Building a physics model in class

We are building a physics model in class. It is one of those chain reaction machines. The kind that one movement activates another function to accomplish another thing. We are using marbles, rolling down a ramp to hit another marble and eventually have one to cross the finish line. This is made out of paper tubes, and other things, all that can be gotten from one's house. Our project has many restrictions like time limit, trials, success, size and complexity. The project is 2 1/2 weeks long.