My timeSpan is considered a nonscalar operator when using if statement. Help.

Hello,

I am having trouble with my code. I tried experimenting with other statements other than if but I can't seem how to make my code work. I want the values to change after 15 seconds in the scenario. I am still a novice to Matlab.

%Differential Equation Parachute

%Terminal Velocity Differential Equation
m = 80;
S = 0.65;
Cd = 0.45;
rho = 1.225;
v0 = 0;
g = 9.8;
timeSpan = [0 50];
   
if timeSpan > 15
 Cd = 1.75;
 S = 11.6127;
end


[Time,Vout] = ode45(@(t,v)(g-(rho/(2*m))*v^2*S*Cd),timeSpan,v0);

f2 = figure;
plot(Time,Vout);

title('Free Fall Terminal Velocity')
xlabel('Time (s)')
ylabel('Velocity v(t)')

Answer 1

[Cris LaPierre]

Your if statement is not doing what you think it is doing. To learn more about how an if statement works, I suggest going through Ch 13 of MATLAB Onramp.

To do what you want, you are going to have to create a function for your odefun and place the if statement inside the function. Ode45 solves the equation one time step at a time. You can use this example as a starting point.

Your final code might look something like this.

v0 = 0;
timeSpan = [0 50];

[Time,Vout] = ode45(@odefun,timeSpan,v0);

plot(Time,Vout);

title('Free Fall Terminal Velocity')
xlabel('Time (s)')
ylabel('Velocity v(t)')

function [dvdt] = odefun(t,v)
m = 80;
rho = 1.225;
g = 9.8;

if t > 15
    Cd = 1.75;
    S = 11.6127;
else
    S = 0.65;
    Cd = 0.45;
end
dvdt = (g-(rho/(2*m))*v^2*S*Cd);
end

[Marcos Dominguez]

Thank you very much!

The explanation of how ode works and showing me that the if statement has to be in a function statement made things a little clearer. I will definatly check out the resource you gave me to understand if statements better.