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Inflate in a Single Breath

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What kind of experiment is this?

Experimental procedure and explanation:

  • We will inflate a slender bag. First, inflate it by putting your mouth against the opening. The bag only inflates by the amount of air blown in from the mouth, so it will take time to fully inflate the bag.
  • Next, blow air with your mouth approximately 10 cm from the bag opening. It takes only a single breath to inflate the bag. Apparently, more air went into the bag than was blown out from your mouth. How could this happen?
  • This occurred because the blown air pulls along surrounding air by means of viscous friction. As a result, the volume of air entering the bag is larger than the volume of air leaving your mouth.
  • There is another explanation for this phenomenon that is incorrectly based on Bernoulli’s theorem. That explanation goes like this: “Blowing creates a flow that is faster than the surrounding air; so, by Bernoulli’s theorem, the pressure of the flow is less than the surrounding atmospheric pressure. This lower pressure draws along surrounding air and increases the volume of the flow.” It is apparent from the Sprayer 2 experiment that this explanation is incorrect. If, in fact, the surrounding air gets drawn along, the flow speed at the center section will become even greater, thereby further decreasing the pressure, drawing even more surrounding air. According to this explanation, the flow speed will keep increasing as the flow goes further downstream. This makes no sense.
  • Note: Many introductory science books explain that per Bernoulli’s theorem, pressure reduces to below atmospheric pressure when you blow air rapidly or when air or water spouts out from straws and hoses. However, the pressure remains atmospheric when air or water is just blown, and the explanation that pressure reduces is incorrect. Bernoulli’s theorem describes energy conservation in a fluid at points upstream and downstream along one streamline. It is incorrect to compare blown air against surrounding, stationary air. Please look at the reference documents for details.
[Keywords] Viscous friction, Bernoulli’s theorem
[Related items] Sprayer 2
[Reference] “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, pp. 206–209, pp. 212–213.
Logergist “Physics Promenade” (1964), Iwanami-Shoten, pp. 153–157.


Last update: 1.21.2015