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I am having trouble with a problem

  1. Apr 17, 2009 #1
    I am given the second order nonlinear homogeneous DE and I am supposed to find the equilibrium solutions for it.

    u"+(u-1)u=0

    I tried substuting u'=v but got stuck when I wasn't given initial values and couldn't solve for the constants after integrating.
     
  2. jcsd
  3. Apr 17, 2009 #2
    If you use the substitution:

    [tex]u=y[/tex]
    [tex]v=y'[/tex]
    [tex]\frac{dv}{du}=\frac{y''}{y'}[/tex]

    from which we have:

    [tex]y=u[/tex]
    [tex]y'=v[/tex]
    [tex]y''=v\frac{dv}{du}[/tex]

    You end up with a first order DE:

    [tex]vdv=(1-u)udu[/tex]

    Which is separable. The inverse substitution in the solution gives then a new first order equation in [itex]y'[/itex], which can be solved because it is also separable. However the integral is complicated. The end solution will be:

    [tex]x=\int\frac{dy}{\sqrt{y^2-\frac{2}{3}y^3+K_1}}[/tex]

    Perhaps you have boundary conditions which can make life easier, although I think it will stay an unpleasant integral. Hope this helps a bit.
     
    Last edited: Apr 17, 2009
  4. Apr 17, 2009 #3
    There seems to be something wrong with the latex generation, so here my edit:

    I rewrote the equation as:
    [tex]\frac{d^2y}{dx^2}+y(y-1)=0[/tex]
     
  5. Apr 28, 2009 #4
    I have tried to solve several times a Laplace transform with I.V.P's. There is what it looks like:

    y" - y' -2y = F(t); where y(0) = 0, y'(0) = 0 and F(t) = 1 if 0<= t < 2
    = t^2 + 1 if 2 <= t < 5
    = t^2 + t if t => 5
    I'm not having problem setting up but having trouble once I have everything on the right hand side. Unless I'm not using me brain the partial fractions look insane.
     
  6. Apr 28, 2009 #5

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Please do not take over someone else's thread to ask a separate question. Start your own thread.

    Solve the problem with F(t)= 1. Calculate the values of y and y' from that function at t= 2 and solve the initial value problem with F(t)= t2+ 1. Calculate the values of y and y' from that problem at t= 5 and solve the initial value problem with F(t)= t2+ t.
     
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