The Physics of Catapults

When approaching the “Matter in Motion” unit in my physics class this semester, I thought a lot about how to make the concepts we would be learning (acceleration, average speed, velocity) meaningful and experiential for my students.  

I was inspired by a neat little projectile one of my students made in his free time.  Using a ballpoint pen and a spring, the student explained that the plastic tubing of the pen served as a guide for the projectile (the smaller ink tube) while the spring, usually used to pop the pen out for use, could be held in tension with a clip, storing enough force to send the ink tube a few feet across the room.  image03-copyHe tried adjusting his design to increase the distance of the projectile and ended up with several different types of pen ballistics.  Here was a student already experimenting with the concepts we would be studying and so we created the Physics of Catapults unit.  

Our class spent several weeks creating a variety of catapults starting with smaller, table top varieties. The students followed plans and also created their own designs.  We tested each design and, with the help of analyzing videos for research, were able to determine which type offered the best results for distance, speed, and acceleration.  We tested different hypotheses for the types of projectiles – Did the density of the object we launched have any effect on the distance?  Did size or shape of the object make a difference?  image05-copy

Finally, we built a larger, sling-shot style catapult.  This project was wrought with difficulties.  During the initial build, we found the instructions to be full of errors and we had to go back several times to correct the design mistakes.  In our first attempt to launch it, the catapult collapsed upon itself and it was back to the drawing board. After a lot of trial and error, we were able to shoot a projectile about ten feet.  Not exactly the huge payout we were expecting in return for all our time and energy, but the students were still extremely proud of their creations and the catapult has become a favored attraction for visitors to my room.  

The interest in the catapult unit helped to keep my students motivated through many small (and large) failures.  Working through the formulas seemed exciting because the data we generated gave us feedback about which designs had been most successful and how to adjust them for even better results.  Although our catapult may have been anticlimactic, the experience gave my students a real understanding of the concepts we had read about in class, many opportunities to work together to solve complex problems, and quite a few hilarious moments thrown in for good measure!

Sarah Champ

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