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If you make a hole in the middle of a straw (answer)

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

  • The answer is "(3) Liquid below the hole flows out".
  • The pressure of the liquid where the hole is made reaches atmospheric pressure. Therefore, the pressure below the hole is almost atmospheric pressure both above and below, and the weight of the liquid cannot be supported, so the liquid flows out. Meanwhile, in the part above the hole, the atmospheric pressure is reached at the height of the hole, but because the top of the straw is sealed, the air pressure at the top is lower than the atmospheric pressure because of the height of the liquid, and it can support the weight of the liquid.
  • Watch the answer video and you'll notice a little liquid left at the bottom. This is because the weight of the liquid is supported by the effect of surface tension acting on the water surface above and below that part. This happens with relatively thin straws.
  • In this video, the answer was (3), but it may be answer (1) (all liquid flows out). For example, if the straw is slightly thicker, all the liquid may flow out. The effect of surface tension is reduced, and the water surface at the lower end (that is, the position of the hole) of the upper liquid collapses with a slight trigger (such as a slight impact), so the upper liquid also flows out. There are also other cases where the answer could be (1) if the water was made easier to flow, for example, by dissolving dishwasher detergent (surface tension becomes weaker) and applying a water-repellent spray on the inside of the straw (repels water).
[Keywords] Depth and pressure, surface tension
[Related items] Scoop liquid with a straw, Plastic Bottle with Holes
[Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P38-41.
“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P18-19.

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Last Update:4.13.2021