So I have never solved a second order NON LINEAR ode before, but my current project has thrown one at me. I am going to use matlab to solve it when I have it in the correct state, but at the mo, I cant get it to that... Its three coulped equations, in terms of x,theta and x2. They are in a matrix, ma + cv + kx = force for each one. The way they are coupled though is a trigonometric relationship between x and theta and x2. I.E theta = f(x) x2 = f(x) and x changes with time. What should I do to include this relationship?