Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy and Power Question

  1. Apr 2, 2006 #1
    Question is.. how much power does a 45 kg person develop climbing up a vertical ladder 7.0 m high if the average acceleration is 0.25 m/s^2?

    This is where I'm at... (so far anyways)

    calculate t from d = vt + 1/2at^2 and get value of 7.5 s to climb ladder... but now, how do I calculate Work in order to sub into P=W/t equation...

    I have tried this... W = (Fg + Fa)*d = [(45)(9.81) + (45)(0.25)]*7 = 3168.9

    and then P= W/t = 3168.9/7.5 = 423 W

    Answer in book is 423 W but I am confused as to why I am right.. don't Fg and Fa act in opposite directions on the FBD??

    Any help is appreciated
  2. jcsd
  3. Apr 2, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The correct way to think about this is [itex] F_{person} - m g = m a_y [/itex] (where I am working in the y direction). So the force applied by the person is [itex] F_{person} = mg + m a_y [/itex] (makes sense, the person must apply a larger force than mg in order to accelerate upward). Now you multiply this by the distance to get the work done by the person.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook