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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


PENCIL SHOOT

Figure I. Figure II. Figure III.


Figure IV. Figure V. Figure VI.

Description

Equipment: Rectangular box with sides made of plexiglass and back and bottom made of aluminum. A hole has been cut in the front. The dimensions of the box are 12 inches by 12 inches by 12 inches. Inside the box, a small vice has been mounted on the bottom.

5 pound cylinder filled with carbon dioxide at a pressure of 900 pounds per square inch. At this pressure, most of the carbon dioxide has condensed to liquid.

Wooden dowel (6 inch long cylinder with diameter of 0.25 inches)
Square wooden board (6 inches by 6 inches by 0.5 inches)
47 inch long 2X4 with 50 inch brass tube mounted on top
Reinforced tygon tube

Step 1: The equipment is displayed to the audience and discussed. Although the experiment is called 'Pencil Shoot', a wooden dowel of approximately the same dimensions of a pencil is used. A wooden dowel splinters less than an actual pencil and is therefore safer. It is in other respects very similar to a pencil. The square wooden board is mounted inside the rectangular box using the small vice. The 'pencil' is inserted into the end of the brass tube and the reinforced tygon tube is connected to the brass tube and the carbon dioxide cylinder. (See Figure I., Figure II., Figure III., and Figure IV. for images of the setup.)

Step 2: The demonstrator puts on his safety goggles and is careful to face away from the experiment. He then presses very quickly on the handle of the gas cylinder. The 'pencil' goes shooting down the brass tube and busts through the wooden square! (This step happens much too quickly to actually watch.) (See Figure V., Figure VI.)

Explanation

Basic Ideas: A gas exerts pressure on all sides of the container which holds the gas. The amount of pressure is related to the energy of the gas and the amount of gas. The higher the energy, the more pressure is exerted, and the more gas is contained, the more pressure is exerted.

Step 1: During the setup, nothing exciting is happening. The important aspects of this step are to be sure that the small wooden board is securely fastened inside the vice, that all the tubes are properly connected, and that the brass tube is aiming at the wooden board.

Step 2: The gas inside the cylinder is being held under a very large amount of pressure. When the demonstrator presses the handle of the cylinder, some of the gas inside is released. This gas travels through the plastic tube until it reaches the small wooden dowel. The pressure inside the container is so great that there is enough force to propel the wooden dowel down the brass tube. The wooden dowel is accelerated so much, that it goes shooting through the small wooden board!


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Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia