PURPOSE: To illustrate the third law of motion.

DESCRIPTION: 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.

SUGGESTIONS: 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.

REFERENCES: This might be a controversial demonstration, in that any result can be obtained through the appropriate choice of sail geometry. See Demonstration Reference File for several interesting papers and demonstration references.

EQUIPMENT: Fan cart with plastic sail, as photographed.

SETUP TIME: None.

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