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Homework Help: Real quick question on 2nd order differential equation

  1. Apr 23, 2007 #1
    1. The problem statement, all variables and given/known data

    How do I go about solving [itex]d^2\theta/dt^2+ (g/L) \theta= g[/itex]? It's been 2.5 years since I had diff eq.

    2. Relevant equations


    3. The attempt at a solution

    I don't know. I've spent the past 2 hours going through old books and searching online and still can't figure it out :frown:
    Last edited: Apr 23, 2007
  2. jcsd
  3. Apr 23, 2007 #2
    Heres a quick and decent review of homogeneous and nonhomogenous second order diffyq's

    http://www.haverford.edu/physics-astro/MathAppendices/Differential_Eqs.pdf [Broken]
    Last edited by a moderator: May 2, 2017
  4. Apr 23, 2007 #3
    also since this looks a lot like a pendulum problem, curious as to the problem that gave rise to it, ie the pendulum would be the homogeneous soln where right side eqn=0.
  5. Apr 23, 2007 #4
    It's the problem a few posts below about a father pushing a child on a swing. If you have any other help I'd greatly appreciate it.
  6. Apr 23, 2007 #5
    i see, i'd just repost your query there. I'd help but gotta earn a living today
  7. Apr 23, 2007 #6


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    This is more math than physics but here is how: first ignore the "g" on the right hand side to get the "associated homogeneous equation" [itex]d^2\theta/dt^2+ (g/L)\theta= 0[/itex]. It's "characteristic equation" is [itex]r^2+ (g/L)= 0[/itex] which has imaginary roots: [itex]r= \pm \sqrt{g/L}[/itex] and so the homogeneous equation has general solution [itex]Ccos(\sqrt{g/L}t)+ Dsin(\sqrt{g/L}t)[/itex].

    Now, since the righthand side of the orginal equation was a constant, g, we "look for" a constant solution, y= A, to the entire equations. y'= 0 so the equation becomes 0+ (g/L)A= g and so A= L. The general solution to the entire equation is [itex]y(t)= C cos(\sqrt{g/L}t)+ D sin(\sqrt{g/L}t)+ L[/itex].
  8. Apr 23, 2007 #7
    Thank you so much, I can't even tell you how much I appreciate your help! I finally got it; using the initial conditions, C ends up as 1-L, and then taking the derivative and using the initial conditions again, D has to equal zero, leaving only the cosine term and the L. Thanks again!!!!
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