Two Plastic Bottles

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• Place two plastic bottles on their ends next to each other, and blow air in between using a dryer. (A guide is also used so the bottles will not move backwards.) You can also blow air using a straw.
• The two bottles are pulled closer to each other.
• A similar phenomenon takes place with a single plastic bottle, but the principle is different.
• With two plastic bottles, the flow narrows, and at the gap, the flow is faster (more kinetic energy) while the pressure is reduced by the same amount (lower pressure).

[Note]	The explanation that pressure at an area where flow exists is lower (than the ambient pressure) because velocity there is higher, in accordance with Bernoulli’s theorem, is incorrect. Even if you blow air with a dryer or with your mouth into an empty space, the pressure will remain almost at the atmospheric level. Bernoulli’s theorem describes the energy conservation law in a fluid. Forcing air to flow increases its energy relative to that of the surrounding air. Therefore, Bernoulli’s theorem cannot be used to compare the moving air with the surrounding, stationary air. Please be careful about this as several books have used this erroneous explanation.
[Keywords]	Bernoulli’s theorem, equation of continuity
[Related items]	A Single Plastic Bottle (Note the difference in principle)
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp.102-107 “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, pp. 194-195 and 126-127