Physics

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Video

Materials

• 2 drinking straws of equal width
• Electrical tape
• 1 pair of scissors
• Mounting putty

Short explanation

The air you blow comes out through the two corner holes on the propeller. As the air shoots out of there, it also pushes back on the propeller and makes it spin.

Long explanation

The air flow that is formed when you blow into the straw leaves the other straw at its two ends.

When you blow into the straw, you fill both straws with air molecules. These molecules will bounce around inside the straws and thus stretch the walls of the straws. This in itself is not strange, because even the air molecules in the air outside the propeller bounce against the walls of the straws, and want to compress them. But since there are many more air molecules inside the straws (the air pressure is higher), there will be more collisions there and the straws are not compressed but stretched.

Each air molecule exerts a force on the inner wall of the straw in the event of a collision. If the propeller had been completely closed off, there would have been the same number of air molecule collisions in all directions. There had been no net force in any particular direction.

But when there is an opening in a straw, the air molecules will not collide with the inner wall of the straw at that point anymore, but will move out through the hole instead. However, air molecules will still collide with the inner wall of the straw opposite the hole. This means that there is now a net force directed at this inner wall. This force pushes the straw in that direction.

The straw propeller illustrates Newton's third law of motion. It states that for each force there is an equally strong and opposite reaction force. In this case, the air inside the straw, through the collisions of the air molecules (air pressure), exerts a net force on the inside wall of the straw. The opposing force is the push back on the air from the straw wall, since the straw wall, due to its chemical composition, has a built-in resistance to stretching.

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 blow harder?
• What happens if you make the propeller straw shorter?
• What happens if you make the holes in the propeller straw smaller?
• What happens if you make the air gaps where the straws meet smaller?

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