A Balloon that Inflates when Lifted

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• There is a balloon inside a plastic bottle.
• When you lift the plastic bottle, the balloon inside inflates. Why does this happen?
• The key is that there is a hole in the bottom of the plastic bottle, connecting the water inside with the water outside.
• Lifting the bottle creates a difference in water levels inside and outside the bottle. Where the water level is low, the pressure is high; where the water level is high, the pressure is low. As a result, the air pressure around the balloon becomes lower than atmospheric pressure (the pressure at the water surface outside), causing the balloon to inflate. This lower-than-atmospheric pressure is called negative pressure.
• The balloon inflates due to negative pressure around it, following the same principle as the “Balloon in a plastic bottle” experiment.
• The air pressure inside the plastic bottle can be calculated by the formula
  (Pressure difference) = (Density of water) × (Gravitational acceleration) × (Difference in water level).
• For example, if the difference in water level is 20 cm (0.20 m), using the density of water as 1000 kg/m³ (which is 1 g/cm³ when expressed in g and cm, but I will use 1,000 kg/m³ to be consistent with other values) and gravitational acceleration (approximately 9.8 m/s²), the pressure difference can be calculated as
  Pressure difference = 1000 × 9.8 × 0.20 = 1960 pascal (Pa) below atmospheric pressure.
  This pressure is about 1/50th of atmospheric pressure, a small but sufficient difference to inflate the balloon.
• Tip: Inflating the balloon a little before attaching it will weaken the rubber, making it easier to inflate during the experiment.
• This video was produced with the support of the JSPS Grant-in-Aid for Scientific Research (18K03956).

[Keywords]	Depth and pressure, negative pressure
[Related items]	When the plastic bottle is upside down, Balloon in a plastic bottle
[References]	Ryozo Ishiwata and Mitsumasa Nemoto, “The Wonder of Flow,” Kodansha Blue Backs, pp. 38–41. Ryozo Ishiwata, “Illustrated Fluid Dynamics Trivia,” Natsume Publishing, pp. 18–19.

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