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Homework Help: Work and Potential Energy Problem

  1. Oct 18, 2008 #1
    1. The problem statement, all variables and given/known data

    A 140 Watt motor (70% efficient) is available to raise a load 5 meters into the air. If the task takes 5 seconds to complete, how heavy was the load in kilograms?

    I first determine the motot output: 140W * 70% == 98 Watts

    I then multiply 98 Watts by 5 seconds to get PE = 490Joules

    Now, setting 490Joules = mgH and solving for m, I get 10 kilograms

    The answer is supposed to be 110kg according to my professor but I cannot get to that number. What am I doing wrong?

  2. jcsd
  3. Oct 18, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your answer looks good to me. I don't think you did anything wrong. (Ask your professor to show her solution.)
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