why is my plot not showing anything

function BTLnhom11
clear;
close;
syms t v;
disp('Chon chieu duong huong len')
disp('Chon goc toa do tai mat dat');
disp('Phuong trinh dinh luat II Newton cho ten lua');
fprintf('\t\tm*dv/dt = -v0*dm/dt - mg\n');
k=input('Nhap toc do dot nhien lieu dm/dt = ');
m0=input('Nhap khoi luong ban dau cua ten lua m0 = ');
y0=input('Nhap vi tri ban dau cua ten lua y0 = ');
v0=input('Nhap van toc day khi cua ten lua v0 = ');
g=9.81;
v=v0*log(m0/(m0-k*t))-g*t;
t1=m0/k;
disp('Gia toc cua ten lua a=');
a=diff(v,1);
disp(a);
disp('Phuong trinh chuyen dong ten lua y = ');
y= y0+int(v,t);
disp(y);
fprintf('Ten lua het nhien lieu tai thoi diem t= %0.2f s\n',t1);
fprintf('Tai t= %0.2f s ten lua khong con chuyen dong\n',t1);
fplot(y,[0 t1],'r');
title('Do thi bieu dien phuong trinh chuyen dong cua ten lua');
xlabel('Thoi gian t');
ylabel('Do cao(h)');
grid on;

I want to plot y, but it not working.

[Walter Roberson]

We would need to know the input values in order to test this.

[Quang Nguyen]

the input values are:

m0= 300000

k=2000

v0=1200

y0=0

Answer 1

[Walter Roberson]

m0 = 300000;
k = 2000;
v0 = 1200;
y0 = 0;
BTLnhom11(m0, k, v0, y0)

Chon chieu duong huong len
Chon goc toa do tai mat dat
Phuong trinh dinh luat II Newton cho ten lua
		m*dv/dt = -v0*dm/dt - mg

v = 

Gia toc cua ten lua a=

Phuong trinh chuyen dong ten lua y = 

Ten lua het nhien lieu tai thoi diem t= 150.00 s
Tai t= 150.00 s ten lua khong con chuyen dong

ans = 

NaN

function BTLnhom11(m0, k, v0, y0)
    syms t v;
    disp('Chon chieu duong huong len')
    disp('Chon goc toa do tai mat dat');
    disp('Phuong trinh dinh luat II Newton cho ten lua');
    fprintf('\t\tm*dv/dt = -v0*dm/dt - mg\n');
    g=9.81;
    v=v0*log(m0/(m0-k*t))-g*t
    t1=m0/k;
    disp('Gia toc cua ten lua a=');
    a=diff(v,1);
    disp(a);
    disp('Phuong trinh chuyen dong ten lua y = ');
    y= y0+int(v,t);
    disp(y);
    fprintf('Ten lua het nhien lieu tai thoi diem t= %0.2f s\n',t1);
    fprintf('Tai t= %0.2f s ten lua khong con chuyen dong\n',t1);

    figure
    fplot(real(y),[0 t1],'r');
    title('real -- Do thi bieu dien phuong trinh chuyen dong cua ten lua');
    xlabel('Thoi gian t');
    ylabel('Do cao(h)');
    grid on;
    
    figure
    fplot(imag(y),[0 t1],'r');
    title('imag -- Do thi bieu dien phuong trinh chuyen dong cua ten lua');
    xlabel('Thoi gian t');
    ylabel('Do cao(h)');
    grid on;

    limit(y, t, t1)
end

[Walter Roberson]

syms m0 k t
term = log(m0/(m0 - k*t))

term = 

int(term, t)

ans = 

Now look at the integral: it has t times the initial expression, but it also has log() of the negative of the denominator of the original expression -- the denominator m0 - k*t in the first expression becomes log(k*t - m0) in the integral. But as long as m0 is positive and m0-kt is positive, then the first one is log() of a positive number, but the second one is log() of a negative number. So there is inherently a complex result for the integral for positive m0, k and m0 > k*t. (And you build your k to be m0 >= k*t)