# How could i extract the mass matrix and the stiffness matrix from equation M *X''+K*X=0 what i am asking for is M(5X5) AND K(5X5) c

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Mohamed Amer on 28 Oct 2017
How could i extract the mass matrix and the stiffness matrix from equation M *X''+K*X=0 what i am asking for is M(5X5) AND K(5X5) ci tried but what i get the matrix combiend betweeen M and K to M *X''+K*X=0 so i tried to make 2 equations but i think its worng because the stiffness matrix is not symmetric using the equationsToMatrix order from matlab
Karan Gill on 30 Oct 2017

Vaidyanathan Thiagarajan on 2 Nov 2017
Edited: Vaidyanathan Thiagarajan on 2 Nov 2017
Hi Mohamed,
I am assuming you want to extract M and K matrices of a free vibration problem in MATLAB's PDE toolbox. You can use 'assembleFEMatrices' function to get the stiffness and mass matrix of a free vibration problem. For more details on 'assembleFEMatrices' please see the following link :
The following sample code extracts the M and K matrices of a free vibration problem using 'assembleFEMatrices' function :
N = 3;
model = createpde(N);
importGeometry(model,'Plate10x10x1.stl');
figure
hc = pdegplot(model,'FaceLabels','on');
hc(1).FaceAlpha = 0.5;
title('Plate with Face Labels');
E = 200e9; % Modulus of elasticity in Pascals
nu = .3; % Poisson's ratio
m = 8000; % Material density in kg/m^3
c = elasticityC3D(E,nu);
a = 0;
specifyCoefficients(model,'m',m,'d',0,'c',c,'a',a,'f',[0;0;0]);
applyBoundaryCondition(model,'mixed','Face',1:4,'u',0,'EquationIndex',3);
hmax = 1.2; % Maximum element length for a moderately fine mesh
figure
pdeplot3D(model);
FEM = assembleFEMatrices(model);
%Mass Matrix
M = FEM.M;
%Stiffness Matrix
K = FEM.K;

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