Sinking a Japanese Penny Underwater

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Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• When you drop a Japanese penny into water, it will flutter as it sinks. If you repeat this several times, you will see that it rarely lands at the same location.
• When an object encounters a flow, a separation generally takes place (vortices form behind it). These vortices, called “separation vortices,” usually detach from the object and flow downstream.
• With a disc shape like a Japanese penny or with a ball-like object, the formation of vortices becomes irregular and the object will receive inconsistent forces from the fluid. Therefore, it flutters as it drops.
• The knuckleball in baseball and the no-spin shot in soccer obey the same principle, resulting in inconsistent changes in the trajectory of the ball.

[Keywords]	separation vortex
[Related items]	Sinking Japanese Penny in Oil
[Reference]	"The Wonders of Flow," Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 174-179