Physics

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Video

Materials

• 1 plastic or paper cup
• 1 pair of scissors
• 1 plastic bag
• 1 rubber band
• 1 candle
• 1 matchbox or lighter

Short explanation

When you tap the plastic, you trigger a domino effect of air particles hitting each other - an "air puff" that propagates forward. The candle goes out because the air puff blows away the flame from the candle.

Long explanation

Air consists mostly of nitrogen molecules and oxygen molecules, but also of some water molecules, argon atoms, carbon dioxide molecules and more. We can summarize all of these particles as air particles, because in this context they work in principle the same way.

When you tap the plastic, you push the air particles on the other side of the plastic. These fly forward and in turn push the air molecules in front of them, and so on. This chain reaction continues towards the opening in the bottom of the cup, where the force from the pushing air particles is concentrated on a smaller amount of air particles, which then gain a higher speed. And then the chain reaction continues outside the cup.

How can the air puff go so far without being dispersed in the rest of the air? The puff is held together thanks to Bernouille's principle. This principle says that the faster air moves, the less it pushes on its surroundings. And vice versa - stagnant air pushes the most. In this demonstration, the air puff moves rapidly forward, while the surrounding air is stagnant. This means that the stagnant air acts as a large hug that holds the air puff together, which is thus not dispersed.

The air puff is actually not a round puff of air. It is more like a disc.

Why does the candle go out? The thing that burns in a candle is wax (or whatever the candle consists of) which has evaporated. The heat from the burning wax melts, evaporates and then ignites more wax from the candle and in this way the flame survives. But a puff of wind transports the burning wax vapor away from the candle, and prevents it from heating up new wax.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you make a bigger hole?
• What happens if you make a smaller hole?
• What happens if you use a shorter and wider cup?
• What happens if you tape two cups together into a single long cylinder?

Variation

You can build air cannons from everything. If you want to build a larger one, you can, for example, use a paint can and a large plastic bag.

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