function ydot = ptf(t, y)
% PTF :  function for the paratrooper ODE


g = 9.81;
k1 = 1/15;
k2 = 4/15;
m = 80;

if t < 5
    alpha = k1 * y(2)^2;
else
    alpha = k2 * y(2)^2;
end;

ydot(1) = y(2);
ydot(2) = -g + alpha/m;
ydot = reshape(ydot,2,1);


