# Problem tracking a value to plot.

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Chett Manly on 7 Oct 2021
Hi, I am trying to plot an accel/decel curve but am struggling to record it. the program seems to run in its two phases but if I try and change V to a vector to follow the program won't run. Apart from that I believe it is correct...
clear; close all; clc
t(1) = 0.001; % time incriment in seconds
C = 0.008; % Drag constant
A = 30; % Accelleration part 1
Ad = -2; % Decelleration part 2
V = 0; % Velocity
N = 1; % iteration ticker
while true % Accel phase
Vnew=(V+t*(A-C*V.^2));
if V > 60
break
end
V = Vnew;
N = N+1;
end
while true % Decel phase
if V < 0
break
end
V = Vnew;
N = N+1;
end
plot(V);

Jakob B. Nielsen on 7 Oct 2021
You have a problem with indexing. Every time you say V=Vnew, your V is simply replaced with a new overwritten value. You have an iteration ticker you call N, use it!
Now, your Decel phase has no time factor associated with it, so you'll need to look at that. Right now your test subject accelerates to a full stop in one iteration. Imagine the forces involved :D
clear; close all; clc
t(1) = 0.001; % time incriment in seconds
C = 0.008; % Drag constant
A = 30; % Accelleration part 1
Ad = -2; % Decelleration part 2
V = 0; % Velocity
N = 1; % iteration ticker
while true % Accel phase
Vnew=(V(N)+t*(A-C*V(N)^2));
if Vnew > 60
break
end
N = N+1;
V(N) = Vnew;
end
while true % Decel phase
if Vnew < 0
break
end
N = N+1;
V(N) = Vnew;
end
plot(V);
##### 2 CommentsShowHide 1 older comment
Jakob B. Nielsen on 7 Oct 2021
At least you have something to go by now :) have fun with it! :D