Physics

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Video

Materials

• 1 balloon
• At least 10 m (33 ft) of string, fine enough to fit inside a drinking straw
• 1 drinking straw
• Tape

Short explanation

When the air shoots out through the balloon opening, it also pushes back on the balloon. This causes the balloon to move in the opposite direction as the air.

Long explanation

When you inflate the balloon, you fill it with air molecules. These molecules will bounce around inside the balloon, thus stretching its walls. This in itself is not strange, because the air molecules outside the balloon also bounce against the balloon walls, and want to compress the balloon. But since there are many more air molecules inside the balloon (the air pressure is higher), there will be more collisions there and the balloon will not be compressed but stretched.

In the event of a collision, each air molecule exerts a force on the inner wall of the balloon. When a balloon is tied shut, the same number of air molecule collisions occur in all directions. There is no net force in any particular direction.

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

The balloon rocket 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 rocket, through the collisions of the air molecules (air pressure), exerts a net force on the front inside wall of the balloon. The opposing force is the push back on the air from the balloon wall, since the balloon wall, due to its chemical composition, has a built-in resistance to stretching.

Real space rockets work according to the same principle as this one. But there the pressure inside the rocket is not from air, but from another gas that is released through a chemical reaction of the fuel.

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 use a balloon with a different shape?
• What happens if you blow more air into the balloon?
• What happens if you increase the inclination of the string?

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