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Homework Help: Power Help

  1. Oct 16, 2007 #1
    1. The problem statement, all variables and given/known data
    Water falls onto a water wheell from a height of 2.0m at a rate of 95kg/s (a) If this water wheel is set up to provide electricity output, what is the max power output. (b) What is the speed as it hits the wheel?

    2. Relevant equations

    Ep= mgh Ek= 1/2mv^2

    3. The attempt at a solution

    I know that the water initally has both potential and kinetic energy as it falls but i am unsure as to how i use the rate in these equations and also which power equation(s) I use once i get to that stage.
  2. jcsd
  3. Oct 16, 2007 #2


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    Staff Emeritus
    Science Advisor

    Well the elevation of a mass m in a gravitational field provides for gravitational potential energy (GPE) by virtue of mgh, where h is the elevation above some reference point.

    If an object at h is released and it falls through distance h, then GPE decreases by mgh and that GPE is transformed to kinetic energy 1/2 mv2, and neglecting air resistance mgh = 1/2 mv2.

    If one has a mass flow rate [itex]\dot{m}[/itex], then the rate at which energy (described above) is delivered is simply [itex]\dot{m}[/itex]gh, and the rate of energy delivery/transformation is power.
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