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Calculate the efficiency of the electric motor.

  1. Dec 2, 2009 #1
    1. The problem statement, all variables and given/known data
    An 8.5 X 10^3 kg elevator is pulled up at a constant velocity of 1m/s by a 10kW electric motor.
    Calculate the efficiency of the electric motor.


    2. Relevant equations
    P=w/t
    %efficiency=work out/work in X 100%
    W=mgh
    d=vit+1/2at^2
    v=d/t

    3. The attempt at a solution
    The answer in the book is 83%, but I don't understand how they got that answer. Please help.

    In the meantime, this is how far I got (not too far):

    vi=1m/s
    m=8.5X10^3kg
    vf=0
     
  2. jcsd
  3. Dec 2, 2009 #2
    The work done can be expressed in gravitational terms: mass * g * height. The work in can be calculated by power * time. Height can be found with velocity * time.

    mass * g * velocity * time / (power * time) * 100 = mass * g * velocity / power * 100
    8.5X10^3kg * 9.81 * 1 m/s / ( 10000 W) * 100 = 833%. It's off by a multiple of ten, so you gave the wrong initial values, the book published a faulty question, or (i sure hope this isn't it) i'm partly wrong. I hope this helps.
     
  4. Dec 4, 2009 #3
    Is the time 1 second? Is the height 1 metre? Yes, I got the mass wrong, it was suppossed to be to the power of 2 not to the power of 3.
    How do I know whether to use power out/power in vs. work out/work in? Thanks!
     
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