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Homework Help: Power and Energy of a motor problem

  1. Jan 24, 2010 #1
    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


    User Avatar
    Homework Helper
    Gold Member

    The correct equation for power is: P=W/t

    What is work? (W=Fd)
    What is force? (F = ma)

    In this problem, there are 2 accelerations to consider: (1) The problem says the speed is constant so the acceleration of the mass as it travels through space is 0. (2) assuming that this motor is operating near the surface of the Earth, the acceleration due to Earth's gravity is acting on the mass.
  4. Jan 24, 2010 #3
    Fv should work as well as wd/t, because d/t = v. To find the force, draw a force body diagram. What forces are acting on the 400kg object? Remember, force does not equal acceleration. A NET force equals an acceleration.
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