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Homework Help: Power and Energy

  1. Jan 24, 2010 #1


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    1. The problem statement, all variables and given/known data

    What is the average power developed by a motor
    as it lifts a 400.-kilogram mass at constant speed
    through a vertical distance of 10.0 meters in
    8.0 seconds?
    (1) 320 W (3) 4,900 W
    (2) 500 W (4) 32,000 W

    2. Relevant equations
    P=Wd/t= Fd/t= mad/t

    3. The attempt at a solution
    How does the fact that acceleration is 0 change the way I should approach this question. The question says acceleration is constant.
    On the other hand I tried using this formula.... P=Fv..... but because I dont know acceleration, I can't find 'F'.
  2. jcsd
  3. Jan 24, 2010 #2
    The question says that the SPEEDis constant, so what is the acceleration?
  4. Jan 24, 2010 #3


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    Doesn't constant speed mean 0 acceleartion??
  5. Jan 24, 2010 #4


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    that means its 0, and that would also mean my answer will turn out 0. Now that cant be. Am baffled.
  6. Jan 24, 2010 #5


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    Yes, constant speed is 0 acceleration.

    This one is approached easier if you do it in terms of energy. How much potential energy does the 400kg block pick up? That energy must be the work done by the motor right (or else, where else would that energy come from)?

    This is the simplest way.

    To do it your way, you need to realize that the NET force is 0. The other force acting on the block is gravity. In this case, the Motor has to act against gravity.
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