function Yprim = pendel(t,Y)
% Y(1)= theta, Y(2)=d(theta)/dt, Y(3)=r
% 
%  dr
%  -- = f(t) is specified, it is controlled by the winch speed
%  dt
%
% the ode becomes
%
%  dY(2)     f(t)        r
%  ----- + 2 --- Y(2) + ---  sin(Y(1)) = 0
%   dt       Y(3)       Y(3)
%
% which becomes  first order system
%  dY(1)
%  ----  = Y(2)
%   dt
%
%  dY(2)      f(t)          g
%  ----  = - 2 ---- Y(2) - ---- sin(Y(1))           
%   dt        Y(3)        Y(3)
%
%   dY(3)
%   ----  = f(t)  % to be specified by the programmer!
%    dt
%
g     = 9.82;
alpha = 0.5 ; % alpha is the winch speed

th = Y(1);
dth = Y(2);
r = Y(3);

Yprim(1,1) = dth;

Yprim(3,1) = ...;

Yprim(2,1) = - 2*Yprim(3,1)/r*dth - g/r*sin(th);