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Homework Help: Average Power

  1. Dec 7, 2012 #1
    A glider is launched from the ground by means of a winch, which rapidly reels in a towing cable attached to the glider. What average power must the winch supply in order to accelerate a 200 kg glider from rest to 25 m/s over a horizontal distance of 75 m. Air resistance and friction are negligible and tension in the winch is constant.

    1. The problem statement, all variables and given/known data
    v2=25 m/s
    d=75 m
    m=200 kg

    2. Relevant equations
    Power=change in work/change in time (p=w/t)
    change in work=change in kinetic energy (w=k2-k1)

    3. The attempt at a solution
    w=k2-k1 (where k1=0 because initial velocity is 0)
    w=k2 (0.5)(200)(25)^5 =6.25e4 J

    now to get time: v=d/t ... t=d/v=75/25= 3s
    sub w and t into the equation for power
    p=6.25e4/3=20.8e3 W

    but this isn't the right answer, the actual answer is 1.0e4 W .. i don't see where i went wrong.. help please
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 7, 2012 #2


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    Science Advisor
    Homework Helper

    Your determination of the time is wrong. You solved the problem as if the speed were a constant 25m/s. You know that's not true, right?
  4. Dec 7, 2012 #3
    ah! now that you say it, thanks a lot
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