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Momentum against a wall

  1. Sep 29, 2007 #1
    1. The problem statement, all variables and given/known data
    Question: A stream of water from a hose is hitting a window. The window is vertical, the stream is horizontal, and the water stops when it hits. About 20cm(cubed), that is 20g, of water with speed 1.5 m/s strikes the window each second. Find a) the impulse on the window in time t and b) the force exerted on the window.

    2. Relevant equations
    I= delta p = mv -mu = Ft
    f= delta p/t = mv-mu/t
    s=ut +1/2 at (the last t is squared)

    3. The attempt at a solution
    well i originally tried to split this problem up into it vertical and horizontal components, saying that the water would follow a hyperbelar (i forget how to spell it) shape.

    then i used the s=ut for the horizontal component (the rest of the equation isn't needed as the water doesnt accelerate horizontally and therefore is a 0 value) but then found that s and t arnt known.

    i have a feeling it has something to do with the 20g of water with a force of 1.5m/s every second but i dont know what to do!

    please help :)
  2. jcsd
  3. Sep 29, 2007 #2

    Doc Al

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    Staff: Mentor


    What's the momentum of the water that hits the window every second? What's its change in momentum, thus impulse. If that's for 1 second, what would it be for t seconds?
  4. Sep 29, 2007 #3
    delta p = mv-mu
    -20 x 1.5 = -30

    okay thats for 1 second,
    but its the t seconds thing that gets me

    and thanks for helping
  5. Sep 30, 2007 #4

    Doc Al

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    Think about it. If the impulse is 30 kg-m/s for 1 sec, what would it be for 2 seconds? 3 seconds? t seconds? (It's easy.)
  6. Sep 30, 2007 #5
    so 30t

    haha right, i cant believe i didn't get that before :P

    one problem; the answer in the back of the book is 0.03t N s
  7. Sep 30, 2007 #6


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    That's because Doc and you were talking about 30kg of water and the problem says 30g.
  8. Sep 30, 2007 #7
    lol thanks dick
  9. Oct 1, 2007 #8

    Doc Al

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    Staff: Mentor

    D'oh! :redface: Thanks, Dick.
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