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Homework Help: Power question?

  1. Aug 12, 2005 #1
    A 2.0 hp electric motor on a water well pumps water from 10 m below the surface. The density of water is 1.0 kg per liter. How many liters of water does the motor pump in 1 hr?

    im not sure what equation to use on this problem, so if anyone could let me know that would be good thanks!
  2. jcsd
  3. Aug 12, 2005 #2
    Power = Energy/Time

    Energy = mgh in this case

    1 hp = 745.8 Watts
  4. Aug 12, 2005 #3


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    Homework Helper

    A motor is working on increasing the potential energy of water (calculated by PE = mgh) by bringing it up the well 10 meters below. The motor is using 745.8 Joules per second (745.8 Watts). In one hour or 3600 seconds, how many kilograms of water will it be pumping up?

    Does that make the question easier?
  5. Aug 12, 2005 #4
    yea i understand the question a tiny bit more, but dont you have to find the volume of water being pumped up?

    this is what i got so far but im completely not sure since its 2 hp..

    745.8w*2*3600s= is the energy of the motor in 1 hour.
    h= -10 or 10 depending on where u place your axis
    g= 9381

    745.8w*2*3600=mgh? is that right?
  6. Aug 12, 2005 #5


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    g = 9.79 meters per second per second

    And hint about volume. The volume doesn't matter in this energy problem. Just know for later reference that 1000 liters of water = 1000 kg = 1 cubic meter.
  7. Aug 12, 2005 #6
    oo thanks alot i got it now =]
  8. Aug 12, 2005 #7


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    Not 9.81? Where do you live?

    Well, volume does matter because the problem asked for volume!
    I presume you meant that since, for water, 1 litre is 1 kg, it's easy to convert.
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