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Homework Help: Motor efficiency question

  1. May 14, 2012 #1
    1. The problem statement, all variables and given/known data
    A lift motor is 70% efficient. It dose 450 000j of work lifting a load to the top floor of a building.

    2. Relevant equations
    What electrical energy input is required to do this?

    3. The attempt at a solution

    I first did output/input * 100 I transposed the formulae

    so I did 450 000* 0.7

    which gave the wrong answer. When I looked at the answer is say this is the correct way is 450 000 * 100/70

    Am I right in saying that when you do it this you are calculating what it is at 100% efficiency and then calculating the difference?
  2. jcsd
  3. May 14, 2012 #2
    70% of the energy input is 450000J, so 1% is 450000/70 = 6429J
    So 100% of the input energy is 100 x 6429 = 642900J
    You had it 'upside down'....do you see
    Last edited: May 14, 2012
  4. May 14, 2012 #3
    Write it out as an equation. Of the total electrical energy consumed, 70% of it went into lifting the weight.

    E * 70% = 450,000 J

    Where E is the unknown energy input.

    How do you solve for E?
  5. May 14, 2012 #4
    E = ??????
    just rearrange your equation....you have it !!
  6. May 14, 2012 #5
    Realize that if it is only 70% efficiency, more work is actually required than if it was 100% efficient.
  7. May 15, 2012 #6
    Just to say thanks for everyone clearing it up for me big help.
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