MATLAB: ODE function does not seem to work

Question

Want to run this script to solve the function but it doesn't seem to work no matter if i use ode45 or ode23

t0=0;tf=20;

x0=[0;0.25];

[t,x]= ode45('sdof',[t0 tf],0);

plot(t,x)

function xdot = sdof(t,x)

k=3; c=1 m=3;

A=[0 1;-k/m -c/m];

xdot=A*x;

function xdot = sdof(t,x)

↑

Error: Function definitions are not permitted in this context.

Error using feval

Undefined function 'sdof' for input arguments of type 'double'.

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);

Answer 1

You need to make a few adjustments in your code.

This:

k=3; c=1; m=3;
sdof = @(t,x) [0 1;-k/m -c/m]*x(:);
t0=0;
tf=20;
x0=[0;0.25];
[t,x]= ode45(sdof,[t0 tf],x0);
plot(t,x)
grid

works for me, and produces this plot: