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Project Overview
This project is one of my earlier ones, completed in a group of four as part of my physics class. The objective was to design and build a tennis ball launcher that could accurately launch a projectile into a small bucket at a specific distance. We were tasked with researching how to approach this challenge, developing a viable design for the launcher, and constructing it ourselves. Along the way, we encountered numerous challenges, requiring multiple revisions and new ideas. The project was constrained by the need to rely almost exclusively on elastic potential and mechanical energy. Additionally, we had to use readily available materials, and the launcher needed to fit within a 1m by 1m by 1m space.
Designing
Before beginning the design process, we performed some basic calculations to determine the energy required to launch the tennis balls over the specified distance. We then cross-referenced these calculations with measurements on the potential energy available from elastics to estimate the resources we would need. During the design phase, I took the lead in creating the parts, measuring lengths and cuts, and building nearly every component on my own. The remaining tasks were delegated to my team members. For the overall concept, I chose a ballista-style launcher, and my team agreed with the idea. I immediately began designing the structure of the launcher, including the size and angles of all the cuts. We used standard 2x4 lumber for the construction.
Building
Several key decisions needed to be made during the construction of the ballista. The first was determining the method for launching the tennis balls, and the second was figuring out how to adjust the strength of the launching mechanism. We chose to run an exercise band through a PVC pipe where the tennis ball would sit. The band was attached to a hook at the back to "load" the ballista. There were some challenges with this design, as the band would often slip under the ball, but after making some adjustments, this issue was resolved. To control the launch distance, we decided to place multiple pegs on the crossbar, allowing us to change the power quickly and reliably.
Design Choices
Several design choices were made during the planning and construction of the projectile launcher. To build the frame, we used a table saw and a mitre saw to cut the necessary lengths and angles. The wood was then secured using wood glue and screws. For the elastic component, we experimented with different types of exercise bands, ultimately selecting the least tense one, which provided more than enough power. To secure the band once loaded, we used a gate hook with an attached string. Loading the launcher involved pulling the band back from the sides into the gate hook, which would close automatically. To release the tension, a string attached to the hook allowed us to simply pull and shoot the ball. For the pipe, we found a PVC pipe that was small enough for the ball to fit but large enough to minimize friction.
Results and Final Thoughts
During testing, we found that our ballista performed very well, consistently shooting the balls into the bucket (about 1m in diameter, roughly 25m away). However, we noticed that the shooting distance degraded over time. We discovered that this was due to the tape we had applied to cover the rough cuts on the PVC pipe (where the band passed through), which wore away with use. To address this, we redesigned the firing mechanism by attaching the band to a smaller, shorter PVC pipe, which acted as a "scoop" for the ball. While the concept was difficult to explain, it worked too well. On the day of the competition, our launcher shot the tennis balls at least 5m past the target, even on the lowest setting.
Here are some photos of the construction process of the ballista
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