Physics Puzzler: In the Newton's Cradle, why do two dynamic balls eject two static balls
I was actually going to post something on this and then I figured I'd do a quick check of the available literature. Seems someone has explained the cradle in far more detail than I would care to so you can check out his explanation when you think you have your answer. Oddly enough there are differing opinions in physics on why things work the way they do.
http://www.lhup.edu/~dsimanek/scenario/cradle.htm
When you start to think about the dynamics try to explain in your own mind a 3 ball system with one ball swinging, a 3 ball system with 2 balls swinging, and finally a 5 ball system with 2 balls swinging. In any system, the number of swinging balls at contact will equal the number of balls at ejection. One of the things noted in the article is that all solutions to the problem tend to take into account balls of equal mass, size, density, and material and that things might change drastically if you start to introduce different compositions of balls. As an aside, I also found this article on half-bakery that talks of an extreme experiment.
http://www.halfbakery.com/idea/Gravity_20Waves2