function yprime = cannon(t,z);
z1 = z(3) * cos(z(4));
z2 = z(3) * sin(z(4));
z3 = -D*z(3)*z(3)/15. - g*sin(z(4));
z4 = -g*cos(z(4))/z(3);
yprime = [z1 z2 z3 z4]'; %don't know why i need this after making canode23
% what in the world changed from ode23.m to canode.23?