What it shows:

A straightforward demonstration of Newton's 3rd law, that forces are interactions and thus come in pairs.

How it works:

Two people, each sitting (cross-legged) on their own board, position themselves in the center of the track facing each other. Upon pushing against each other with their hands, they glide apart down the length of the track. Repeat this with one person turned around — the other person pushes on his/her back instead of pushing against each other with their hands. The ensuing motion down the track is exactly the same as before.

Alternatively, you can have one person on a board throw a heavy medicine ball ... the reaction force propels the person down the track. Catching the medicine ball is a nice example of an inelastic collision and conservation of momentum.

If the two people are significantly different in weight (mass), their accelerations down the track will also be significantly different, even though the force on each was the same — an example of Newton's 2nd law

Setting it up:

The two 10-ft-long tracks are laid down on the floor end-to-end and coupled together.

Comments:

The tracks are lightweight aluminum Skate Wheel Conveyors, 10 feet long and 18" wide (McMaster-Carr # 56755K58). The boards are solid oak and measure 15"×24"