Pressure Exerted by the Atmosphere on a Sheet of Newspaper

The Idea

How much pressure does the atmosphere exert on a sheet of newspaper? As with the previous experiments, the force exerted by air pressure can be surprisingly powerful.

What You Need

• section of newspaper
• table
• piece of wood about 12 to 24 inches long and roughly 1 to 2 inches wide. (The wood should be thin enough so it can be readily snapped in half by someone who is not a black belt. It should also be stiff enough so it will break rather than flex if stuck. A ruler that you are willing to dedicate to the cause of science usually works.)

Method

1. Place the piece of wood on the table, extending approximately one-half its length.
2. Place a few layers of the newspaper over the wood. Lay it out so it is as flat as possible. Remove any "air pockets" that you can under the paper and make sure the edges are flat.
3. Using your best Maxwell Smart karate chop, strike the wood. Hit it hard enough to break the wood. Show it no mercy (Figure 37-1).

Expected Results

Many people would expect the wood to push the paper up and throw the paper partway across the room. However, if the paper is properly sealed over the wood, striking the wood results in the wood being pinned to the table and breaking apart as if it were clamped to the table.

Why It Works

Let's say you have a 1-inch wide ruler that extends 10 inches under the paper. This means that 147 pounds of air pressure is pressing down on the ruler. About the same as a medium-sized person standing on the paper holding down the ruler. Air pressure is that strong. If this doesn't work, it isn't because of insufficient air pressure. The ruler may not break if: air leaks under the paper from the edges, the wood is too flexible to break, or the wood is too thick to break. It is unnecessary, of course, to actually break the ruler to demonstrate the strength of air pressure on the paper.

Other Things to Try

Air pressure on paper can also be observed pressing down on the pages of a book. After interleaving the pages of two similar books, it will be very difficult, if not impossible, to pull the two books apart. This is not the result of the friction of the pages, but it is a direct effect of the air pressure holding the pages together.

The power of air pressure can also be demonstrated using a suction cup such as those used to pop out minor dents in car side panels. To do this you will need an object with a smooth surface. One good example is a laboratory stool.

1. Attach the suction cup to the top surface of the stool, as shown in Figure 37-2. Make sure the suction cup seals to the top surface of the stool.
2. Pull up on the suction cup.

The suction cup should be able to lift an average-sized stool up off the ground. There is a common misconception that a vacuum somehow pulls or "sucks" objects to it. This is not the case. Suction cups work because of a difference in air pressure between the outside of the suction cup and the little air trapped under the suction cup. The pressure on a suction cup that is 4 inches in diameter (assuming a perfect seal) would be greater than 150 pounds.

The Point

Air pressure exerts a force on a surface in proportion to its area.

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