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Homework Help: Work Energy therem

  1. Nov 21, 2012 #1
    1. The problem statement, all variables and given/known data

    A pump is required to lift 800 kg of water (about 210 gallons)
    per minute from a well 14.0 m deep and eject it with a speed
    of 8ms-1 (a) How much work is done per minute in lifting the
    water? (b) How much work is done in giving the water the kinetic
    energy it has when ejected? (c) What must be the power output of
    the pump?

    2. Relevant equations

    Work Energy theorem

    3. The attempt at a solution

    The book attempt was that work done = mgh... Why don't we add tension as well... Or force applied by pump???
  2. jcsd
  3. Nov 21, 2012 #2


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    Science Advisor
    Homework Helper

    hi ehabmozart! :smile:
    you can calculate the work done in two ways …

    by integrating the tension T times the displacement, total ∫ T dh :yuck:

    or (applying the work energy theorem) by simply finding the difference in energy! :tongue2:​

    (if the acceleration is 0, so that T is constant, then T = mg, and ∫ T dh = mgh :wink:)

    the whole point of these work-energy exercises is to show you how to avoid using the usual work done formula …

    which would be very difficult if the acceleration wasn't 0!​
    Last edited: Nov 21, 2012
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