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Homework Help: Graviational potentioal energy

  1. Oct 11, 2009 #1
    1. The problem statement, all variables and given/known data

    A 40 N toy is placed in a light swing that is attached to ropes 2.0m long. Find the gravitational potential energy associated with the toy relative to its lowest position when (a) the ropes are horizontal (b) the ropes make a 30 degrees angle with the vertical and (c) at the bottom of the circular arc.

    2. Relevant equations

    PEg = mgy (note: y is distance)
    Wg = -mg(yf-yi)=mgyi-mgyf=PEi-PEf

    3. The attempt at a solution

    I do not know what to start. Well, as my prop show a similar problem with (b) problem, I tried to draw free body diagram. like vertical = y axis and horizontal = x axis. So the the rope and y axis makes a sort of triangle. and the unknown length of the one side of triangle must be length of cord multiplied by cos30. And I cant' go further. Maybe I went down all wrong. Will you help?
    If I am asked solve the length between the maximum point reached by toy or something like that, I think I can do it. However, about energy, I am confused.
    Last edited: Oct 12, 2009
  2. jcsd
  3. Oct 14, 2009 #2


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    Homework Helper
    Gold Member

    As usual I'd begin by drawing a picture!:wink:

    What is the minimum potential energy of the toy?
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