PURPOSE: To illustrate the third law of motion.

DESCRIPTION: Place a fan cart on a dynamics track and release the cart; it moves in the direction opposite to that of the blowing air. Mount the a 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:

EQUIPMENT: Fan cart with plastic sail, as photographed.

SETUP TIME: None.

Keywords: Newtons' third law