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Homework Help: Waterfall Energy Question

  1. Nov 21, 2009 #1
    1. The problem statement, all variables and given/known data

    The highest waterfall in Canada is the Della Falls in British Columbia, with a change
    in elevation of 4.4  102 m. When the water has fallen 12% of its way to the bottom,
    its speed is 33 m/s. Neglecting air resistance and fluid friction, determine the speed
    of the water at the top of the waterfall.

    2. Relevant equations

    E before = E after
    1/2 mv^2 + mgh = 1/2 mv^2 + mgh
    v^2 + 2gh = v^2 + 2gh
    v^2 = v^2 + 2gh - 2gh
    v = √v^2 + 2gh - 2gh

    3. The attempt at a solution


    waterfall distance from ground = 440m
    V2= 33m/s
    water fallen at 12% distance = 387.2m

    v = √v^2 + 2gh - 2gh
    v = √(33m/s)^2 + 2(9.8 m/s^2)(387.2m) - 2(9.8 m/s^2)(440m)

    now the answer in the book is 5.0m/s and i was wondering what is the mistake that is made ?? or is the book just incorrect?

    thank you for your help in advance..
  2. jcsd
  3. Nov 21, 2009 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Start with kinetic + potential at 440 m is equal to kinetic plus potential at 387.2 meters. Use different symbols for the speed v and the height h. Your problem is you plugged the numbers in the wrong places because everything looks the same.
  4. Nov 22, 2009 #3
    thanks will work on it n message back
  5. Nov 24, 2009 #4


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    your answer 7.35 m/s is correct
    the only reason it says 5.0 m/s in the book is because the book uses
    10m/s for gravity because it is for easier calculations
    thanks for helping me with a waterfall physics question
  6. Nov 25, 2009 #5
    i actually went to my physics teacher and he said the same thing you said cad.
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