v=1;
dt=0.05;
x=[0 0 10 10];
y=[0 10 10 0];

for i=1:4
   plot(x(i),y(i),'.'),hold on
end

d=20;
while(d>0.1)
   x(5)=x(1);y(5)=y(1);
   for i=1:4
      d=sqrt((x(i+1)-x(i))^2+(y(i+1)-y(i))^2);
      x(i)=x(i)+v*dt*(x(i+1)-x(i))/d;
      y(i)=y(i)+v*dt*(y(i+1)-y(i))/d;
      plot(x(i),y(i),'.'),hold on
   end
 end