I decided to have students discuss this question before starting forces this year:
I chose this question because I knew students would have lots of good ideas about it, and it’s complex enough that we wouldn’t fully figure it out in one discussion. Plus, I knew we could revisit it a few times this semester and refine our explanations.
Problem was – pretty much all students agreed that the answer was A, so it felt a little artificial to debate. I asked students to think of reasons someone might say B or C, and they came up with good explanations, but it still felt forced.
What I realized throughout the discussion, though, was that students did disagree about the specific path of the ball, even if they all thought the answer was A.
One disagreement had to do with the shape of the path – curved vs. straight. This I wouldn’t have noticed if these 2 groups hadn’t spontaneously drawn the path when they wrote their explanations:
Another question that came up was whether the ball loses any forward motion to gravity – in other words, does gravity “redirect” the forward momentum of the ball to be more straight down?
After the in-class discussion, I asked students to do the following for homework.
- Today we heard lots of ideas in class. Choose one idea that you heard that you disagreed with and describe it here in as much detail as possible. Explain that person’s reasoning. (note: you’re just describing the idea, not saying why you disagree with it)
- Why do you disagree with that idea? Why does it not make sense to you?
- What is your current thinking on the answer to the question on the other side of this sheet? Why? (If you changed your mind, explain why. If you think pretty much the same thing, explain any new insights you have into the situation)
Note that I made this homework before class, and I expected more students would disagree about the answer (A, B, or C). But since so many chose A initially, and still thought A at the end, a lot of them said they had no new insights. This was disappointing to me, because I know there were things that came up that they hadn’t thought of – stuff about the path shape, whether the ball will “keep up” with the plane or not – but not many of them mentioned that on the homework.
Next year I want to reword the original question so more of their thinking is made visible, which will provide more fodder for discussion and reflection. I’m thinking about providing a blank film strip (do students know what these are??) and asking them to sketch the airplane + bowling ball from when it’s released to when it hits the ground.
One question I still have though is – whether to tell them to ignore air resistance or not. It seems weird to say that before we’ve talked about forces at all in class. But if there’s air resistance, it would be hard to tell if their ball is falling behind the plane because of air resistance or because gravity “redirects” the forward motion. The only way to sort those two out is to remove air resistance – maybe I could get the students to see this. I hate saying “we’re going to ignore air resistance because physicists like to start with simple models” and would love to move toward the students deciding we should ignore air resistance for a specific reason (like distinguishing between two different explanations)