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Air Pressure- Paper and Ruler (Jenny Green)







Author

Jenny Green

Principle(s) Illustrated

  1. Air Pressure Forces
Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in theEarth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air one square centimetre in cross-section, measured from sea level to the top of the atmosphere, has a mass of about a kilogram and a weight of 63N (and a column one square inch in cross-section would weigh about 14.7 lb).

Materials: 

    1.  One or two full sheets of an ordinary newspaper.
    2. A stick of pine wood (ruler, paint stick, wooden pencil)

Procedure:

                    1.  Place the stick on a table with a smooth surface and let it protrude over the edge about 8 cm.
                    2.  Ask students to hypothesize what will happen when you hit the stick.
                    3.  Strike it and let students catch the flying stick.
                    4.  Place the stick back on the table like in point 1, and cover it with the newspaper flush with the
                        edge of the table.  
                    5.  Ask students to now hypothesize what will happen if you hit the stick now? (anticipated answer:
                        ' the paper will fly up' or 'paper will tear')
                    6.  Smooth down the paper with your left hand and strike the protruding end of the stick with your 
                        fist (a sudden sharp blow):  The stick will break.
                    7.  have students hypothesize how much stick you could leave out each time before the pressure is
                        not enough to hold the paper down.  

Standards

  • 1.1.1 - Science Standard
  • 1.1.2 - Science Standard
  • 1.1.3 - Science Standard

Questioning Script

Prior knowledge & experience:

Students know that newspaper is easy to rip and not very strong.  Paint sticks are thin and easy to break as well.  

Root question:

What will happen if I hit this protruding end of the stick?

Why was it necessary to smooth the paper down before hitting the stick?

What would happen if the protruding stick was slowly pushed down? Why?

How much weight was actually holding down the stick?

What would happen if you did this on the top of the mountain?

What would happen if you did this below sea level?

Target response:

By smoothing the paper down, there was almost no air under it, but a whole column of air exists above the paper, pushing down on the paper with atmospheric pressure.  This is about 1kg/cm squared. (14.6 or rounded 15 lbs per square inch).  In total weight or force pushing down on a 60 x 80 cm or 20" x 30" paper is roughly:  60 x 80 x 1kg = 4800 kg, or 20 x 30 x 15 lbs = 9000 lbs, which is close to the weight of two large station wagons!  It is therefore impossible to lift it with a thin stick. 

Common Misconceptions:

Most students think the stick will fly up and that you are stronger than the paper.

Photographs and MoviesVideo with background information on air pressure and newspaper strength


References

Reference 1 (give the title of the page and insert a link.  Don't just paste URL)

Reference 1 (give the title of the page and insert a link.  Don't just paste URL)
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