During my senior year of high school, as the final project of my AP physics class, students were expected to design and construct a catapult using any existing, researchable catapult designs with the goals of throwing a tennis ball the furthest, and throwing it a set distance, to prove the accuracy and precision of the catapult. My group consisted of me and two friends.
We considered a few main designs before settling on our final design, first looking at gravity-controlled catapults. One design that usually leads to consistent throwing distances consists of a controllable mass weighing down one end of a lever, quickly lifting the other end and throwing the projectile from that end. Throwing distances are generally controlled through adjustments of the mass weighing down the lever but could also be changed lengthening or shortening the arm.
We passed this option up in favor of using springs or large rubber bands to exert force on the arm because we didn’t think that we could achieve as fine control or as much power with a gravity-fed catapult as a spring-fed model. To save money, we pooled our resources and gathered a large number of spare trampoline springs as well as wood from which to build our frame and fasteners to hold it together, then got to work building the catapult.
The project constraints required that the entire catapult in its most compact state fit into a 1x1x1 meter envelope. None of us were particularly fond of having to spend our own money purchasing components for the project, so we mostly used things that we already had including scrap wood, galvanized pipe, power tools and long nails and screws. We quickly discovered how difficult and dangerous it was to pull back the arm during loading, so we purchased a few ratchet-straps to help pull the arm down slowly and without a human directly in front of it.
Every catapult ended up working pretty well, with some groups opting to use gravity-based throwing methods, some large rubber bands and some with arrays of springs. If the purpose of the assignment was to teach students to think mechanically and have fun planning and building something with friends, it was a success, since we all thoroughly enjoyed the building and testing process.
Getting the arm to consistently throw the same distance with a good amount of precision made up half of the credit of the assignment, but it was fairly difficult to achieve compared to maximizing throwing distance. This was mainly because the extreme pulling force generated by our trampoline springs deformed the holes that we created for a release-pin to be pulled from. In order to determine experimentally which initial arm angles corresponded with which throwing distances, we installed a board at a 45-degree angle from horizontal directly next to the catapult arm, marking with sharpie where the arm had been pulled down to, and after shooting and measuring the distance that the ball was thrown each time, we brought the arm back to that position then drilled and marked a release-pin hole to reproduce that throwing distance.
We used pine for our structure which is a fairly weak wood, so the release-pin holes began to widen over time and function poorly, leading to difficulties when pulling out the pin to fire. The catapult arm itself, which was a 4x4 pine board, broke in two while we were testing the catapult under the extreme loads due to the abrupt stopping that the arm experienced at the end of its short swing. We used steel and copper perforated duct straps as well as several wood screws to clamp the arm back into one piece. Had the arm broken in a brittle fashion, at 90 degrees, we would have had to replace the wood and re-drill most of the components. Luckily the break was along the grain of the wood, or the length, as it is a composite material, and binding it with duct straps worked very well to keep it held together.