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Homework Help: Elevator problem?

  1. Feb 20, 2007 #1
    1. The problem statement, all variables and given/known data

    A skier of mass 69.7 kg is pulled up a slope by a motor-driven cable.
    (a) How much work is required to pull him a distance of 59.8 m up a 29.9° slope (assumed frictionless) at a constant speed of 1.90 m/s?
    (b) A motor of what power is required to perform this task?

    2. Relevant equations

    (a) w=f*change in distance*cos angle
    (b) power=work/change in time

    3. The attempt at a solution

    gravity is the only force in play. so the equation for work would be

    take that value and divide it by change in time to get power

  2. jcsd
  3. Feb 20, 2007 #2


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    Homework Helper

    "cos angle" refers to what angle? With respect to your given angle cos is the wrong trig function. You are also missing a mass factor which would have been obvious if you had checked the units on your answer.
  4. Feb 20, 2007 #3
    No. w=f.s=m.a.s
    You havent taken the mass of the skier into account. The acceleration will be along the incline (which youve taken incorrectly).
  5. Feb 20, 2007 #4
    Power can be calculated by p=fv
  6. Feb 20, 2007 #5
    Ok so the F in the problem is 9.8m/s^2. I would use cos0. However, the change in distance is unknown. Using the formula d=time * speed, I found it to be 8.75m. So, now would I use the work formula with the numbers (9.8m/s^2)(8.75m)(cos0). Which equals 85.57J. After I find that I would divide work by time (5secs) to find average power which is 17.15 W...just my thinking??
  7. Feb 20, 2007 #6
    F is not 9.8 m/s^2. That is a. You are forgetting to take into account mass of the skier. force = mass * acceleration
  8. Feb 20, 2007 #7
    Ok-so hopefully this is it:
    W=(mass of skier*acceleration)(change in distance)(cos29.9)
    Then take that value and divide by the time which is 31.55 seconds.
    Which equals 217.6 hp
  9. Feb 20, 2007 #8
    The SI unit of power is watts. There are about 746 watts per h.p. but you better look up that number, i am pulling it out of my head.
  10. Feb 20, 2007 #9
    Is the way I worked the problem out correct? I just need to convert the 217.6 watts to hp? I have had trouble solving this problem. It would be nice to know if this thinking is correct.
  11. Feb 20, 2007 #10
    The reasoning seems okay to me.
  12. Feb 20, 2007 #11
    UGH! I'm still stuck. Any help would be greatly appreciated!!
  13. Feb 20, 2007 #12
    Where are you stuck?
  14. Feb 21, 2007 #13
    alright...i think i've got it. I have to run to class, but I will post later witht the answer I finally came up with! YAY!
  15. Feb 21, 2007 #14
    actually, nevermind that answer was also incorrect. sorry to get our hopes up!:frown: Still working...
  16. Feb 21, 2007 #15
    whoops, you shouldnt be looking at the cosine of theta. Sorry, just caught that.
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