Balloon Car 1 (Mechanism)

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What type of experiment is this?

Experimental procedure and explanation:

• Let’s make a simple toy called a balloon car.
• Attach a balloon to a small dolly or cart (in this video, a 22 cm balloon is used).
• If available, insert a pipe or a similar object into the balloon’s opening to keep it from collapsing—though this is optional.
• Inflate the balloon and then release it. The car will start moving on its own.
• 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).
• Therefore, the greater the amount of air expelled in a short time, the stronger the propulsive force—although this also means the force acts for a shorter duration, so the car accelerates quickly but only briefly.
• It can be fun to hold races to see which balloon car moves the fastest or travels the farthest!
• A related experiment is the “Wind-powered vehicle,” which uses the same jet propulsion principle. Instead of a balloon, it employs a propeller that blows air backward to propel the vehicle forward.
• 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, Wind-powered vehicle, Balloon Car 2
[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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