If I am not giving the FCI, then I base my decisions about how to teach on my own beliefs and judgments about what "works" -- what supports my personal vision of what it means to teach and learn physics. But if I give the FCI, and I accept that gain on that test is a referendum on my teaching, then I start to base my decisions about how to teach on what I believe will raise those scores. Now, I am just one little high school physics teacher. Maybe my beliefs, judgments and "vision" are not so hot. But are they a better or worse guidepost than the FCI gain?
In my other job, I spend a lot of time helping students raise their SAT scores (something I have never mentioned on the list). So I have a lot of experience looking for efficient ways to chase "gain". JD suggests that we should help students with the chain of reasoning that leads back and forth from momentum conservation to the 3rd law. I agree...but once the goal becomes "gain" and as RT point out, you are trying to keep the course moving along, it sure it tempting to say something like this in class:
"Objects interact in lots of contexts: train cars colliding, ice skaters pushing, ball meets bat...
When they do, they exert forces on each other.
Though you may not believe it or agree with it today, those forces are always equal in size and opposite in direction.
This is true every time, no matter what. Here are some things that you might think matter, but they don't:
Which object has more mass
Which object is experiencing other forces
Whether the objects are speeding up or slowing down
Which object "initiated" or is responsible for the interaction occurring -- physics doesn't care about that.
Anything else that you think might somehow cause one of the forces to be bigger than the other. So if you are looking at what you believe to be an exception to this rule, you are mistaken."
Then, give a bunch of examples. Maybe even do that (somewhat bogus) demo where you push two force sensors against each other and show that the force vs. time graphs are reflections of each other...
This doesn't take long. And it should address 4, 15, 16, 25 and 28 on the FCI. I don't think 17 is a 3rd law question.
But of course, I may be all wet. I have not yet used the FCI. Was planning on starting next fall. So I may find that my approach will fail. But even if it "succeeds" -- produces gain -- I don't know if that's a good thing. This is not the SAT, where the higher number is all you want.