Objects in Motion, Chapter 5 part 1, Conservation of Momentum.
I turned to a favorite book of mine, Gizmos and Gadgets, and found a bunch of activities relating to this chapter. Simple things, like giving them three coins--two pennies and a nickle--and challenging them move one penny using the other penny without them touching each other. They tried all kinds of things before finally figuring out how to use then conservation of momentum to do it.
And more complex things, like creating Newton's Cradle using marbles and an egg carton ramp. There's just something fascinating about watching spheres collide!
We finished up building a simple momentum tower, another idea from the book. I had two 20 oz bottles filled with water. The kids tied a string between them and then attached two strings with paperclips to the first string, and then stuck modeling clay onto the paperclips. They held the bottles so the string between them was taut and then carefully swung only one of the clay suspensions. Guess what happened to the other clay swing?