In doing Newton’s 3rd Law, here was the process that seemed to work pretty well, but will need a little tweeking:
1st : Introduce idea of multiple objects of interest, and multiple free-body diagrams. Worked through drawing FBDS for case of pushing two unequal mass blocks on a frictionless surface.
2nd: Introduced the idea of a force pair, show the example in the Free-body diagram, and talk about the property of forces pairs: They are “two” perspectives on the same “interaction”; they are forces that act between two different objects; for contact force pairs, they occur at a single location (the boundary between where two object meets), and they always occur on different free-body diagrams
3rd: We did one example clicker question about identifying force pairs (I would do a few more next time) or even have them work whiteboards. It helped having a list of properties of force pairs, because we used them to argue about it.
4th: I told them that we already know the rules about how single forces acting on one object behave (they superpose to generate a net force, which causes acceleration). Now we needed to determine the rules for how force pairs behave.
5th: I introduced, diagrammed, and demonstrated three cases where the force pairs are equal: “Tug-o-War” Scenario where no one wins (linked to carts with hooks, and pulled on equal mass carts equally hard on both sides with identical rubberbands ). Collision scenario where two equal mass carts head on collide with equal speed with equal stiffness bumpers. And a “pushed together” scenario where equal mass carts are pushed together at constant speed. For each situation, the force pair was identified and then measured using force probes. We saw that in each case, the force pairs were identical in magnitude.
6th: We brainstormed changes that we could make that would make the force pairs unequal. Students suggested a tug-o-war where one side wins, collisions with unequal speeds, or unequal masses, or unequally stiff bumpers, and “push together” scenario with unequal mass as well as speeding up.
7th: Every group had to try out at least one variation from each of the three categories (tug, collision, push together)… Some groups already kinda guessed what they would see, but many groups were honestly (and pleasantly) surprised. I let a lot of groups really show me what they had found and shared in their cool findings. Given the different place of each group, my conversations were very different. I talked with some groups about what they were observing, and what that implied. With other groups we talked explicitly about Newton’s 3rd Law, and when it did (and didn’t apply, haha!). With other groups, we talked about how could it be that the forces were the same (tug-o-war). At the end, I did a lot of the work of synthesizing (at the whole-class level) our findings due to time constraints.
8th: We ended, by watching Frank’s Newton’s 3rd law Video, and talked about the squishing as evidence for force pair equality. We could have spent more time here, but also given them more explicit tasks. Need to rework how to engage this task for sure.
From there we took a dive into problem-solving. If I did it again, I probably wouldn’t do it right now. I would do more scenario representing and reasoning. The problem we did was pretty hard, too–two blocks in an accelerating elevator. Students worked well through it, it was definitely productive struggle. But there was a lot of struggle, still, around normal force and weight. That doesn’t surprise me. Vertical stuff is just hard, and to pile on vertical toughness with the newness of Newton’s 3rd Law and multiple objects is just a bit much. They still did pretty good.
Part of what I learned they needed a little more scaffolding on managing just thinking about 2 (or 3) net force vectors simultaneously–that each net force gets separately attached to the free body diagram for that object. Some of it was just labeling issues, but it was also more than that. I’ll have to think more about what other scaffolding they need. But all and all, the Newton’s 3rd Law day was a good day.
My approach here was very much “elicit confront resolve”… And I know there can be some criticisms of such an approach, but what matters is how we as a class frame what we are doing. Students aren’t perceive in my class the activity as “tricked you” (you were wrong!), because they have learned to have a response that’s more “so cool” — being pleasantly surprised by circumstances when nature is different than you expect. The activity is also just fun because we are brainstorming and testing our ideas… I’m not presenting a specific situation for them to be wrong about. They are proposing scenarios, that they themselves are tricked by. That feels more playful, I think. It’s harder to feel like you got suckered, when you are the one who proposed the trick in the first place! Anyway, I’ve just been thinking about this more… about how a lot of “elicit confront resolve” criticisms are criticisms not of the instructional technique, but of an instructional framing that is commonly developed around them. It can unproductive for students to frame it as “guessing”, or “being tricked”, “or always being proved wrong”, or “intuition is never right”, but that’s definitely not the only framing that can happen. Anyway, this last rant was a little unrelated, but it’s been on my mind. I also did very elicit confront resolve tasks for static friction, and that went really well too. With students really engaging their ideas, having a combination of being right and wrong, that led to being intrigued curiously by being wrong (rather than feeling stupid about being wrong). blah blah blah blah…. It’s Friday and Just wanted to keep typing my thoughts…