The balloon carousel

As soon as the air flows out of the balloons, the rotor starts to move. The carousel turns. If you change the orientation of the balloons by 180 degrees, the carousel rotates in the other direction. (If you turn only one balloon around so that both balloons point in the same direction, the carousel also rotates, but less strongly).

The rod with a cross hole in the middle is placed on the pin of the wooden stand. Attach a wire loop to each end of the stick to secure the balloons. The balloons are attached to this. Make sure that the wire loop is not too wide, otherwise the balloons will not be held tight enough and will fly away. Make sure that the outlet openings of the balloons are perpendicular to the rod axis and point in the opposite direction. Now inflate the balloons, squeeze them with your hands and open them as synchronously as possible so that the air can flow out of both balloons at the same time.

• 1 wooden stand with pin,
• 1 wooden stick drilled through the middle (rotor),
• 2 balloons,
• wire loop (for attaching the balloons),

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 balloons. The taut balloon exerts a force on the air inside it. The air escapes from the balloon (actio) and the balloon experiences a force in the opposite direction (reactio). The special orientation of the balloons ensures that the wooden stick is set in rotation. Another explanation of the principle of recoil is the conservation of total momentum (law of conservation of momentum). Inside an inflated balloon, the air molecules move back and forth randomly at different speeds. In the process, they repeatedly collide with the rubber skin and are reflected by it. These impacts are the cause of the pressure inside the balloon, which ensures that the balloon retains its shape. Each particle undergoes a change in momentum due to the reflection. The total momentum is = 0, which means that the balloon does not move. If the opening of the balloon is released, air molecules escape from the opening of the balloon, taking momentum with them in the outflow direction. However, in order for the total momentum to remain = 0, the balloon must experience an equally large momentum in the other direction. The balloon therefore moves in the opposite direction to the direction of ejection when the air escapes.

What the experiment has to do with everyday life

One example of the recoil principle is the jet propulsion of airplanes, spaceships or rockets and the propulsion of motorboats. Some animals also move forward according to this principle. For example, octopuses hurl water backwards to propel themselves. The recoil can also be felt when a rifle bullet is fired. In everyday life, you can try out the recoil principle as follows: if you stand on a boat in a stationary position on a lake and then jump into the water, the boat experiences a recoil. The boat moves in exactly the opposite direction to the jump.

You can also easily demonstrate this principle by standing on a skateboard and throwing things away.

Carolin Schneider & Bastian Fleck