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Mysterious tangent line on my plots

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Frank Kostecki
Frank Kostecki 2021-4-8
编辑： Fangjun Jiang 2021-4-9
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?
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Khalid Mahmood
Khalid Mahmood 2021-4-9
and Fangjun Jiang, how can you say that.... That's very strange... Did you know values of variables before you commented that way? At least I found some values which cause the problem. Furthermore these are variables which are causing this, not MATLAB bult in error
Khalid Mahmood
Khalid Mahmood 2021-4-9
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

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Fangjun Jiang
Fangjun Jiang 2021-4-8
编辑：Fangjun Jiang 2021-4-9
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 个评论
Fangjun Jiang
Fangjun Jiang 2021-4-9
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);

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Khalid Mahmood
Khalid Mahmood 2021-4-8
编辑：Khalid Mahmood 2021-4-8
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

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