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 (click for mpeg). Mount the sail and blow into it with your breath or with a fan; it moves in the direction which you are blowing (click for mpeg). Finally, with the sail in place turn on the fan while holding the cart. Notice the flexing of the sail (click for mpeg).

Q: What will the cart do with the sail in place and the fan operating when it is not held? Will it: (a) move in the direction that it moved with the sail off, (b) move opposite to the direction it moved with the sail off, or (c) remain at rest. A: It will remain at rest (click for mpeg).

SUGGESTIONS: Activate the fan with the sail in place while holding the cart to show the flex of the sail and increase your students' uncertainty. Ask the students to predict what will happen before doing the experiment.

See Question of the Week #46 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1H10.20) 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.