To illustrate the third law of motion.
Fan cart with plastic sail, as photographed.
Activate the fan with the sail in place while holding the cart to show the flex of the sail and make your students think. Ask the students to predict what will happen before doing the experiment.
With the sail unmounted, turn on the fan and release the cart; it moves in the direction opposite to that of the blowing air. Mount the sail and blow into it with your breath; it moves in the direction which you are blowing. Finally, with the sail in place turn on the fan and release the cart. Q: What will the cart do with the sail in place and the fan operating: (a) move in the direction of the air, (b) move opposite to the direction of the air, or (c) remain at rest. A: It will remain at rest.
Meiners, Physics Demonstration Experiments, Section 9-4.5, page 195. Lewis Epstein, A Self-Propelling Mechanism, TPT 8, 332 (1970). Steven R. Smith and Jerry D. Wilson, A New Design to Demonstrate Newton's Third Law, TPT 10, 208-209 (1972). Jerry Wilson, Newton's Sailboat, TPT 10, 300 (1972). Richard S. Murphy and Jack Van Geldren, Letter: A Note from the Real World, TPT 16, 260 (1978). David L. Mott, Letter: A No-Sail "Boat," TPT 16, 426 (1978). B. L. Blackford, The physics of a push-me pull-you boat, AJP 46, 1004-1006 (1978). B. L. Blackford, A push-me pull-you wind vehicle, AJP 49, 28