You'll probably notice about a quadrillion other copies of this Instructable. The reason for that is that posting one was a physics assignment for everyone in our class.... But forget that. Mine's the best, anyway.
Welcome! One day, a bunch of crazy people decided it would be a great idea to fire a C rocket strapped to a matchbox car down a track. And then we actually did it. Are you that crazy?! Sure you are. Get on over, and find out how!
For the experiment, our goals were to successfully fire a rocket-strapped Hot Wheels/matchbox car down a track and take measurements of its position over time, using photogates. Some pictures of our mk.1 car and the photogate track can be found below.
Step 1: Build a Track
1) A track. We used a 4.8-m long 2x4, but anything long and narrow will work. There were ideas to use PVC piping, gutters, metal tubes, and other, stranger things. (Note that using PVC would require a small section to be run through the center of the tube and hold the rocket.
2) Some method of holding your guide-wires in place. Our first choice was two metal plates that we drilled holes into and tied the wires down on. The plates seemed to work pretty effectively as blast shields, too, but if one car gets stuck at the starting line, the rocket will EASILY burn a hole through the metal.
3) Wires. That's wire-s. Plural. Please don't use just one, it won't work. We tried aluminum and copper, and both seemed to work well enough, though copper has a lower melting point, if I remember correctly. The wires should be about 16- to 20-guage, and remember that you'll have to secure them somehow. You might try pulling a wire through a hole, then coiling it around a metal bar so that it would stay taut.
Assembling it should be easy enough, assuming you've the common sense. And if you don't, what are you doing on Instructables? Just run the wires parallel to the track.
Step 2: Endanger Yourself (Build the Car)
Rockets are dangerous. Yes, yes they are. So... be careful, alright? You seem nice enough. I wouldn't want you to suffer horiffically painful burns.
Here are three simple rules. The way you do this is: Do whatever, as long as you follow these rules. You may also want to keep in mind the distance between your photogates, as if the car is of the wrong size, it can mess with the gates' tracking.
Rule 1: DO NOT USE ONLY HOT GLUE. Trust the guys who've done this, if you use only hot glue to secure your rocket motor to the car, it will disconnect. And then you'll have this extremely volatile rocket motor (which is exploding semi-controlledly) flying around of its own will. This is bad. You do not want this to happen.
Rule 2: Be extra-sure. Corrupted data is better than somebody losing an eye, or two, or several patches of skin. I reccomend putting electrical tape (keep it taut, people) around the body of the car such that it will secure the motor without touching the wheels. If there's the slightest chance of your rocket coming loose, fix it.
Rule 3: Use something thicker than straws to secure the car to the wires. The accompanying picture is how to do this wrong. We did try straws, and the results weren't pretty. In the end, we decided on pen barrels. Crap pens are something like $3.00 a box at the supply store. Take out the ink to refill a good pen, and use the barrel to do something extremely dangerous. It's a win-win deal!
Step 3: Set Up the Photogates
Photogates are the essential part of this experiment. If you know of another method for accurately timing an object's position as it passes certain points, then feel free to use it, but the photogates worked extremely well in our experiments.
We used Vernier's photogates: http://www.vernier.com/probes/vpg-btd.html
Vernier also publishes free graphing software which you can use to setup your photogates. It can be used to input the distance between photogates, as well--you can have the computer calculate the car's velocity and acceleration at various points, and graph it for you. The graph of our results is included here. (Note that the graphs are of another car than the one on the intro page. We didn't have any pictures of the Matchbox Missile Mk. II, but it had two pen barrels instead of one straw. The straw managed to snap and kill the poor little Mk. I.)
Step 4: *Bang*
If all went well, you should have plenty of data from your photogates.
Schmadeke's physics class wishes you the best of luck in your future (insane) endeavors.