# Matlab (calculate natural frequencies using eig function)

1. Dec 17, 2015

### Junichirol

1. The problem statement, all variables and given/known data
Determine the corresponding natural frequencies
M = [1 0 0 0;0 1 0 0;0 0 1 0;0 0 0 1];
C = [2 0 0 0;0 0 0 0;0 0 0 0; 0 0 0 0];
K = [10 -5 0 0;-5 10 -5 0;0 -5 10 -5;0 0 -5 10];
2. Relevant equations
A = [zeros (4) eye(4); -M/K -M/C]
[V,D] = eig(A)

3. The attempt at a solution
just need help on A = [zeros (4) eye(4); -M/K -M/C].
-M/C =
NaN NaN NaN NaN
NaN NaN NaN NaN
NaN NaN NaN NaN
NaN NaN NaN -Inf

I also try with -inv(M*C)
-Inf -Inf -Inf -Inf
-Inf -Inf -Inf -Inf
-Inf -Inf -Inf -Inf
-Inf -Inf -Inf -Inf

This matrix caused the [V,D] = eig(A) cannot work.
Any step that i missed up?

2. Dec 17, 2015

### Staff: Mentor

Is that really the complete problem your were given?

3. Dec 17, 2015

### Junichirol

For a system with the mass, damping, stiffness matrices given as below:
M = [1 0 0 0;0 1 0 0;0 0 1 0;0 0 0 1];
C = [2 0 0 0;0 0 0 0;0 0 0 0; 0 0 0 0];
K = [10 -5 0 0;-5 10 -5 0;0 -5 10 -5;0 0 -5 10];
Determine the corresponding natural frequencies by using Matlab eig function.