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Homework Help: Equilibrium solutions of a spherical pendulum

  1. Feb 19, 2013 #1
    1. The problem statement, all variables and given/known data

    A spherical pendulum consists of bob of mass m attached to a massless rod of fixed length R. The end of the rod opposite the bob pivots freely (in two directions) about some fixed point.

    For the conical pendulum (θ=constant) case, show that the conical angle is stable for θ< θ/2. That is, show that if θ=θ0+ε then oscillates about θ0 in harmonic motion. Plot the frequency of oscillation for angles 0<θ<∏/2. Comment on any interesting aspects of the curve.

    g = 9.8, R = 1.8824, m = 1.96706

    2. Relevant equations
    $$H=\frac{1}{2mR^2}\left(p_\theta^2 + \frac{1}{\sin^2\theta} p_\phi^2\right) + mgR(1-\cos\theta)$$

    3. The attempt at a solution

    I don't really know how to type this out too well, but I took the derivative of H with respect to the canonical momentum in theta to get the first derivative of theta, then took the time derivative of that. this yielded
    $$\ddot{\theta} = \frac{\dot{p}_\theta}{mR^2}$$
    The hamiltonian to find the first derivative of pθ and plugging it in yielded
    $$\ddot{\theta} = \frac{p_\phi^2}{m^2 R^4 \tan\theta\sin^2\theta}-\frac{g\sin\theta}{R}$$
    and was from here unable to find an equilibrium solution

    Places where pθ or p[itex]\phi[/itex] appear are meant to indicate pθ and p[itex]\phi[/itex] respectively, I just couldn't figure out how to do subscripts in latex
    Moderator note: I reformatted your equations using LaTeX. Let me know if I made a mistake.
    You guys are my last hope. Thanks!
    Last edited by a moderator: Feb 20, 2013
  2. jcsd
  3. Feb 23, 2013 #2


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    Homework Helper

    hi, welcome to physicsforums!
    You have done most of the hard work already. The question is asking about the behaviour of theta. So try to write the right-hand-side of your last equation in terms of just theta and constants.

    Edit: ah, also, I guess you know that [itex]p_{\phi}[/itex] is a constant. But you will need to write it out explicitly, or finding an equilibrium solution (using the form the equation is currently in) is difficult (as you know already).

    Edit again: no, sorry you don't need to write out [itex]p_{\phi}[/itex] explicitly. For some reason, I thought the equation in it's current form is bad. But it is fine in the form it is in now. So, the right-hand side is a function of theta only. And they want you to describe the behaviour close to [itex]\theta_0[/itex] How would you do this? (hint: it is effectively just a 1d problem now, so think of what you would do in a 1d problem).
    Last edited: Feb 23, 2013
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