Use break instead of return. As when you use return, any code after it won't be executed
"Be careful when you use return within conditional blocks, such as if or switch, or within loop control statements, such as for or while. When MATLAB reaches a return statement, it does not just exit the loop; it exits the script or function and returns control to the invoking program or command prompt."
m = 80; %kg the mass of the sprinter%
F_x= 400; %N%
L = 100; %m the lenght of the race%
p = 1.293; %kg/m^3 the density of the air%
A = 0.45; %m^2 the cross-sectional area%
Cd = 1.2; %the drag force%
w = 0; %air velocity
time = 10;
dt = 0.1;
n = round(time/dt);
D = zeros(n,1);
a = zeros(n,1);
v = zeros(n,1);
x = zeros(n,1);
t = zeros(n,1);
a(1)= F_x/m;
v(1)=0;
x(1)=0;
D(1)=0;
t(1)=0;
for i = 1:n-1
a(i+1) = (F_x - D(i))/m;
v(i+1) = v(i) + a(i)*dt;
D(i+1) = 0.5*p*Cd*A*(v(i)-w)^2;
x(i+1) = x(i)+ v(i)*dt + 0.5*a(i)*dt^2;
t(i+1) = t(i) + dt;
if x(i+1)>=100
break
end
end
f1 = plot(t,x);
f2 = plot(t,v);
f3 = plot(t,a);
