# Trying to plot Eulers, but keep getting straight line

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Elizabeth Ng on 7 Dec 2019
Commented: Elizabeth Ng on 7 Dec 2019
Whenever I try to plot, it tells me that the matrix dimensions must agree and I get a straight line. This is the second part of a question (the loop is from the first part and I know that works) so I'm confused as to where my syntax has gone wrong after the loop D:
m=5;
I=0.03;
%l=R for my own notation convention
l=0.3;
a=4.8;
g=9.81;
c=0.69;
%initial conditions tell us at n=1, t(1)=0, theta(1)=0, omega(1)=0
theta(1)=0;
omega(1)=0;
delta=0.001;
alpha(1)=(m*l*a*cos(theta(1))-c*omega(1)-m*l*g*sin(theta(1)))/(I+m*l^2);
Fy(1)=m.*g+c*omega(1)/l.*sin(theta(1)); %forces acting on point O in y direction
Fx(1)=m.*a-c*omega(1)/l.*cos(theta(1)); %forces acting on point O in x direction
% reusing the Euler's Method from 2a for the required vales
for n=1:10000
theta(n+1)=theta(n)+omega(n)*delta;
omega(n+1)=omega(n)+alpha(n)*delta;
alpha(n+1)=(m*l*a*cos(theta(n+1))-c*omega(n+1)-m*l*g*sin(theta(n+1)))/(I+m*l^2);
t(n+1)=n*delta;
Fy(n+1)=m.*g+c*omega(n+1)/l.*sin(theta(n+1));
Fx(n+1)=m.*a-c*omega(n+1)/l.*cos(theta(n+1));
acmi(n+1)=-a+(alpha(n+1)*l*cos(theta(n+1)))-((omega(n+1)^2)*l*sin(theta(n+1)));
end
%splitting into x and y components from free body diagram using Newton's
%2nd law in ijk vectors results in:
acmj(1)=alpha(1)*l.*sin(theta(1))+(omega(1)^2)*l*.cos(theta(1));
acmi(1:10001)=-a.+(alpha(n)*l*cos(theta(n)))-((omega(n)^2)*l*sin(theta(n)));
%F=ma
Rx=m*acmi;
plot(t,acmi)
axis tight

#### 1 Comment

darova on 7 Dec 2019
Can you attach your initial equations/formulas?

Matt J on 7 Dec 2019
Edited: Matt J on 7 Dec 2019
You're getting a straight line because the right hand side of this line
acmi(1:10001)=-a+(alpha(n)*l*cos(theta(n)))-((omega(n)^2)*l*sin(theta(n)));
is just some scalar number. You have overwritten every acmi(n) calculated in the loop with this number and then plotted it. We have no way of knowing what you intended it to be instead.

Show 1 older comment
Elizabeth Ng on 7 Dec 2019
So basically just toss it into the loop instead of having it outside?
Matt J on 7 Dec 2019
No, that is unrelated to my advice. My advice was, don't overwrite the results of the loop with something trivial. Otherwise, the loop has no purpose.
Elizabeth Ng on 7 Dec 2019
Alright, thank you!