Put the lid on the cup and submerge

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Flip a cup upside down, submerge it in water, and place a rubber board (or any other board that sinks in water) on top.
• When you let go with the hand that is holding the cup, the cup and rubber board floats. You can observe that the buoyant force acting on the air inside the cup is greater than the weight of the rubber plate (magnitude of gravity).
• Next, turn the cup right side up , cover it with a rubber board, and slowly submerge it in water. The rubber board will sink without coming off.
• The pressure exerted by the water on the upper surface of the rubber board is greater than the atmospheric pressure on the lower surface inside the cup. This results in a net downward force on the rubber board, which prevents it from being removed from the cup. The greater the water depth, the greater the pressure difference and the stronger the force on the rubber board.
• If you do not have a rubber board, then any board that sinks in water will work. However, plastic sheets or thin plastic boards cannot be used. They will keep floating under the influence of surface tension when placed on the water surface.

[Keywords]	Depth and pressure, buoyancy
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[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs,  p. 38–41 and p. 42–47. “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, p. 18–19.

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