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Homework Help: Electric pump basic power problem

  1. Aug 12, 2012 #1
    This is a high school work/power problem that I'm struggling with:

    An electric pump is used to pump water from a 40m deep hole to the surface. If 1200kg of water is needed every 2 minutes and the water is released at 12 ms-1 at the surface, what should the minimum power of the pump be?

    I'm not sure whether I can use Wnet=ΔK in this example. Can I assume that the water moves at constant speed? Is the force of the pump present at all times during the movement of the water, or does it just accelerate the water initially? I know that the water gains potential energy, so I would simply use U=m g h to calculate the work. But then, why is the speed of the water at the surface given?

    Once I have the work, the power is easy (just divide the work by 120s, obviously).
  2. jcsd
  3. Aug 12, 2012 #2


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    Focus on the change in energy of the 1200 kg "parcel" of water that is transported in 2 min. How much energy does it have at the start, before it is pumped to the surface? How much more energy (in all relevant forms) does it have after it reaches the surface? So, if that amount of energy was gained in 2 min, what is rate of change of energy due to the pump?
  4. Aug 15, 2012 #3
    Thanks, I sorted it out and my solution was correct :)
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