function spring_pendulum_anim2( varargin )
%integrate equations of motion
[t , X ] = spring_pendulum_int ( varargin {:});
%extract length and angle in time
r = X(: ,1);
th = X(: ,3);
%find position of the pendulum bob:
rm = [ r .*sin( th ) , -r .*cos( th )];
%find bounds:
maxr = max(abs( r ));
xmin = min( rm(: ,1)) - maxr /10;
xmax = max( rm(: ,1))+ maxr /10;
ymin = min( rm(: ,2)) - maxr /10;
ymax = max([0 ,max( rm(: ,2))])+ maxr /10;
%create and clear figure
h = figure(1);
clf( h );
% create a circle and a square:
a = 0:pi/100:2*pi;
s = [ -1 -1 1 1 -1; -1 1 1 -1 -1];
xscirc = maxr /50*sin( a );
yscirc = maxr /50*cos( a );
xssquare = maxr /50* s(1 ,:);
yssquare = maxr /50* s(2 ,:);

%create the pendulum components
%draw the pivot (centered at origin) and the mass:
fill( xssquare , yssquare ,'g');
hold on
bob = fill( xscirc + rm(1 ,1) , yscirc + rm(1 ,2) ,'b');
%connect mass and pivot:
arm = plot([0 , rm(1 ,1)] ,[0 ,rm(1 ,2)] ,'r','LineWidth',2);
%plot track so far:
track = plot( rm(1 ,1) , rm(1 ,2) ,'k--');
%set axes to proper values:
axis equal ;
grid on;
axis([ xmin xmax ymin ymax ]);
%step in time:
for i =1:length( t )
%update all moving elements
set( bob ,'XData', xscirc + rm(i ,1) ,'YData', yscirc + rm(i ,2));
set( arm ,'XData',[0 , rm(i ,1)] ,'YData',[0 , rm(i ,2)]);
set( track ,'XData', rm(max([ i -100 ,1]): i ,1) ,...
'YData', rm(max([ i -100 ,1]): i ,2));
%pause for length of time step
if i < length( t )
pause( t( i +1) - t( i ));
end
end