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Homework Help: Trouble with rotation problem

  1. Apr 27, 2007 #1
    1. The problem statement, all variables and given/known data

    A bicycle weighs 23lbf, including hollow wheels each weighing 2.5lbf. Calculate how much of the bike's kinetic energy is in the wheels. Ans = 36%

    2. Relevant equations
    1. I = mr^2 <----- i think this is 2mr^2 because of two wheels
    2. Kinetic Energy = 1/2mv^2 <---- translational of the bike
    3. Ke = I(omega)^2 <------ rotational of the wheels
    4. Ktotal = 1/2mv^2 + Iw^2

    3. The attempt at a solution

    I tried putting the rotational kinetic energy over the total kinetic then multiplying by 100 but it isnt working... Also im not sure if im supposed to double the rotational energy or not. I think that im only supposed to double the inertia. Either way i cant seem to arrive at the given solution. Am i doing something wrong?
  2. jcsd
  3. Apr 27, 2007 #2
    The wheels carry energy in two ways as you suggest. I would think then it would be 2*(1/2mv^2+Mr^2)/ (the translational kinetic energy plus the rotational of the whole bike)--perhaps you neglected the latter term in your calculation?
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