I use this project as a way to make the mathematics part of physics relevant to my 7th graders. They are calculating mass, speed, velocity, momentum, force, and acceleration and having fun at the same time. They have the freedom to design their own project but are constrained by the materials provided and the time allowed.
The materials are cheap and easy to acquire which is a recurring theme on my teacher budget (materials are on the next page).
I usually give students about 45 minutes of pure build time. This does not include the time taken to hand out materials. I usually hand out the materials and give them some planning time... then start the clock. Because my school has 45 minute periods, we can't do it all in one class period. I force them to build and test in two.
I have an area to drop that is 5.3 meters. You will want to find an area that is at least 4. The higher the better!
*The new national science standards, if your state chooses to adopt them, will place greater emphasis on the process and application of topics than on recall. This is a simple, yet effective way to asses what they know and are able to do (and aligns perfectly with the motion and forces standards).