Plot inside for loop result in error

I am trying to plot kinetic energy as a function of time. I am using fplot inside a foor loop which results in two errors and a warning.

clear; clc
V = 5000;
d = 0.5e-2;
B = 1; 
q = 1.6e-19;
m = 1.7e-27;
% T = 2pi/omega 
% q V = 0.5 m v^2 
% m v^2 = 2 q V
% v = sqrt(2 q V / m)
% q v B = m v^2 / r
% r = 2 V / v B
T(1) = 0;
r(1)= 2*V/(sqrt(2*q*V/m)*B);
n=1;
while r(n)<5e-2
    V = n*5000;
    r(n+1)= 2*V/(sqrt(2*q*V/m)*B);
    T(n+1) = T(n) + (6.67588e-8)/2;
    n=n+1;
end
%2*V/(sqrt(2*q*V/m)*B)
n
%plot(T, r, '.-'), legend('Radius')
%xlabel('Time [s]')
%ylabel('Radius [m]')
V=5000;
t=zeros(1,n);
t(1)=1.03077640640442e-08;
for i=1:n
    vi=sqrt(2*(i-1)*q*V/m);
    vf=sqrt(2*i*q*V/m);
    vm=(vf-vi)/2;
    t(i+1)=t(i)+d/vm;
    ti=t(i);
    Ti=T(i);
    tf=t(i+1);
    Tf=T(i+1);    
    fplot(@(x) (q.*V./ti).*x,[Ti Ti+tf],'b')
    hold on
    fplot(@(x) 0.5.*m.*vm.^2,[Ti+tf Tf],'b')    
end
hold off

So the errors occurs at the end of the code when I'm trying to plot the functions.

If you have any other suggestions please tell as this code probably looks quite bad.

Answer 1

[Walter Roberson]

    fplot(@(x) 0.5.*m.*vm.^2,[Ti+tf Tf],'b')

is independent of the input, x. If you want to create a constant with that value, then you need to replicate the constant by the size of the input to produce a vectorized form:

fplot(@(x) 0.5.*m.*vm.^2 .* ones(size(x)),[Ti+tf Tf],'b')

That is what the warning is about.

The error occurs because on i = 2, Ti+tf > Tf so the interval [Ti+tf Tf] becomes backwards.

Since you are plotting a constant there anyhow, you could consider sort() around that vector.