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Finding Power when no time or velocity are provided?

  1. Apr 25, 2016 #1
    1. The problem statement, all variables and given/known data
    A man weighing 125kg walks 15m up to the top of a plane inclined at 30 degree to the horizontal.
    find A) Work he has done B) the increase in Potential Energy C) If he wants to go to the top of the plane, what is his horsepower?


    2. Relevant equations
    PEo + KEo = PEf + KEf
    P = W/T = Fv
    1HP = 746 W
    3. The attempt at a solution
    I found A) and B) which gives 9188 Joules, for the C) part I'm having more difficulty. I thought that by using the law of conservation of energy I could find the initial velocity and then multiply by mg to give the power to then convert the watts to HP. Although, I doubt that this would be it. Any thoughts? Thanks a lot
     
  2. jcsd
  3. Apr 25, 2016 #2

    gneill

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    Staff: Mentor

    Since HP is a unit of power there's a time component involved. Are you sure that the problem is stated in its entirety? No mention of time or velocity?

    If so, the best you can do is either assume that the man walks at a constant speed and make a reasonable estimate of that speed to use in the calculation, or leave the speed as a variable in the answer (i.e. leave the answer in symbolic form).
     
  4. Apr 25, 2016 #3
    Ok but would it make sense to use the law of conservation of energy?
     
  5. Apr 25, 2016 #4

    gneill

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    Staff: Mentor

    Use it how?

    While the work the man is doing in climbing the the ramp is being transformed into potential energy, it doesn't tell you anything abut how fast that transformation is being carried out. Energy conservation only tells you part of the story.

    Note that the man walking up the ramp implies a constant speed, so there's no net change in kinetic energy taking place.
     
  6. Apr 25, 2016 #5
    I would have used it this way:
    We know that the potential energy is 0 at the beginning and that at the end of the motion it is 9188 joules. Therefore, by using the law of conservation of energy I would equal the initial kinetic energy to the final potential energy like this -> KEo = PEf -> 1/2mv^2 = 9188 J -> v = sqrt( 2m9188) ?

    Thanks for your help by the way, I'm just very puzzled with this question considering that I have no information about time or velocity to find the power.
     
  7. Apr 25, 2016 #6

    gneill

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    Staff: Mentor

    I think you mean v = sqrt(2 ⋅ 9188 / m), if m is the mass. But no, that won't work since the mass is not sliding or rolling up the hill and changing its KE. There is no change in velocity taking place, no net trade between KE and PE.
    Yup, that is a puzzler. Perhaps the question is badly posed. It happens sometimes.
     
  8. Apr 25, 2016 #7
    Ok thanks for your time !
     
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