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Homework Help: Not sure how to start this (Power problem)

  1. Feb 22, 2006 #1
    The loaded cab of an elevator has a mass of 2.6 x 10^3 kg and moves 183 m up the shaft in 22 s at constant speed. At what average rate does the force from the cable do work on the cab?

    I've got the formula for average power, but that requires the work over the time. I only have the time. How can I obtain the work from what I've got?

    I know that for the work itself I need the Force involved, so I think of F = ma, but if the velocity is constant that means the acceleration is 0. Am I doing this right? If so, what do I need to do next? If not, what should I do instead?
  2. jcsd
  3. Feb 22, 2006 #2
    There are two forces working on the elevator, gravity and the force from the cable.
  4. Feb 22, 2006 #3
    Right but that puts me in my original dilemma. How can I figure out what the tension force is? If I can get the tension force I can just take that and plug it into the formula for work, then take that and plug it into the formula for average power, but how do I figure out that upward force?
  5. Feb 22, 2006 #4
    Ahh wait nevermind, I just realized how to get the work. I can just take the Work by fravitational force formula since I know the mass, gravity, and distance, then take the answer and plug it into average power's formula.

    And upon submitting that result to the online quiz it appears to have worked beautifully, though I wonder why?
  6. Feb 23, 2006 #5
    As you mentioned earlier, if the elevator is traveling at a constant velocity it is not accelerating. This means that the net force on it is zero, so the force exerted by the tension in the cable must be equal and opposite to that exerted by gravity.
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