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Remove a ping-pong ball from water

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What type of experiment is this?

Experimental procedure and explanation:

  • Submerge the plastic bottle in the water tank, fill it with water, and turn it upside down.
  • Put a ping-pong ball on the mouth of the plastic bottle.
  • Gently pull up the plastic bottle and remove it from the water. The ping-pong ball does not fall.
  • When the ping-pong ball is underwater, the ping-pong ball has an upward buoyancy and will not leave the plastic bottle.
  • When a ping-pong ball comes out into the air, the same thing happens as in "Stick a round object in a plastic bottle".
  • In other words, the water pressure in the gap between the plastic bottle and the ping-pong ball is almost atmospheric pressure. At the top of the plastic bottle, the water pressure is lower than the atmospheric pressure by the height of the water.
    (Difference in vertical pressure) = (Density of water) × (Gravitational acceleration) × (Difference in height)
  • The water pressure at the bottom is almost atmospheric pressure, but it is slightly lower than atmospheric pressure because of the amount that supports the weight of the sphere. It is the function of surface tension that creates this slight pressure difference.
[Keywords] Depth and pressure, surface tension
[Related items] Water that doesn’t fall, Stick a round object in a plastic bottle

"Trick improvement cheat sheet," TPM Age Editorial Division, Ed., JIPM Solution, P74-75.
“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