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Homework Help: Magnitude of the rate of change of momentum of water

  1. Oct 19, 2007 #1
    Rate of change of momentum

    1. The problem statement, all variables and given/known data

    Hey everybody this is my first post and I really hope you guys can help me out. Ok so here's the problem which is divided into three parts:

    The first part consists of this: A fire hose sends 1464 gallons of water perminutre against a burning building at 92 m/s and does not bounce back. What is the magnitude of the rate of change of momentum of the water. Answer in Newtons.

    The second part which I've figured out asks whether the rate of change of momentum of the water cannot be determined, is positive or negative. The answer is that it's going to be negative.

    The third part consists of finding the once exerted by the water in 1.7 minutes.

    2. Relevant equations
    I know thus far that momentum or delta p = mass x velocity

    3. The attempt at a solution

    So far I've tried converting the amount of water given into a mass given that the density of water is 1kg/L which gave me a value of 3.786 kg/gal which I used with 1 min= 60 sec to convert the gal/min value into 92.38 kg/sec

    I multiplied this by the velocity given (vf-vi= -92) and got a value of -8498.8 N, which when I inputted into the online homework program said was incorrect.

    I've been working on this for several hours now and am currently stuck at this moment. The assignment is actually due the 23rd but the answers need to be submitted well before then. Any insight anybody could give me would be greatly appreciated. Thanks!
    Last edited: Oct 19, 2007
  2. jcsd
  3. Oct 19, 2007 #2


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    I'm getting the same answer as you... I suspect the problem is with the conversion from gallons to kg... I'm trying to find the conversion...
  4. Oct 19, 2007 #3


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    maybe use 3.78541178 kg/gallon ? changes the last few decimal places...

    these are US units right? not UK units I'm guessing...
  5. Oct 19, 2007 #4
    well we've been using the metric system thus far. I'm suspecting that the conversion rate might be off but I've yet to be able to verify that.
  6. Oct 19, 2007 #5


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    Did you enter -8498.8 N or 8498.8 N.

    The first part asks for the magnitude, so you need to enter the positive value...
  7. Oct 19, 2007 #6
    Yes I put it in positive form and it's still wrong. The thing is the program the homework is on only let's you submit an answer a max of 7 times, thus far I've tried three answers and they've all been wrong. They were: -8498.81, 8497.49, -4726.54

    My question now is are we sure this is the right way to convert to mass for the water and the velocities: our inital will be 92 right, so the initial I'm presuming is 0 but is that right?
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