The balloon car
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Test observation
When the valve is opened, air flows out of the balloon. The carriage starts to move against the direction of flow.
Carrying out the experiment
The valve fork is placed on the back of the recoil carriage. The valve to which the balloon is attached is clamped into it. Then blow air into the balloon and close the valve. Once the balloon is inflated, place the vehicle on a flat surface and open the valve.
Experimental setup
Commercial balloon car (left) and home-made car (right)
- 1 recoil car,
- 1 valve fork,
- 1 balloon,
- 1 balloon valve,
Valuable tips for the success of the experiment
If you build the trolley yourself, it should be as light as possible so that the effect is particularly visible. The surface should be as smooth and even as possible.
Explanation for the teacher
What is technically behind the experiment
The experiment demonstrates the principle of recoil. The recoil principle is always effective when something is thrown away from a body or released in a certain direction. The remaining body then experiences a force in the opposite direction. The principle of recoil is a consequence of Newton's 3rd axiom, which states that whenever a body A exerts a force on another body B, body B exerts an equal force on A in the opposite direction. This principle is also known as the law of interaction or "actio = reactio". The two bodies involved here are the air and the balloon car. The stretched balloon of the balloon car exerts a force on the air inside it. This causes the air to flow backwards. According to the law described above, the air must now exert an equal force on the balloon and the car connected to it in the other direction. This is why the car moves in the other direction. The lighter the car is, the easier it will move. (Another explanation of the recoil principle is the conservation of total momentum. See explanation of the balloon rocket).
What the experiment has to do with everyday life
An example of the recoil principle in everyday life is the jet propulsion of airplanes. Some animals also move forward according to this principle. For example, octopuses hurl water backwards to propel themselves. Lawn sprinklers also use this principle to change their position. The recoil can also be felt when a rifle bullet is fired. You see this quite often in movies.
Practical teaching tips
You can also easily demonstrate this principle by standing on a skateboard and throwing things away.