## Friday, November 11, 2011

### The Physics of a Potato Cannon

When I was in middle school my friends and I decided to build our own potato cannon. It was really easy, affordable, and a lot of fun. Recently my friend brought it up and I was curious about how it actually worked. After a little bit of roaming the internet and class lectures it became pretty clear how the cannon works.
How we built or potato cannon was simple. We took two pieces of PVC piping, one piece had a twist off cap and the larger chamber was connected to the smaller chamber. A hole was drilled on the side of the PVC pipe in it we placed a grill igniter. For fuel we used hairspray, gas, ect., basically anything that would combust with a spark. So now how does the design help shoot the potato out?
First we can look at how pressure and fluid motion work together in the chamber of the cannon. We have studied that when fluids flow from larger areas to smaller areas the rate of flow increases in the smaller areas. Thus, the staggered build of the cannon; the design of the large chamber connected to the smaller chamber helps achieve an increase in flow, which pushes the potato forward. The pressure that builds up in the chamber as the gas ignites will push the potato forward out of the barrel.
Next you can look at the energy created due to the explosion of gas. This reaction releases an enormous amount of energy which is going to help propel the potato out of the barrel. At the same time the explosion of gas results in an enormous expansion in volume, which is going to greatly increase the rate of flow in the small tube. The shape of the barrel will focus the energy released by the explosion down the smaller tube. The barrel itself is going to do negative work on the potato through frictional forces of the barrel and air resistance. The greater the pressure in the large chamber the greater the difference in pressure between the two chambers. All of these concepts are going to shoot the potato out of the barrel at a tremendous rate, that’s how a potato cannon works!