Spin around

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Make a ring from polystyrene foam board (2 mm thick, sold at home improvement stores, etc.) and attach a small balloon around it. If you don’t have a polystyrene foam board, you can use thick paper that is not too heavy.
• If you blow it from below with a dryer, it will spin around.
• The spinning in the air is due to the two functions of the flow.
• ①  The flow bends along the balloon, causing a force to be exerted on the flow downward (lower right downward shown in the explanatory figure), and as a reaction, the force acts upward on the balloon. This force acts to buoy the whole structure. This is the same principle as "A Balloon that Floats at an Angle".
• ②  The flow hitting the balloon also generates a force called air resistance. This air resistance is the force that rotates the whole structure.

[Keywords]	Coandă effect, air resistance
[Related items]	A Balloon that Floats at an Angle
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P128-133

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