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Homework Help: Flow of a river power math question?

  1. Oct 20, 2013 #1
    1. The problem statement, all variables and given/known data

    The average flow rate in the Niagara River is 6.0 x 10^6 kg/s and the water drops 50 m over Niagara falls. If all this energy could be harnessed to generate hydroelectric efficiency at 90% efficiency, what would be the electric power output?

    2. Relevant equations


    3. The attempt at a solution

    I did

    (6.0 x 10^6 kg/s) (9.8 m/s^2) (50m) (.9)
    = 2.65 x 10^9 kg*m^2/s^3

    I think I did the math right? But am unsure of the units.
    Is this the answer? Or should my answer be in watts?
    If so how would I get this number to watts?
  2. jcsd
  3. Oct 20, 2013 #2


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  4. Oct 20, 2013 #3

    rude man

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    Good exercise in dimensional analysis.

    What are the dimensions of each of your factors? (Example: dimension of kg/s = MT-1.)

    M = mass
    T = time
    L = displacement

    What is force F dimensionally ? (hint: F = ma).

    Then, what is FL?
    And finally FLT-1?

    Of course, you can dispense with dimensional analysis & just use common sense, knowing that if you stick to the SI system you will wind up with Joules for energy, Watts for power, etc. But dimensional analysis is one of the most powerful and most neglected tools in your physics toolbox to check equations term-by-term, answer reality checks, etc. so I advise getting comfy with dimensional analysis.

    Dimensional analysis can do much more than that, but that's another story.
  5. Oct 20, 2013 #4


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    What is P? If P=Power then this is wrong.

    Energy = mgh

    Power (in Watts) is the rate of change of energy eg

    Power = Δenergy (in Joules) / ΔTime (in seconds)

    In other words

    Power = mgh/t

    However the problem gives the flow rate Fr (= 6.0 x 10^6 kg/s) which is equivalent to m/t. So your starting equation should

    Power = Frgh

    or to be complete

    Power = Frgh * efficiency

    As it happens that's actually what you calculated when you wrote..

    You have the right answer but just need to understand how you got there :-)
  6. Oct 20, 2013 #5


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    PS. This question and your responses on one of your other threads (eg the one on the power output of a solar array) suggests you probably need to practice with manipulating units and dimensional analysis.
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