ID Code: C3-12
Purpose: Dramatically demonstrate inertia
Description: A pencil is accelerated to almost the speed of sound by blasting it through a four-foot tube using a carbon dioxide fire extinguisher. The pencil will readily impale itself on a piece of 3/8" plywood. With a little bit of luck the pencil point will be virtually intact, although sometimes you need to re-sharpen it after the demonstration
Some videos have recently been added to the demo page. To see the action in slow motion, click for 600 frame per second or 1200 frame per second version. Be sure to notice the vibration of the pencil after it is impaled in the plywood.
Analysis of the 1200 fps video allows us to obtain an approximate value for the speed of the pencil as it emerges from the tube. The size of the plywood used in the demonstration was about 4 inches square; the length of the pencil is about 8 inches. A single frame shows the pencil traveling approximately 4 inches (10 cm) in 1/1200 second, or 12,000 cm per second, or 120 meters per second. This is greater than 1/3 of the speed of sound.
A way to place a maximum limit on the speed of the pencil is to use the dynamics of the system. The length of the tube, along which the pencil is being accelerated is about 75 cm. Assume that the force on the pencil is equal to the vapor pressure of the CO2 acting on the area of the pencil to determine the velocity of the pencil at the end of the tube: v = SQRT(2 P A L /m ), where P is the vapor pressure, A is the inside area of the tube, L is the length of the tube, and m is the mass of the pencil.