Demos: 1H-02 Fan Cart (Action-Reaction)

A large fan is placed on a cart with low-friction wheels. The fan is turned on and the cart moves backward, as expected. If a large “sail” (rectangular card) is attached so that the air from the fan strikes it perpendicularly, the system will not move. Finally the fan is removed from the cart and the air flow directed toward the “sail,” which remains attached to the cart. The cart will then accelerate away from the fan.

Directions: The sail and fan are quite heavy, so when only the fan or sail is on the cart, be sure to counterweight the opposite end of the cart to prevent it from tipping.

Suggestions for Presentation: You might start with a situation in which someone is stranded in a boat but has a battery-operated fan. The stranded party decides to direct the fan toward the sail and propel the boat home. Will this work? What if the sail is canted at an angle so that the air strikes the sail at an angle rather than dead on. (This changes the sense of the demo. Initially we’re talking about Action-Reaction only, but if the sail is canted, vector components of the force is involved.

Applications: Compare this demo with other situations in which internal forces in a system cancel each other when the system as a whole is considered.

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