Not sure what's happening in this code and not sure where to come from here.

Question

Steve Avilesmejia 2020-12-2

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https://ww2.mathworks.cn/matlabcentral/answers/674503-not-sure-what-s-happening-in-this-code-and-not-sure-where-to-come-from-here

回答： Alan Stevens 2020-12-3

clc; clear all; close all
L = 30;
m = 68.1;
c_d = 0.25;
k = 40;
gamma = 8;
g = 9.81;
h = 0.1;
t = 0: h: 50;
n = length(t);
v = zeros(2, n);
v(:,1) = [0;0];
y = t(2)
for i = 2:n
  if y(1) <= L
dy = [y(2);
g - sign(y(2))*c_d*y(2)^2/m];
else
dy = [y(2);
g - sign(y(2))*c_d*y(2)^2/m - k*(y(1)-L)/m - gamma*y(2)/m];
v(:,i) = v(:, i-1) + fun(v(:,i-1), L, m, c_d, k, gamma, g)*h; 
  end
end 

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Image Analyst 2020-12-2

Can't you ask the person who wrote the code for you?

Steve Avilesmejia 2020-12-2

what im trying to say is that i dnt know why its not working, i wrote it

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Answer 1

Alan Stevens 2020-12-3

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You are not keeping track of parameters through time. Try

L = 30;
m = 68.1;
c_d = 0.25;
k = 40;
gamma = 8;
g = 9.81;
h = 0.1;
t = 0: h: 50;
n = length(t);
% You need dx/dt = v; and dv/dt = acceleration
% Simple Euler approach
x = zeros(1,numel(t)); % Allocate storage space
v = zeros(1,numel(t)); % Allocate stirage space
x(1) = 0;   % Initial position
v(1) = 0;   % Initial velocity
for i = 1:numel(t)-1
    
    x(i+1) = x(i) + h*v(i); % update position
    % Select acceleration
    if x(i) < L
        accn = g - sign(v(i))*c_d*v(i)^2/m;
    else
        accn = g - sign(v(i))*c_d*v(i)^2/m - k*(x(i)-L)/m - gamma*v(i)/m; 
    end
    v(i+1) = v(i) + h*accn; % update velocity
    
end
% Plot position and velocity vs time
plot(t,x,t,v),grid
xlabel('time'),ylabel('Posn and Vel')
legend('Posn','Vel')