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Little needed with homework

  1. Oct 29, 2004 #1
    A 30.3 N child is in a swing that is attached to ropes 2.14 m long. What is the gravitational potential energy when the ropes make a 30.9° angle with the vertical?

    So far ive tried 6 different answers ranging between 0-64.8 J and no luck. Need a clue to get me on the right track. Thanks in advance.
  2. jcsd
  3. Oct 29, 2004 #2
    You know [itex]E_{gravitational potential}=mgh[/itex], where h is height off the ground and m is mass. I believe you must assume that when the ropes make an angle of 0 with the vertical h=0. Does that help?
  4. Oct 29, 2004 #3


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    Pretend that its like those incline questions.
  5. Oct 29, 2004 #4
    I did assume that when the angle is zero E=0 and when the angle is 90 E=mgh=64.8, so I thought the answer would be (sin30.9)(mgh), but that answer was incorrect. I also tried (sin59.1)(mgh) which was also incorrect, Im not sure where to go from here.
  6. Oct 29, 2004 #5
    Well I think when the kid swings up to make a [itex]30.9^\circ[/itex] with the vertical, his height above the ground is increased. By how much? The answer is

    [tex]R \cos (30.9^\circ)[/tex]

    So the kid's [itex]U=mg(R\cos (30.8^\circ))[/itex]
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