MATLAB: ODE45 Returning Wrong Signed Answer

MATLABode45plot

Hey, I have been struggling with this for the past few days. Basically, I have an equation that models the angular acceleration of a rigid body (pendulum). I want to integrate it twice (to get the plot of angle vs time). The angle should be decreasing, however, in my plot it is increasing. I can't seem to figure out why. The equation is correct, I suspect there is something I have implemented wrong.
Here is a sample of my code, first I have the function in its own file, then I use the main code with the input main. If you have any idea why it is producing the wrong answer please let me know
function dydt = phiddot
g=9.81; L=1; m=1;
dydt = [y(2); ((-6*g*sin(y(1)))/(L*(1+3*(sin(y(1))^2))))];
end
function maincode = main
t0=0;
T=10;
 y0 = [60;0];  
[ts,ys] = ode45(@phi_ddot,[t0,T],y0); 
plot(ts,ys(:,1),'r')
end

Best Answer

Hi h7,
I'm guessing that you want to start at 60 degrees and not 60 radians, so try
y0 = [60*pi/180;0];  
[ts,ys] = ode45(@phi_ddot,[t0,T],y0); 
plot(ts,ys(:,1)*180/pi,'r')   % plot in degrees
which does start out going negative.
In your example your function needed to be called phi_ddot in order to match the ode45 call.
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