Inputs must be floats, namely single or double.

I am trying to solve an ODE about newton's cooling law and plot it:

this is my script called newton.m

function dTdt = newton (t,T)

    t = (0:1:80)';
    syms T(t);
    
    
    dTdt = 0.1*(20-T);

end

and here is another script called newton_plot

function newton_plot
newton


    [x, y] = ode45(@(x,y)newton,[0 80],100);


    plot(x,y,'-o')

end

I tried bringing the newton function to the newton_plot function in the beginning of the newton_plot function. (not sure if it works)

I get errors like:

>> newton_plot
 
ans(t) =
 
2 - T(t)/10
 
Error using odearguments (line 113)
Inputs must be floats, namely single or double.

Error in ode45 (line 115)
  odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in newton_plot (line 6)
    [x, y] = ode45(@(x,y)newton,[0 80],100);
 

How can I get it to work?

Answer 1

[Bjorn Gustavsson]

When you integrate an ODE numerically there is no need whatsoever to introduce a declaration of T as a symbolic variable (the way you do it the input-argument T is even destroyed...). It should be enough to do this:

function dTdt = newton (t,T)

dTdt = 0.1*(20-T);

end

That should be enough to define your cooling-ODE. That is if your cooling characteristics are something like this:

HTH

[Ada-Natalia Luukkanen]

Thanks for your answer. I took off the dedundant lines. Now I get errors like this

>> newton_plot
Not enough input arguments.

Error in newton (line 4)
    dTdt = 0.1*(20-T);

Error in newton_plot (line 2)
newton
 

[Ada-Natalia Luukkanen]

once again, this is now my newton.m script:

function dTdt = newton (t,T)

    dTdt = 0.1*(20-T);
    
end

and this is my newton_plot.m script:

function newton_plot
newton

    [x, y] = ode45(@(x,y)newton,[0 80],100);

    plot(x,y,'-o')

end

[Ada-Natalia Luukkanen]

I got it working! Thank you again for your help :)

[Bjorn Gustavsson]

You're welcome.