What is wrong with my code?

% Itfinds the velocity using RK 4 method
clc; % Clears the screen
clear all;
F = @(t,v) 9.81 - ((0.28/85) * (v^2));
t0 = input('Insert the value of the initial time: ');
v0 = input('Insert the value of the initial velocity: ');
tfinal = 20;
h = input('Insert the value of the step size: ');
v = RK_4(F,t0,h,tfinal,v0);

Answer 1

[Ameer Hamza]

You need to write the RK_4 function. Right now, RK_4 is the name of your current script. You can find one implementation of the Runge Kutta-4 algorithm here: https://www.mathworks.com/matlabcentral/fileexchange/29851-runge-kutta-4th-order-ode

[Ibrahim Butt]

Can you please tell me what is wrong with my code now?

% It finds the velocity using RK 4 method
clc; % Clears the screen
clear all;
h = input('Insert the value of the step size: '); % step size
t = 0:h:20; % Calculates till v(20)
v = zeros(1,length(t)); 
v(1) = input('Insert the value of the initial velocity: '); % initial condition
F_xy = @(v,t) 9.81 - ((0.28/85) * (v^2));
for i=1:(length(t)-1) % calculation loop
    k_1 = F_xy(t(i),v(i));
    k_2 = F_xy(t(i)+0.5*h,v(i)+0.5*h*k_1);
    k_3 = F_xy((t(i)+0.5*h),(v(i)+0.5*h*k_2));
    k_4 = F_xy((t(i)+h),(v(i)+k_3*h));

    v(i+1) = v(i) + (1/6)*(k_1+2*k_2+2*k_3+k_4)*h % main equation
end

[Ameer Hamza]

In function handle

F_xy = @(v,t) 9.81 - ((0.28/85) * (v^2));

You have specified v as first input and t as second. But inside the for-loop, you are doing the opposite. Apart from that, your equations seems correct.

[Ibrahim Butt]

Thank you very much for helping me in correcting my mistakes and clearing my doubts.

[Ameer Hamza]

I am glad to be of help!!!