Using ODE45 to solve a system of 2nd order ODEs equation

I am trying to solve

This is what I have written as my finction script. everytime I try to solve it once I type the initial condiitions to the Command Windown, I get "Show complete stack trace." How can I solve my problem?

function theta_dot = pendulum(t,x)

theta_dot = [x(2);
             -8.8*x(1)-1.8*x(3);
             x(4);
             1.8*x(1)+8.8*x(3)];
end

Answer 1

[Bjorn Gustavsson]

After fixing the sign-errors in your ODE-function this:

thetathetadot0 = [0 0 0 -1];
[t,theta] = ode45(@(t,y) pendulum(t,y),0:0.1:10,thetathetadot0);
plot(t,theta)

works just fine.

HTH

[Missael Hernandez]

I get:

Error in odearguments (line 90)

f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.

Error in ode45 (line 115)

odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in rg5y (line 2)

[t,theta] = ode45(@(t,y) pendulum(t,y),0:0.1:10,thetathetadot0); - Show complete stack trace

[Missael Hernandez]

fixed it! thank you!