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Homework Help: Speed of Water Flowing

  1. Oct 7, 2008 #1
    Hello, I would appreciate any help!
    1. The problem statement, all variables and given/known data
    A river narrows at a rapids from a width of 12m to a width of only 5.8m. The depth of the river before the rapids is 2.7m; the depth in the rapids is .85m. Find the speed of water flowing in the rapids, given that its speed before the rapids is 1.2m/s. Assume the river has a rectangular cross section.

    2. Relevant equations
    I'm using bernoulli's equation:
    and P=F/A

    3. The attempt at a solution
    I figured I would have to find the Pressures at the top and the bottom with P=F/A. The area I can find with the depth and width given but how do I find the force. Or is the pressure on the top of the river just atmospheric pressure?
  2. jcsd
  3. Oct 7, 2008 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi dislike! Welcome to PF! :smile:

    You don't need the pressure.

    Just use conservation of mass. :wink:
  4. Oct 7, 2008 #3
    I'm still confused; could you explain a little more.
  5. Oct 7, 2008 #4
    l understand what you mean now. Thank You.
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