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Potential Energy problem homework

  1. Nov 23, 2009 #1
    Hello guys I need your help T__T I'm new here.. ^__^

    1. The problem statement, all variables and given/known data
    A girl weighing 470N hangs suspended on the end of a rope 8 meters long. What will be her gain in potential energy when a boy swings her to one side so that the rope makes an angle of 35º with the vertical? gravity= 9.7 m/sec²

    I need the answer to be exactly 679.5 Joules.. My professor wants that I use an equation with Fg? (Free body diagram)? Or any solution that will leads to the exact answer..


    2. Relevant equations

    PE=mgh

    3. The attempt at a solution
    PE''=mgh mg (470N) x h (8 meters)
    =3760

    PE'=mgh 470 x (cos 35º) x 8
    =3080

    3760-3080=680
     
  2. jcsd
  3. Nov 23, 2009 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hello aspirare21! Welcome to PF! :smile:
    Well, I get exactly the same answer as you (not 679.5).

    (btw, using 1- cos35º in the first place would have been quicker and neater :wink:)

    Also, I don't understand what your professor thinks g has to do with it (nor why it's only 9.7 !! :rolleyes:) … the weight is given in newtons.

    (I assume the equation your professor is thinking of is PE = (minus) work done = force "dot" distance = Fg.x = Fgh :confused:)
     
  4. Nov 23, 2009 #3
    Hello tiny-tim :)

    I myself is confuse on how he derive the g=9.7 m/sec². He wants us to show a free body diagram on how to solve it.. Maybe putting tension,normal force,etc. I don't know if the formula or the pendulum is applicable here.. But this problem doesn't have a velocity. :(
     
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