That's a nice model, I just have one criticism about the motion. It appears the balls slow down before striking their neighbours, and this is not correct from a physics point of view. In fact, the ball would be travelling at its fastest at the exact point of impact, transferring its energy through the series of balls and sending the other outer ball up and out. The best way to think of it is like there was only one ball, swinging like a pendulum. At the lowest point of the pendulum's arc it is travelling fastest, and in the Newton's Cradle model it is at that point that the ball stops dead, and the ball at the other end continues the arc. It's a relatively minor thing, and shouldn't be too hard to fix (remove any temporal smoothing from those keyframes), but I think it is important to get the physics right in an animation of a model originally constructed to demonstrate a physics concept. Great work, and and excellent idea for an animation btw.
