Hi guys,
You can talk about collisions with the ground. There is a happy ball (small elastic) and a sad ball on the cart. The sad ball does not bounce (at all). You can talk about elastic, totally inelastic, and coefficient of restitution. Its also fun to trick a student with these balls by secretly switching the sad for happy.
There is also a large rubber ball and a smaller volley ball. The large ball has horns (its a spherical cow). Drop the large ball with small ball placed on top. Small ball bounces very high and big ball stays essentially on the ground. Can explain by elastic collision. Calculate assuming small ball is 1/3 mass of big ball. You can point out that this is the same trick that is used to "slingshot" spacecraft off of Jupiter: the big ball is Jupiter, and the small ball is the spacecraft.
There is a wooden block standing upright on the cart. A happy and sad ball are tied to strings and can swing and collide with the block. Note happy ball has red marker. Start at 90 degrees and let the happy ball swing into the block. It should knock it over. The sad ball cannot knock it over with similar swing. Again momentum conservation.
The air track is set up with 2 carts. Elastic and inelastic collisions can be shown. Turn carts so that velcro is facing in to get inelastic. Can also add 100 g to one of the 300 g carts and repeat elastic collision. This is done by attaching the 2 small 50 g masses to the screws on the sides of the cart.