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Homework Help: Rotational Kinetic Energy of merry-go-round

  1. Apr 1, 2010 #1
    1. The problem statement, all variables and given/known data

    A 31.0 kg child runs with a speed of 2.80 m/s tangential to the rim of a stationary merry-go-round . The merry-go-round has a moment of inertia of 520 kg\cdot m^2 and a radius of 2.51 m. When the child jumps onto the merry-go-round, the entire system begins to rotate.

    A) Calculate the initial kinetic energy of the system.

    B) Calculate the final kinetic energy of the system.

    2. Relevant equations

    3. The attempt at a solution

    I got the answer to part A by simply doing KE=1/2*m*v^2. The answer was 122 J.
    I'm not sure how to approach part B, however. I thought that because of conservation of mechanical energy that the initial energy would equal the final and I could use my answer from part A to solve part B. But this isn't working. Any help would be appreciated. :D
  2. jcsd
  3. Apr 1, 2010 #2


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    Homework Helper

    The KE energy is not the same as it was initially. There must be some interaction between the child and merry-go-round to set it into rotation and this consumes some energy. You can use conservation of angular momentum to calculate angular velocity, and calculate the rotational energy from that.

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