One of the really cool things about teaching physics is that you’re not limited to theoretical scenarios or paper-and-pencil problem-solving. You can physically build or recreate the problems that you’re working on in real life. The few times when you can’t, there’s probably a really good simulation out there.
As much as possible, I like to work on problems where my students can observe, over and over, the motion and forces that they study. Better yet, they can measure different aspects of that motion, and confirm their theoretical work experimentally. Almost always, we find that our calculations are well within 10% error, and often much less than that.
This is one of the lessons that just worked seamlessly…
Algebraic kinematics, Day 2. A ball is launched vertically from the top of a lab table with a speed of 5.2 m/s. How long does it take to hit the ground?
Before we attempted anything that resembled math, we observed the motion several times using the PASCO Projectile Launcher. We discussed (think-pair-share!) the motion using every-day language, no fancy physics vocabulary.
After a few students shared with the class, we came up with: “The ball leaves the launcher and moves upward. It slows down on the way up until it comes to a stop then immediately falls down towards the floor, speeding up as it goes.”
Language and vocabulary are big barriers for beginning physics students. For example, many of them don’t differentiate between speed, velocity, and acceleration. By starting with simple language, we ensure that we fully understand the motion of the object. As our understanding of physics vocabulary improves, we work on including it in our descriptions.
Once we had the motion down, we began our problem-solving procedure:
- Draw a labeled diagram of the motion, indicating “important time points”
- Make a table labeling all time points
- Write kinematics variable tables
- Pick an equation and solve for the unknown
- Plug in values with units and direction
Rinse and repeat until you have what you need.
Below you can see the work, with the different problem-solving pieces labeled. I write these solutions up for myself as I get ready to present a demo in class and also as sample problems for the students. I scan and upload these solutions for the students to look back on.
In class, I didn’t actually give them the initial height of the ball, so a little ways in, we found that we were missing some information. After some prodding on my part (think-pair-share!), we worked out that we could calculate the distance traveled by the ball on the way up (we had found that the day before) and measure the height above the floor from which it launched (1 m). Add ’em up and you get the distance traveled by the ball on the way down.
Whenever we do demos like this, I try to incorporate a missing quantity that we can physically measure, like the mass of a disk or the length of the string. This brings the hands-on laboratory mindset into what would otherwise simply be a pencil-and-paper problem.
Not bad for day 2 of 1D kinematics!
At the end, we calculated 1.21 seconds. I launched the ball and had a student time it with the stopwatch on her phone – 1.39 seconds. We had a brief discussion about human reflex time. Not bad, but we can do better. We recorded the motion with a slow motion camera and measured 1.22 seconds.
What followed was a tiny pause of teenage wonder.
“That was so satisfying.”
Then the bell rang. I love my job.
What lesson have you taught this year that just worked? What lesson fell short and how will you improve it next year? Let me know in the comments! It’s all about getting better!
Author’s note: This is my first year teaching science, so like all good first-year teachers, my mantra has been “beg, borrow, and steal.” I got the idea for this AP Physics 1 lesson and for the use of “quantitative demonstrations” from Greg Jacobs over at jacobsphysics.com.