With a trip to the drug store, the grocery store, and the hardware store, you can make a cannon that shoots potatoes over a hundred yards. All you need to do is know a little about gas laws. And promise you won't use this for evil.
Top image: RodimusPower on Flickr.
Here's a simple way to arm yourself. Grab a couple of several-foot lengths of PVC pipe, one with a three or four inch diameter and one with a one and a half to two inch diameter, and an adapter that lets one of the lengths be screwed into the other. Use a relatively small, perhaps two foot, length of the wider pipe and about four to five feet of the narrower one. Screw them together and carefully seal junction. Cap the wider end of the long pipe. You can buy a cap or make one, as long as it screws in tight, and can come off and on. Make a hole in the cap and stick a barbecue or fire lighter through, securing it carefully. You're done with your cannon.
Potatoes are key to this. If you leave the front end of the cannon sharp, you can literally cut a potato so that it perfectly fits in the barrel. Just jam a potato against the front and let the pipe slice off any parts of the potato that don't fit until the barrel is exactly filled. Use a pole to tamp the potato down nearly to where the pipe gets wider (but not actually in the wider pipe).
Finally, you need your fuel. Some people need gun powder in cannons. Not us. We just need a few sprays of flammable hair spray in the wide end of the cannon. Hold the cannon upright and spray in, then quickly cap it. The airborne particles of hairspray are now charging around the cannon. Light them, and the potato flies.
Why does this work? A couple of reasons. The easy physics is right at the surface. When particles heat, they move faster. When gas particles heat and move faster, they dramatically expands outward, increasing their volume. This literally pushes the potato out of the cannon.
There's another, less obvious reason that the gas expands. There is more gas. Avogadro's Law states that one mole (a mole is a set quantity of atoms) of any gas will be approximately the same volume as one mole of any other gas. So if there are more moles of gas at the end of the ignition, the overall volume of the gas will be larger than the gas before the ignition. Image by Sweejak on Flickr.
Of course, it's impossible that there is 'more' gas even though nothing has been added in the ignition. There are the same number of atoms. It's just that if you have a bulky gas, like butane, which is in many hairspray products, and you break its large molecules into smaller gases, like carbon dioxide and water, suddenly you have more moles than you do before. Take a look at this chemical equation:
2(C4H10) + 13(O2) ➞ 8(CO2) + 10(H2O)
The big term, far to the left is butane, loaded up with carbon atoms and hydrogen atoms. The next term is the oxygen in the atmosphere. When the two combine and combust, the butane is split into a lot of smaller carbon dioxide atoms and water, which here exists as a gas. The moles (which are denoted by the numbers just outside the parentheses) go from fifteen combined moles on the left side of the equation, to eighteen combined moles on the right side of the equation. And these new moles take up more space. The expansion caused by this larger number of moles launches the potato out of the cannon and at whatever hapless passerby walks into its path.
Please spud responsibly. There are videos of potatoes going through car fenders.
Top Image: USDA