My favorite Make projects all seem to have something to do with things that other people might say "Don't try this at home." In this case we went out to the Make Magazine "Test Lab" to learn how to make a small steel ball fly across the room using magnets... good clean fun in my book. This Make project called "Gauss Rifle" by Simon Quellen Field is actually a really good way to demonstrate the transfer of kinetic energy from one object to another. When each nickel-plated steel ball hits one of the lined up magnets, its kinetic energy is passed on to the next ball in the line, making it move to the next magnet. The energy builds up with each collision until the last ball bearing is shot across the room. I keep thinking about when my brother and I played croquet in our backyard growing up and I'd send his croquet ball flying across the yard.
Probably the hardest thing to get your hands on for this project will be the four gold-plated neodymium-iron-boron magnets. Not something you usually find at the local 5-And-Dime. (Or maybe I was just looking in the wrong aisle.) But I'm sure Make Magazine can point you where to get them. Once you do, here's a safety tip: The magnets are very powerful, so make sure they are securely taped down or they might slam together and shatter. Then you'll have to go out and find more gold-plated neodymium-iron-boron magnets.
Do try this at home. But be careful out there. Adult supervision is always a good idea. And make sure to aim your Tabletop Linear Accelerator away from your little brother.
Download Instructions for the Tabletop Linear Accelerator (419.3 KB .pdf)