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Homework Help: Quick Lagrangian of a pendulum question

  1. Mar 5, 2012 #1
    1. The problem statement, all variables and given/known data

    Use the E-L equation to calculate the period of oscillation of a simple pendulum
    of length l and bob mass m in the small angle approximation.

    Assume now that the pendulum support is accelerated in the vertical direction at a rate
    a, find the period of oscillation. For what value of a the pendulum does not
    oscillate? Comment on this result.

    2. Relevant equations

    3. The attempt at a solution

    I've got the first bit:
    E.O.M.: d2θ/dt2+(g/l)sinθ=0
    d2θ/dt2+(g/l)θ=0 in the small angle approximation,
    which is S.H.M. with ω^2=√(g/l) (though I'm not sure about this as there's no minus sign in the E.O.M.) so T=2pi√(l/g)

    For the next bit, I just need help setting up the equations:
    So the generalized coordinates are θ and a.
    Are the following correct?:
    (taking the origin as the point from which the pendulum is swinging)
    Last edited: Mar 5, 2012
  2. jcsd
  3. Mar 5, 2012 #2


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    Science Advisor
    Gold Member

    Careful here, [itex]\omega[/itex] is that angular velocity in radians per time unit. Your T is the time to travel one radian (of the oscillatory cycle) not time per cycle. You need to multiply by [itex]2 \pi[/itex].
  4. Mar 5, 2012 #3
    Thanks for pointing that out, I'll correct that in the first post. :-)
  5. Mar 6, 2012 #4
    Any help would be great. :D
  6. Mar 6, 2012 #5
    Should y be -lcosθ+0.5at^2 instead?
  7. Mar 6, 2012 #6

    I like Serena

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    Homework Helper


    "a" is not a coordinate.
    Actually only θ is a generalized coordinate since the y coordinate of the support is constrained to be y=0.5at^2.
    The angle θ is the only freedom that the system has.
  8. Mar 13, 2012 #7
    Great, I've got it now.

    Thanks! :-)
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