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Homework Help: Lab Questions on gravitational potential energy and Kinetic Energy

  1. Jul 16, 2011 #1
    In this investigation I had to calculate my maximum power as I run up a flight of stairs. These are a few questions that I have for my lab. I want to know if my calculations are right.


    1. mass: 70kg
    2. h = 1.17m
    3. d= 1.78m
    4. t = 2.02s

    Use the information from 1 and 2 to find the change in gravitational potential energy.

    ΔEG = mgΔh
    ΔEG = (70kg) (9.8 N/kg) (1.17m)
    ΔEG = 802.62 J

    Use the information in steps 1 and 3 to find average kinetic energy.

    v = Δd/Δt
    v = 1.78m/2.02s
    v = 0.88m/s

    Ek = mv²/2
    Ek = (70kg) (0.88m/s)²/2
    Ek = 27.10 J

    Using the information in steps 4 and 7 find your power.

    W = (27.10J) (1.78m)
    W = 23.08 W

    P = W/Δt
    P = 48.24 J/2.02s
    P = 23.88 W
  2. jcsd
  3. Jul 16, 2011 #2
    Gravitational energy looks fine. I'm pretty sure your kinetic calculation is all right too, unless they want you to find the total distance traveled (using the height and the horizontal distance). There seems to be some confusion in the third part. Remember Work=FORCE*distance, not energy*distance, and remember the units of force and work are newtons and joules (units of energy are the same as for work).
  4. Jul 17, 2011 #3
    Is this right for the third part

    P = Ek/Δt
    P = 27.10 J / 2.02s
    P = 13.42 W
  5. Jul 17, 2011 #4
    Well...this is what I think. Remember that the full equation for work is F*d cos theta. If theta is 90 degrees (the force is perpendicular to the distance) cos theta is 0. So, I think you shouldn't use the kinetic energy for this, it is changing perpendicular to the distance you are looking at (the height). Use the change in gravitational energy for this, and you'll be done.
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