsd
  3. Jul 12, 2004 #2
    I'm not sure, but until someone better at physics responds, this'll probably tide you over:

    When the water is going forward, I guess it's pushing the hose/person holding it backward

    the momentum going forward is .6kg*25m/s, and you get the force by taking the change in momentum over the change in time, 0.6kg/s * 25m/s => 15kg*m/s^2 = 15N

    I dunno if my underlying assumption is correct there, tho
  4. Jul 12, 2004 #3
    thanks, that's exactly the info that i needed to get going.
  5. Jul 12, 2004 #4
    confused: can anyone tell how can i ask my own problems?/creat threads?
  6. Jul 12, 2004 #5
    I think you'd also need to know H20 pressure, or at least, I guess, differential pressure, because if you think about the idea of a culvert through which a river runs, there is no force because the water just flows through the bottom part. So if you had a hose and you could get flow rate of .6kg/s through it without the hose being full, theres little or no pressure drop as the water leaves the hose. does that make sense?

    All you are measuring with flow rate and mass is the force the water can exert on an object it hits, which doesnt help you that much in answering your question.
  7. Jul 12, 2004 #6
    Phantom...you don't need to know that. The question only involves Newton's third law. The force the water can exert on an object, in fact, can't be known unless you know how much speed the water recoils with.
  8. Jul 13, 2004 #7
    But you DO know the speed. I just misread the question. The answer was correct. The momentum equation works fine. Zero momentum before you turn on the hose and 15 kg-m/s2 afterwards. Got it.
  9. Jul 14, 2004 #8


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  10. Jul 16, 2004 #9
    (ignore this message)

    Hi people
    I'm just testing to see if this thread gets posted. This is the first time ive used theis site.
  11. Jul 19, 2004 #10
    Yeah KillaMarcilla, you are precicely right. As water has a density of 1.00 kg/L you dont need to account for it's density change. So, to hold the hose steady, you would have to apply a force of 15N onto the hose.
  12. Jul 19, 2004 #11


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    Staff Emeritus
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    Killamarcilla is correct.

    Anyways, a useful trick to help you get started on any problem is dimensional analysis. For example, in this problem you are given 2 quantities whose dimensions are kg/s and m/s. You are asked to find a quantity whose dimensions are Newtons or kg.m/s^2. Clearly the product of the dimensions of the given quantities gives you the dimensions of the required answer. This suggests that multiplying the 2 numbers may be the way to go.

    However, let this not be the only thing you do to solve a problem. It does not consider dimensionless constants or involve an understanding of the underlying physics. It's just a useful trick for checking solutions.
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