The Gauss rifle

Now, this is a cute little device.

It's science project season again! I use the opportunity to torture my boy with some actual science! This involves:

• formulating a question whose answer he doesn't actually know (and which isn't easy to look up somewhere);
• attempts at rigor;
• focus, careful practice, patience, and dealing with failure and/or unexpected results;

and so forth. Sadly, I'm not very good at this myself—I thought physics was interesting, but I could not solve the problems in college physics correctly to save my life!—so it takes us a while just to figure out what to do. The biggest challenge, I've realized, is acquiring reliable measurements.

Last year, his project struck me as quite original. You know how there's always a little bit of soda left at the bottom of the bottle when you're finished drinking? You could get it out if you were patient and determined enough, but you just toss it into the recyclying bin (or, if you're naughty, the trash can). Well, he looked at a bottle one day and asked, How many soda bottles would I have to save in order to collect enough of that liquid to make a free bottle? Admittedly, the usefulness of the solution might not be especially appealing, on account of the hygiene and all, but it was at least original. Sadly, it didn't impress the judges, who were more pleased by projects that I have seen done over and over again. Anyway, as I said above, taking measurements was an immense pain. It took us a few hours to figure out how to cut open the plastic bottles and collect the liquid inside (harder than you might think!)

This year, he wanted to work on electricity. Somehow, we wandered from there to the Gauss Rifle instead. The Gauss Rifle is a series of magnets, placed in a line, with balls arranged on one side. One places a ball on the opposite side of the first magnet and lets go. The magnets pulls the ball towards it; once it strikes the magnet, the energy is transferred to the last ball on the opposite side, which hurls off towards the next magnet. In this way, the magnets accelerate the balls to an impressive speed. The photo above is the Gauss Rifle that we built using neodymium magnets that I bought online.

Now, the boy just wanted to do a basic Gauss Rifle activity we found on the net: put it together, fire it, write something, that's all, folks! But I wasn't satisfied with just doing that, noooooo, I had ideas about doing "real" science rather than a prefabricated activity, and running a "real" experiment by changing a variable. So, he decided to determine what effect the distance between the magnets has on the projectile. If the magnets are spaced farther apart, will they accelerate the ball more or less? Actually, the website mentioned that question, too, but it didn't answer it, so I was okay with it. I don't know the answer myself (again, I struggled with college physics), although I think I do.

Again, the biggest problem has been figuring out how to measure the effect of the distance between the magnets. We first tried to measure how far the projectile travels. The study has a perfect floor for this sort of thing: flat, smooth vinyl. It was too perfect, actually; the ball bounced off the opposite wall. Argh.

Next, we tried the upstairs room, then the living room. These have carpeted floors, which provide enough friction to slow the thing down pretty quickly. Sadly (and as I expected) the carpets aren't consistent. We'd get wildly different distances. Each time we prime the gun, we have to lift it off the floor, then set it back down. If the angle's even slightly off, ball fires higher sometimes than others, which can mean anything from less friction on the carpet, to bouncing it against the floor, and so forth. For example: with one setting, the projectile traveled only 20cm on one firing, and more than 50cm on another. So, that didn't fly, either.

Back to the study. This time, we used the previous problem to our advantage, thanks to the stopwatch. We timed how long it took the ball to roll across the room. Great! ...but even then the measurements are inconsistent. For instance, when the magnets are two and a quarter inches apart, results vary from a low of 2.9 seconds to a high of 4.3 seconds. That was without any obvious problems like the tape breaking and setting a magnet loose, thereby stealing a little of the balls' energy. Arghhhhh...!

What should we take, then: the lowest figure? the median? the average, once we throw out the lowest and highest? quartiles? should we calculate some confidence intervals? margins of error? redo the experiment, greasing the floor each time?

You see the problem? My poor boy just wants to shoot a gun, and maybe learn about magnetism in the process. Me, I want to re-invent the theory, and maybe develop a maglev train in the process. :-)