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Homework Help: Graviational potentioal energy

  1. Oct 12, 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.
  2. jcsd
  3. Oct 12, 2009 #2


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    Homework Helper

    At the bottom PE = ...?
    a) The rope is horizontal. So the displacement is L ( length of the rope). So PE = ....?
    b) The displacement is (L - LcasĪø). So PE =....?
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