Understanding Projectile Motion Misconceptions

I am reading an awesome book on Cognitive Sciences and how it applies to teaching students in the classroom: Schools for Thought – a Science of Learning in the Classroom by John Bruer. I will write more on this book later, but I wanted to focus on an idea I got from it to teach in the classroom. One chapter of the book relates to teaching science, and specifically, teaching Newtonian Physics… exactly what I’m teaching right now to my grade 11 physics class.

Anyway, the author emphasizes the importance of understanding the concepts, and demonstrates that students, when coming into a physics class, have sporadic knowledge of physics, and that some of their knowledge is scientifically accurate, but other intuitive knowledge is false, and has to be adjusted with a type of “aha moment” – usually a demonstration or experiment that blows away their arguments / their intuitive beliefs completely.

I agree with the author, and even before I read this book, I did exactly this. For instance, recently we did the pendulum experiment. I got the students to investigate what factors affect the period of a pendulum. After the investigation, a lot of the students came up to me and said that they were surprised that the amplitude really had no effect on the time of one swing. This was mind boggling to them, because they thought that if the pendulum has to go through more distance (amplitude is greater) the time would also increase – yet the time stayed the same. The kids were sincerely surprised with their results, and thus the experiment was one of those aha moments.

However, ever since I read this chapter on teaching physics, I realized that I have to do this kind of mind boggling experiment / demonstration / discussion every time I start to teach a new concept.

For instance, in physics we were doing Projectile Motion in 2-D. I wanted to show the students that the two motions (vertical and horizontal) are completely independent from each other. I found some sites to demonstrate this visually. It worked for most of the students. The demonstrations were clear and visual enough to explain what I was getting at.

Unfortunately, when it came to explaining that an object dropped and thrown horizontally would take the exact same time to land on the ground, some students were stunned! They were in complete disbelief:

“Do you mean that if I drop a bullet and shoot the bullet from a gun, they will land on the ground at exactly the same time? Really? Not true!”

The students started having a discussion /debate about this, and some even started to demonstrate it by throwing their erasers horizontally and dropping them to show they land at the same time. But there was no way of convincing one student – especially when talking about bullets! “But the bullet goes so far – how can it take the same amount of time to reach the ground as to drop from my hand?!” I think the only way I could have convinced him at that instance was to actually do an experiment with a real gun / bullet / very precise timing device – unfortunately I had nothing of the sort prepared.

This led me to think about this misconception. He could picture the two times being equal with an eraser being thrown horizontally and dropping it, with a marble being rolled off the desk at high speed and the dropping the marble from the same height (we actually did this experiment instead of the bullet), but there was something about the gun/bullet scenario that didn’t make sense to this kid. I can understand that the bullet travels very far, and therefore the student might have thought that this distance takes a long time to travel – way longer than the bullet just being dropped from the same height. But on the other hand the student must realize that the bullet leaving the barrel of a gun travels extremely fast… And this is where I think the misconception lies!

I think that with all the movies (e.g. The Matrix) and shows on TV (e.g. Heroes) with awesomely cool special effects of people dodging or stopping bullets, makes this generation of kids not realize the actual speed of a bullet. Kids don’t see bullets being shot from a gun every day, so the closest they get with speeds / distances / times and bullets is in movies and shows. They don’t realize that dodging a bullet is impossible, and therefore don’t realize that the bullet travels extremely fast – that being the only reason the bullet can get really really far before it reaches the ground.

Although the explanation to this misconception is only speculation on my part, I am pretty confident that this is why my student had such a hard time understanding that the time for a bullet to drop and fall as shot from a gun is exactly the same. For me, the Matrix was so cool, because of the “stopping time” effects and the bullets dropping in mid air. For the kids I teach now, the Matrix type of scene is common place and if a show/movie doesn’t show a character dodging bullets, its not real show.

Talk about Hollywood influencing kids – it even leaks into the physics class!