Plotting the results of 2 ODE45 in the same figure

Hello all,

I would like to plot the results of 2 ODE´s in the same figure however, I keep getting an error saying:

the first ODE is ok, but the second gives me the above error.

I saved 2 .m files

1° file: I named it forced.m

function yp = forced(t,y)
yp = [y(2);(((0.01/0.5)*cos(6.28*t))-((1/0.5)*(y(2)^2)-((20/0.5)*y(1)))];
    

2° file: I named it forcedd.m

function yp = forcedd(t,y)
yp = [y(2);(((0.01/0.5)*cos(6.28*t))-((0.011/0.5)*y(2)-((20/0.5)*y(1)))];
    

And then I wrote the main file to solve it

tspan = [0:1800];
y0 = [0;0];
[t,y] = ode45 ('forced',tspan,y0);
plot(t,y(:,1));
grid on
xlabel ('time')
ylabel ('Displacement')
title ('System Response')
hold on;
[t,y] = ode45 ('forcedd',tspan,y0);
plot(t,y(:,1));
hold off;

I just get the result of the first ODE, I tried changing variables but without sucess.

Any help ?

Thanks in Advance

Answer 1

[Alan Stevens]

You need to check the placing of your parentheses in the forcing functions. Try

tspan = 0:1800;
y0 = [0;0];
[t,y] = ode45 (@forced,tspan,y0);

plot(t,y(:,1));
grid on
xlabel ('time')
ylabel ('Displacement')
title ('System Response')
hold on

[t,y] = ode45 (@forcedd,tspan,y0);
plot(t,y(:,1));
hold off;

function yp = forced(t,y)
yp = [y(2);
     0.01/0.5*cos(6.28*t)-1/0.5*y(2)^2-20/0.5*y(1)];
end

function yp = forcedd(t,y)
yp = [y(2);
      0.01/0.5*cos(6.28*t)-0.011/0.5*y(2)^2-20/0.5*y(1)];
end

[Rodrigo Pena]

Problem solved !

Thank you !

[Rodrigo Pena]

Hello Alan, sorry to disturb you again.

what these y(2) represents ?

I want to make sure that I am writting the correct equation.

Thanks !

[Alan Stevens]

y(1) is X, y(2) is dX/dt.

[Rodrigo Pena]

Ok... Is my equation on matlab corresponds to the same equation as I wrote in the paper ? The results that I am getting from the plot looks wrong.

[Alan Stevens]

Your hand-written 2nd equation doesn't follow from the first as you didn't change the sign of the 20/0.5*X term.

[Rodrigo Pena]

yeah, my bad...

[Rodrigo Pena]

The idea is to solve this guy here:

[Alan Stevens]

The equations in my listing do that (for two different values of a).

[Rodrigo Pena]

Ok, thank you !