Equilibrium solutions in double well potential duffing oscillator

Ratpigeon
Messages
52
Reaction score
0

Homework Statement



I am trying to show that for a duffing oscillator described by
x''+2g x'+ax+bx^3=0
with a<0, b>0
the equilibria at x=+- \sqrt{-a/b} are stable

Homework Equations


I used coupled equations, and the characteristic equation of a linear system

The Attempt at a Solution



Coupled equations, x and x' related by
d/dt(x,x&#039;)=[[0,1],[-a-bx^2, -2g]](x,x&#039;)

Setting x=+-\sqrt{-a/b} gives the characteristic is

\lambda^2+2g \lambda+a+b(-a/b)=0
But this can't be right since, i know from literature that the eigens are
\lambda 1=-g + \sqrt{g^2+2a}
\lambda 2=-g- \sqrt{-g^2+2a}
and this gives eigens 0 and -2g
 
Last edited:
Physics news on Phys.org
After you have reduced the equation to the system and found the equilibrium point, you have to linearize the system about the equilibrium point, and then find the eigenvalue of the linear system.
 
Since no one has chimed in yet, I'll throw in my 2 cents (and it might not be worth even that). I believe that one way to examine the behavior near an equilibrium point is to "linearize" the equation in the neighborhood of the equilibrium point. So, you could let z = x-xo where xo is an equilibrium point. Rewrite the differential equation in terms of z but keep only terms linear in z. Then find the characteristic equation for this linearized differential equation.

[Edit: Wow, I did not see voko's response. Sorry.]
 
I'd done all the linearising stuff - I found my problem; I needed to use a Jacobean matrix, instead of just a system
 
To solve this, I first used the units to work out that a= m* a/m, i.e. t=z/λ. This would allow you to determine the time duration within an interval section by section and then add this to the previous ones to obtain the age of the respective layer. However, this would require a constant thickness per year for each interval. However, since this is most likely not the case, my next consideration was that the age must be the integral of a 1/λ(z) function, which I cannot model.
Back
Top