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Cut the water film

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What type of experiment is this?

Experimental procedure and explanation:

  • When water is applied to a board (a chopping board in this experiment), a wide film of water is formed.
  • If you insert a knife into this film, the film will be cut from it. Once a cut is made in the membrane, the surface tension acts to collect the water and the cut widens. At this time, water moves in the direction that reduces the surface area, so it is called "the work of surface tension".
  • If you insert a small object such as a bamboo skewer, the film is connected behind it, but the film starts to break a little downstream. This is because if the object is small, the membrane will be connected behind the object because of the Coandă effect (the function of the water jet trying to wrap around behind the round object), but the flow is slower than in other places. This is because the function of inertia is weakened and the film is easily broken. Therefore, the film cuts faster than in other places.
  • Water is an important resource. Be careful not to waste water.
[Keywords] Inertia, surface tension
[Related items] Make a film of water
[Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P15, P62-67
“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P208-209, P48-49

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Last Update:3.3.2017