Second order ODE application question

[cue928]

We are doing mass spring problems that stem from second order ODE's. I think my lack of linear algebra is hurting me once again in this section so any help would be greatly appreciated.
We are using a stiffness matrix of K = [ -(k1+k2), k2 (row 2) k2, -(k2+k3)]
Our first problem has the following values: m1=m2=1; k1=0, k2=2, k3=0
Setting up the stiffness matrix I got the following:
-2 2
2 -2
So this is my first stopping point: I thought you were supposed to then take the inverse of that but can you do that with a matrix that has a determinant of zero?

 

[tiny-tim]

hi cue928!

not really my subject, so I'm going to risk asking what could be a really dumb question …

if there's only two degrees of freedom, how come there's three parameters?

(and your general formula doesn't have rows summing to 0 …

maybe these are just the top triangle of a 3x3 matrix?)​

 

[cue928]

I don't know how to answer that. Since there is a zero value for two of them, maybe that has something to do with it? I honestly don't know.