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Homework Help: Semi-circle with small angle harmonic oscillations

  1. Oct 12, 2011 #1
    1. The problem statement, all variables and given/known data

    Consider a uniform semicircular disk of radius R, which rolls without slipping on a horizontal surface. Recall that the kinetic energy of an object is the sum of the translational kinetic energy of the centre of mass (point C) and the rotational kinetic energy about the Centre of Mass.

    Using Lagrangian methods, show that the angular frequency of small oscillations is

    ω = sqrt([8g]/[R(9π -16)])

    2. Relevant equations

    .5mR2 = Ic + mh2 where h = 4R/3π

    L = K - U

    dL/dq = (d/dt)(dL/dq')

    3. The attempt at a solution

    First thing to do is find the center of mass of the object

    so i solved for that to get (x,y) = (o, 4R/3π)

    Then using that I solved for the moment which equals .5mR2

    Using the parallel axis thheorem i found that

    .5mR2 - mh2 = Ic

    K = .5mv2 + .5Ic2

    U = mghcos(∅) ( i think)

    This has no R dependance so the Euler Lagrange only acts on theta and omega

    problem is it's not work, so i obviously messed up on my kinetic and potential energies.
  2. jcsd
  3. Oct 13, 2011 #2


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    How are you defining the angle φ? If it's the way I think you're doing it, the potential energy has the wrong sign.

    Your expression for the kinetic energy is wrong, though I think it's just a typo. It should be
    [tex]K=\frac{1}{2}mv^2 + \frac{1}{2}I_c \omega^2[/tex]You left out the ω. What is your expression for v in terms of ω?
  4. Oct 13, 2011 #3
    Yeah i know i didnt transcribe it from my notes well sorry. I did manage to figure it out though. Not sure how to write out phi. Eaaier to draw it
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