Balloon Car 2 (Nozzle Size)

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Let’s experiment to see how changing the size of the nozzle on “Balloon Car 1” affects its performance.
• Attach a balloon to a dolly (in this video, a 45 cm balloon is used).
• Insert a vinyl hose inside the balloon’s opening to keep it from collapsing. This time, we will compare two hoses: one with an outer diameter of 18 mm (inner diameter 15 mm) and one with an outer diameter of 11 mm (inner diameter 9 mm).
• Inflate the balloon and then release it. The car will start moving on its own. The car with the thicker hose runs faster and travels farther.
• This movement demonstrates the principle of “Jet Propulsion”: when air is expelled forcefully backward from the balloon, the reaction force pushes the car forward. This is an example of Newton’s third law of motion (action and reaction).
• The propulsive force depends on both the velocity of the airflow and the flow rate (the volume of air flowing per unit time): (Propulsive force) = (air density) × (flow velocity) × (flow rate).
• When the nozzle is narrow, energy loss due to viscous friction increases, which slows down the flow velocity. Additionally, the flow rate decreases because the cross-sectional area (the area of the nozzle’s circular opening) is smaller. Since flow rate = flow velocity × cross-sectional area, a smaller area means less air passes through over time. Therefore, a narrow nozzle reduces the propulsive force for these two reasons, leading to poorer balloon car performance.
• To improve performance, the nozzle should be relatively large to maximize flow rate and velocity. However, if it is too large, the air escapes too quickly, shortening the duration of the jet and reducing effective propulsion.
• This video was produced with the support of the JSPS Grant-in-Aid for Scientific Research (18K03956).

[Keywords]	Jet propulsion, law of momentum, action–reaction
[Related items]	Jet Propulsion, Balloon Car 1
[References]	Ryozo Ishiwata and Mitsumasa Nemoto, “The Wonder of Flow,” Kodansha Bluebacks, pp. 114–117. Ryozo Ishiwata, “Illustrated Fluid Dynamics Trivia,” Natsume Publishing, pp. 156–157.

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