# Mysterious tangent line on my plots

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Frank Kostecki on 8 Apr 2021
Edited: Fangjun Jiang on 9 Apr 2021
I have no idea why, but every time I plot something like this, there is an unwanted straight line that shows up unexpectedly in the graph. Any reason this keeps happening?
Khalid Mahmood on 9 Apr 2021
How last value of t ca be zero, i.e t(end)=0? Values of variables are not provoided. This means, this code is incomplete portion of what is causing the problem.environment variables... possibly biggest culprit, clear all, especially t is essential. But after clear all, this incomplete code will not run ... as values of dt, D, rho, etc are not provoided in this code. The section of code that initialises the variables may have logical error. Other possibilities are still there, but I am waiting for values of variables, especially how are those initialised? through a code? Not yet told by Frank

Fangjun Jiang on 8 Apr 2021
Edited: Fangjun Jiang on 9 Apr 2021
Your code has problem. If you have "clear all" at the begining to clear the variables in the memory, then your code has error.
t has n elements. v has n+1 elements. So plot(t,v) will have error.
The resulting curve is due to whatever data left in t or v.
It is a good practice to pre-allocate memory for your variables.
t=zeros(n,1);
v=zeros(n,1);
and then do your for-loop.
##### 1 CommentShowHide None
Fangjun Jiang on 9 Apr 2021
This example code could illustrate the problem.
t=0:155;
v=sin(t*pi/400);
plot(t,v);
t(end)=0;
figure;
plot(t,v);

### More Answers (1)

Khalid Mahmood on 8 Apr 2021
Edited: Khalid Mahmood on 8 Apr 2021
v(1)=2; %initial v=v0=vi
a(1)=9.8; %g
n=40; dt=2.5; %use values like dt 0.1, 0.5 1.1,1.5, 2, 2.5, 5 and see the result
D=1;rho=0.75;area=.2;m=7.5;
s(1)=0;
t(1)=0;
for i=1:n
t(i+1)=dt*(i-1) %t0=0;
v(i+1)=v(i)+a(i)*dt; %vf=vi+at
a(i+1)=9.8-(D*rho*area/(2*m))*(v(i+1)^2);%acceleration
s(i+1)=s(i)+v(i)*dt; %position
end