Water that won't fall (cup with a hole)

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Experimental procedure and explanation:

• Previously, we conducted an experiment called “Water that won't fall”. Let’s study this a bit further.
• Fill a plastic cup with water, cover it with a lid, and turn it upside down. Even if you let go with the hand that is holding the lid, the lid will not fall, and the water inside also stays put.
• The reason that the water does not fall is often explained as “the water is being held in place by the atmospheric pressure from the bottom” or “it is supported by surface tension”, but the correct answer is slightly different.
• The force that keeps the water in the cup from falling is the pressure difference between the top and bottom of the water in the cup (= water density × gravitational acceleration × height). In other words, the water pressure on the upper surface of the lid is almost equal to the atmospheric pressure (more precisely, it is slightly lower than atmospheric pressure), and the water pressure on the bottom of the cup (i.e., upper part) is higher than this pressure by the amount equal to “water density × gravitational acceleration × height”.
• The force due to surface tension is small and does not provide much direct force to support the weight of the water. However, surface tension does play a role in supporting a small pressure difference between the outside air pressure (atmospheric pressure) and the water pressure near the rim of the cup where the outside air comes into contact with the inside water.
• Next, we experiment using a plastic cup with a small hole at the bottom (in this experiment, a pin vise was used to make a hole about 2 mm in diameter).
• The pressure at the top of the water inside the cup is also atmospheric pressure. Therefore, there is no pressure difference between the top and bottom of the water, and the force that supports the weight of the water is lost. As a result, the lid will separate, and the water inside will fall.

[Note]	Please be careful not to hurt yourself when creating a hole in the plastic cup. Please do this with the help of an adult.
[Keywords]	Depth and pressure, surface tension
[Related items]	Water that won't fall
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 60–61.p. 38–41 and p. 62–67. “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, p. 48–49 and p. 18–19.

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