%-------------------------------------------------------------------
% defining the ODE system
%   all equations in dimensionless form
%-------------------------------------------------------------------

function dydt=track_ff(t,y,t_scale,r_scale,u_scale,g_r0)

% radial coordinate of mass shell

  dydt(1) = y(2)*u_scale   *(t_scale/r_scale);

% velocity

  dydt(2) = - g_r0/y(1)^2  *(t_scale/u_scale); 
  
dydt = dydt';

%-------------------------------------------------------------------
