[ODE] more air friction

[barbara.yersin at urbanet.ch]

Ok guys. As you told me, I've added a force computed like this:
damping coeff * velocity.

I don't know why, but the ball's velocity seems to update (slow down) only if I click on the window . Otherwise the ball continues rolling and rolling... When I look at the evolution of the variables the "air friction like" force has an appropriate value, but the velocity still stay the same as long as I don't click on the window... 
Any idea?