How Potato Cannons Work
What does a party balloon have in common with a baking potato?
Well, if that tater is deep in the barrel of a potato cannon, quite a bit: They both demonstrate the principle of stored energy.
A potato cannon.
Stored energy, or potential energy comes in many forms – a taut rubber band, a battery, an apple about to be eaten. In the case of our balloon and potato, the energy is in the air – air that is under pressure.
Take a look at our simple U-shaped potato cannon – made of ordinary PVC pipe. The design is simple: a closed chamber on the right, a barrel where the spud is loaded on the left, and a valve in between.
The open-ended barrel is at normal atmospheric pressure. But with an air compressor, we can increase the pressure inside the closed chamber. We force more air into the same space, which means forcing more oxygen and nitrogen molecules into that space. The more you pump in, the more those molecules will push to get out. It's like cramming dozens of people in your bedroom … they will climb the walls, elbow each other and bang on the door to get out.
The air compressor raises the pressure in the 353-cubic-inch chamber to 80 pounds per square inch. That turns out to be a little more than 1 kilojoule of stored energy. A joule is a unit of energy – roughly the equivalent of what's required to pick up a potato. So with the energy stored in the potato gun, you could pick up 1,000 potatoes.
To release the cannon's stored energy, one flips a switch, making the valve between the chamber and the barrel open. That pent-up energy is then converted it into kinetic energy. A surge of air molecules propels the potato some 200 yards at 300 feet per second. The speed with which the 1-inch valve opens – in just 100 milliseconds – allows the compressed air to escape very quickly and forcefully. It's not unlike the surge of energy you get when you pop a balloon with a needle: molecules come streaming out with great force, colliding with other air molecules. Those countless collisions make the loud "boom" you hear after a balloon pops, or after the potato cannon is fired.
Related Links
Thanks to MagLab Research Associate and Cannonmaster Tim Murphy, the scientific advisor on this article.
Classroom Outreach
