Tae Kwan Do Discrepant Event

Authors: Laurie Aiello and Troy Labnow
Discrepant Event - Teacher's Guide
SED 695B; Fall 2005
Detailed Explanation of Discrepant Event

Principles Illustrated:

  • Inertia
  • Impulse
  • Air Pressure
  • Surface Area


  • Two pieces of newspaper
  • A piece of wood 1 1/4 in wide 18 in long and less than 1/4 inch thick


  • Lay the wooden slat half off the edge of a table.
  • Place the newspaper over it, smooth the paper so there are no gaps.
  • Strike the end of the wooden slat with a karate type blow (quick and hard).


  • The wooden slat should break into two pieces without moving the paper off the table.

Standards Addressed:

Students know a force has both direction and magnitude.
Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion.

Motion & Forces
1c Students know how to apply the law F=ma to solve one-dimensional motion problems that involve constant forces ( Newton's second law).


Questioning Script

Prior Knowledge & Experience:
Students are often unaware of the pressure exerted by the air around us. They know that massive objects can be employed to break the stick. They also know that one can easily lift the newspaper with a stick.

Root Question:
How can we break a piece of wood using only a piece of newspaper?

Target Response:
Inertia - Can the students relate the effect of air pressure (14.7 lbs per square inch) to the surface area of a newspaper (575 square inches) in the breaking of a stick?

Common Misconceptions:
Students often overlook Impulse - the change in momentum which is related to force and time as Impulse I = Ft where there is an inverse relationship between the amount of force and how much time it is applied.


Here is a good way to begin the demonstration.
Show the students how easy it is to lift the newspaper with the stick. If the stick is lifted slowly, air has plenty of time to get under the newspaper, equalizing the pressure on both sides, so that the only weight you must lift is that of the newspaper.
Here a stack of books weighing 20 lbs is quite obvious to the students that the stick will break if you apply pressure to the end of it.

If the newspaper is lifted quickly, air has little time to move underneath so that you are moving not only the newspaper but the column of air above the paper as well since there is no compensating pressure underneath.
The result is that with a quick (little time) force (a hard karate chop) illustrates impulse. Less time and a lot of force breaks the stick.

The rapid acceleration of the paper when the stick is struck attempts to accelerate the column of air above the newspaper. Here the students can calculate the area of the newspaper and relate it to the pressure of the air on the newspaper. Pressure = force over area P = F/A. Air pressure is 14.7 psi and the area of an average newspaper in 575 sq inches. That is a total of 8450 lbs or about 4.22 tons of force on the newspaper.

References & Links:

Newspaper Drawing

Tae Kwan Do Discrepant Event

UCB Physics Air Pressure Demo

Science and Mathematics Initiative for Learning Enhancement SMILE

University of Illinois "Atmospheric Pressure"

Wikipedia's definition of "Impulse"

Glenbrook South Physics Page "Inertia"