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Homework Help: Help with mechanical energy dissipation, relative to potential and kinetic energy

  1. Dec 13, 2009 #1
    1. The problem statement, all variables and given/known data
    A 42 kg child on a 2.3 m long swing is released from rest when the swing supports make an angle of 32 degrees with the vertical.
    The acceleration of gravity is 9.8 meters per second squared. If the speed of the child at the lowest point is 2.34094 m/s, what is the mechanical energy dissipated by the various resistive forces (e.g. friction, etc.)? Answer in units of J

    2. Relevant equations
    PEg=mgh
    KE=.5(m)(v^2)


    3. The attempt at a solution
    Attempted to solve using the lowest point of the swing as zero for potential energy. PEg at release point - KE at lowest point = dissipated energy.

    PEg=mg(vertical component of swing length)
    KE=.5(m)(v^2)

    PEg=42(9.8)(2.3sin(32 degrees)
    KE=.5(42)(2.34094^2)

    PEg=501.66 J
    KE=115.08 J

    501.66-115.08=386.58 J =Answer

    This however did not work..please help?
     
  2. jcsd
  3. Dec 13, 2009 #2

    PhanthomJay

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    Gold Member

    You did not calculate the initial PE correctly. Draw a sketch and check your trig in determining how high above the low point the child starts.
     
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