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Homework Help: Finding Potential Energy of a Swing When the Ropes Are Horizontal?

  1. Apr 14, 2012 #1
    1. The problem statement, all variables and given/known data

    A 45.0 kg child is in a swing that is attached
    to ropes 1.90 m long.
    The acceleration of gravity is 9.81 m/s2 .
    Find the gravitational potential energy as-
    sociated with the child relative to the child’s
    lowest position under the following condi-
    a) when the ropes are horizontal.
    Answer in units of J.

    2. Relevant equations

    Height of a pendulum:

    Potential Energy


    3. The attempt at a solution

    So first I found the height of the swing at a horizontal position which I assumed is as at 90°.


    Then I used that height and put it into the formula for potential energy


    1213.99 J? = (45)(9.81)(2.75)

    but the online homework interface I have to use to input my answer tells me this isn't correct. I was wondering if maybe my answer isn't in Joules, do I need to convert something. Any help would be greatly appreciated.

    I realized my problem, i had my calculator in radian's sorry for the stupid question, I'd delete the question if i can find out how
    Last edited: Apr 14, 2012
  2. jcsd
  3. Apr 15, 2012 #2


    User Avatar
    Homework Helper

    The ropes are 1.9 m long while the height you used is 2.75 m. How exactly did you get 2.75 m using h = L(1-cosθ)?
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