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Homework Help: Power on an elevator

  1. Apr 13, 2008 #1
    1. The problem statement, all variables and given/known data

    An elevator weighing 2100 N ascends at a constant speed of 8.0 m/s. How much power must the motor supply to do this?

    2. Relevant equations
    P = Fv

    3. The attempt at a solution
    F = 2100 N
    V = 8 m/s
    P = (2100) * (8) = 16800

    in significant figures thats 1.7 x 10^4 w
    in kilowatts thats 17
    why is this wrong?
  2. jcsd
  3. Apr 13, 2008 #2
    Hello onestarburns,

    2100 N = 214.2 kg

    F = m*a = 214.2kg * 8m/s^2 = 1713.6 N

    W = F*change in dis. = 1713.6N * 8m = 13,708.8 J

    P = W/change in time = 13,708.8N/1s = 13,708.8Watts
    P = Fv = 1713.6N * 8m/s = 13,708.8 Watts

    I believe this is correct.
    If the elevator 'weighs' 2100 N than its mass is 214.2 kg.
    So the force it takes to move this mass 8meters in one second every second is: 1713.6 N
    The work that you accomplish by applying 1713.6N of force over 8meters is: 13,708.8 N*m

    I am assuming that the question is asking for the work output per second. However if the question elaborates more than what you have given the answer could be different.
    So the power required to do this amount of work in one second is: 13,708.8 J*s
  4. Apr 13, 2008 #3
    that doesnt work either
    the answer is asked for in 2 s.ds and in KW, so i assumed that would be 14, but thats not working
  5. Apr 13, 2008 #4
    Well personally i hate significant figures so i can't help you there. And without more information in the question i don't really know what else can be done. Let me know if im missing something obvious here. sorry i couldn't be of more help.:frown:
  6. Apr 14, 2008 #5


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    Homework Helper

    Your answer of 16800 W seems correct to me.


    Hi robertm,

    There is an error in your second equation. The elevator is moving at constant speed so a=0.
  7. Apr 14, 2008 #6
    thanks alphysicist thats why i was confused. It looked as if the op's first answer was correct so i assumed the question contained additional info that was missing in the post.
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