## Natural frequency of large degree of freedom system

I have a spring-mass system with 200 masses and 199 springs. All masses are 100 tonnes and stiffness 20 MN. The boundary conditions are fixed-free.

I have constructed a lumped mass matrix and stiffness matrix and calculated the lowest natural frequency. Including the boundary conditions I calculated this to be 0.0124 Hz.

However, when I calculate the wave speed, I can calculate the period of the wave starting at one end, reflecting at the free end and returning. This frequency (1/T) is more than double the natural frequency I calculated. The wave speed I calculated was 141.4 m/s assuming the distance between masses is 10 metres.

Am I using the wrong approach here for calculating the natural frequency both ways?
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 How did you calculate the speed of the wave in this system? And what kind of wave?

 Quote by dxdy masses are 100 tonnes and stiffness 20 MN.
What does it mean stiffness 20MN? You, maybe, meant 20 MN/m?
And how are they connected? In a simple chain mass-spring-mass-sping-mass....mass-spring-mass, or rather in some mesh?

## Natural frequency of large degree of freedom system

Yes I did mean 20 MN/m.
Solved.

 Tags frequency, natural, speed, wave