# Why do i get the error "Dimensions of arrays being concatenated are not consistent"

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Why do I get this error for one but not the other even though the two codes are the exact same.

This one runs fine:

clear all; close all

% Initial Conditions

th= 0.01*pi/180; % initial theta position (rad)

w=0; % inital theta velocity (rad/s)

g=9.81; %gravity constant (m/s^2)

L=0.3; % length of bar (m)

mB=2; % mass of bar (kg)

mA=1; %mass of collar (kg)

dt=.001;

I = (mB*L^2)/12;

Wa=mA*g;

Wb=mB*g;

x=(mB*L)/2;

z=(L/2);

for ii = 1:5000

A = [-1 0 0 -mA 0

0 -1 1 0 0

1 0 0 -mB -x*cos(th)

0 1 0 0 x*sin(th)

sin(th)*z -cos(th)*z 0 0 -I];

B = [0; Wa; x*(w)^2*sin(th); Wb-x*(w)^2*cos(th); 0];

X=inv(A)*B;

w = w + X(5)*dt;

th=th + w*dt;

time(ii)= ii*dt;

theta(ii)=th;

alpha(ii)= X(5);

omega(ii)=w;

Rx(ii)=X(1);

Ry(ii)=X(2);

N(ii)=X(3);

d_acc(ii)=X(4);

end

But this one gives me an error:

clear all; close all

% Initial Conditions

theta= 0.01*pi/180; % initial theta position (rad)

theta_V=0; % inital theta velocity (rad/s)

g=9.81; %gravity constant (m/s^2)

L=0.3; % length of bar (m)

mB=2; % mass of bar (kg)

mA=1; %mass of collar (kg)

dt=.001;

I = (mB*L^2)/12;

Wa=mA*g;

Wb=mB*g;

x=(mB*L)/2;

z=(L/2);

for i = 1:5000

A = [-1 0 0 -mA 0;

0 -1 1 0 0;

1 0 0 -mB (-x*cos(theta))

0 1 0 0 x*sin(theta)

(sin(theta)*z) (-cos(theta)*z) 0 0 -I];

B = [0; Wa; x*(theta_V)^2*sin(theta); Wb-x*(theta_V)^2*cos(theta); 0];

X=inv(A)*B;

theta_V = theta_V + X(5)*dt;

theta=theta + theta_V*dt;

time(i)= i*dt;

theta(i)=theta;

theta_a(i)= X(5);

omega(i)=theta_V;

Rx(i)=X(1);

Ry(i)=X(2);

N(i)=X(3);

d_acc(i)=X(4);

end

I was changing my code from the one that worked because I realized I messed up my dynamic equations and I ended up with the same error I mention in the title When I was trying to debug it by backtracking I realized that even my original code was giving me an error. I had another file saved with my orignal code and it runs.

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### Accepted Answer

William
on 23 Mar 2021

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