Learning Projectile Motion through Air Launchers (Safely!)

August 31, 2017: The first day of class

What better way to study projectile motion in STEM 11 than to launch projectiles? It makes so much sense; in Physics, the first semester is devoted to the study of displacement, velocity, acceleration, forces, momentum, and impulse. In Precalculus, we cover vectors, parametric equations, vertical motion, and a million textbook force problems that have diagrams about Physics.  We decided to hinge the first quarter on the mission to make a t-shirt cannon that students would launch at school events. Getting to the math and Physics is easy, but what about making a safe air cannon?

Summer 2017: Prototyping
We began this summer making the same type of PVC cannons you see on YouTube (see here or here or here).  We’ll skip to the fun, finished launchers:


Success, right? T-shirt launched! Well, when we began researching the pressure ratings for PVC air chambers, we found that OSHA’s stance is pretty clear: do not use it for compressed air, ever, at any pressure. All pressure ratings on PVC are for liquids like water, not compressible fluids like air. These people on YouTube operate under the assumption that everything will probably be fine since everyone else does it that way. But there are plenty of stories online about PVC exploding and causing serious injury. We did not want to put ourselves or our students in any danger. So how could we still do the project, but do it safely?

It’s not the cheapest way to launch a tennis ball, but it’s the safest (we are thankful to the Greenwich Alliance for Education for making this R&D possible). This is the same stuff used for gas lines in homes and buildings. It’s rated to 48,000 PSI. The weakest point in the air chamber is the valve stem, which will leak around 140 PSI. The bike pump and air compressor we’re using to fill the chambers hits its max at 130 PSI. After the air is released, it pushes the ball through the PVC, which is perfectly safe to carry the projectile.

Fall 2017: Students’ first launch
Today, students will launch their small air-powered launchers for the first time. They will use the projectile motion equations seen in every Precalculus and Physics course:v motionv motionWe generated these equations during the first two weeks using student-gathered data from a video-recorded drop test.

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graphs

Data credit: Jess T., Jody B., Julian R., Santiago M.

Now, students will calculate how fast their air launcher can shoot a projectile using the same equation they generated before. We’ll look for relationships between pressure, velocity, distance, and eventually, angle of launch. But for now? Images from class today!

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