Breaking A Disposable Chopstick

Let's take a look!

Watch the video on YouTube.

What type of experiment is this?

Experimental procedure and explanation:

• Place a disposable chopstick on a table so that it is slightly overhanging the edge.
• First, we hit the chopstick on the table with another stick, and the chopstick on the table did not break.
• Next, we laid a sheet of newspaper on top of the chopstick on the table, pushing the newspaper down so that there were no gaps. This time, when we hit the chopstick on the table with another stick, the chopstick broke.
• The principle behind this phenomenon is often explained simply as “atmospheric pressure acting on the top of the newspaper”, but this explanation is not correct. Even if the newspaper is laid on the chopstick properly with no gaps, the underside of the newspaper is still at atmospheric pressure. An appropriate explanation is required to describe why the pressure on the underside of the newspaper drops at the moment when the chopstick is hit with the stick. (This video proves that the chopstick can be broken even if there is a gap between the newspaper and the table.)
• At the moment when the chopstick is hit with the stick, a gap is created between the newspaper and the table, and the surrounding air flows into this gap. Viscous resistance acts as a brake on the air flowing into the gap, and a pressure difference is generated under the newspaper because the parts of the newspaper in contact with the outside are at atmospheric pressure, and the parts near the chopstick are at low pressure. Air accelerating in the gap also increases this pressure difference. The low-pressure areas that are formed inhibit the newspaper’s inversion.
• The acceleration of the surrounding fluid also produces forces that act to increase the air pressure on the upper side of the newspaper, further inhibiting the newspaper’s inversion (Added mass: see “Master of Karate”).
• In other words, the viscous friction of flow in a gap between the newspaper and the table and the acceleration of the air around the newspaper (both top and bottom) slow down the movement of the newspaper, and the chopstick breaks.

[Note]	This experiment can be dangerous if the chopstick or stick hits a person or object, so be sure to conduct a safety check with an adult before performing the experiment. When considering the forces acting on an object under atmospheric pressure, be sure to correctly understand the situation and consider the pressure on both the front and the reverse sides, rather than on one side only.
[Keywords]	Flow in a gap, Viscous friction, Added mass
[Related items]	Quickly Lifting a Plastic Sheet, Master of Karate, Added Mass
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P180－181. “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P182－185、P198－203.