A Simple Air Cushion

Let's take a look!

Watch the video on YouTube.

What type of experiment is this?

Experimental procedure and explanation:

• Seal the plastic bag, make a round hole in the side resting on the table, and attach a piece of cardboard to the other side with a string.
• When the string is pulled up and down, air enters the bag. When there is sufficient air inside, the air cushion is completed.
• Air enters the bag when the string is pulled up and down because when the string is pulled up, the pressure inside the bag decreases, and air flows into the gap between the bag and the table. Viscous friction acts on the flow in the gap, which creates a pressure difference between the outside air, which is at atmospheric pressure, and the low-pressure area at the center of the gap. The pressure difference also accelerates the air flow in the gap.
• In other words, when the string is pulled up quickly, air flow is generated in the gap, and a low-pressure area is formed between the plastic bag and the table, preventing the plastic bag from rising up off the table.
• When the plastic bag is pushed from above, the pressure inside the bag becomes larger than the atmospheric pressure, and the gap disappears as the bag is pushed against the table, preventing the air from escaping.

[Keywords]	Flow in a gap, Viscous friction
[Related items]	Quickly Lifting a Plastic Sheet
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P180－181. “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P34－37, P182－185.