In my last post I talked about how I finally reenvisioned collisions and explosion problem solving for my on-track physics. It went so well I’m definitely going to integrate more of it into AP.
The goal of the reenvisioning was to set students up for a meaningful tennis ball cannon launch lab at the end of the lesson sequence.
If you’re unfamiliar, you create a tennis ball cannon, launch it, and have students calculate some quantity based on momentum conservation. To be honest, I haven’t run this lab since my first few years teaching for a few reasons. One was that my cannon got stolen at my first job. Then I decided that whole class labs are less effective than small group work and I hate when it looks like everyone is copying answers. The activity just wasn’t meaningful enough.
But after talking to several friends, everyone was excited about the idea of a cannon launch, so I spent my weekend rebuilding a cannon.
To open the lesson I set up and demonstrated an “explosion” with our car-track system. I ensured that one car had more mass than the other and we had some conversations about what to expect. We also talked about what the equation would look like based on our previous experiences with elastic and inelastic collisions. Students were able to correctly determine that it’s basically the opposite of an inelastic collision.
Next, I gave them the scenario where the cannon had a mass of 4.0 kg, the ball had a mass of 1.0 kg and the cannon’s launch velocity was 5 m/s. These numbers were strategically chosen. I wanted to keep whole numbers and also have a cannon-ball ratio that was similar to the actual cannon-tennis ball.
Students then completed the four representations as we’d previously done earlier in the week. Below is a student work sample.
The great thing about this was that students were able to accurately represent and predict the outcomes of the cannon-ball system before we got into the muck. This got students thinking individually and talking in small groups. We also discussed why the results made sense.
To launch the cannon I let it go through a photogate to snag the post explosion velocity and then students completed the calculations.
For the post-lab analysis I threw in a few thinkers. They included:
- Find the average force on the ball
- How would a longer cannon change the ball’s launch speed? Explain in terms of impulse-momentum
- If we used the same cannon but filled the tennis ball with rice, what would happen to the speeds of the ball and cannon post explosion?
You can see a sample student response below:
These questions led to some really great conversations that brought us back to equal forces, equal momentum changes and where time falls into the mix.
Physics.Teacher.Momma. 