A couple years back I asked for advice about teaching Newton's Third Law. John Burke, Jason Sterlace, and Chad Hodgkins chimed in with what boiled down to, "set N3L up from the beginning by describing every force in terms of interactions."
They were so right, but the method took enormous patience, especially with the brightest students.
I start the year in honors / AP physics with equilibrium problems.* Following John Burke's explicit recommendation, I expanded my free body diagram rules to include a description of every force in words: not just w or mg or weight, but "w = the force of the earth on the hanging mass" and "T = the force of the string on the hanging mass."
* I'll be demonstrating these demonstrations tonight, 6 June 2012, at 9:30 online at the Global Physics Department.
Consider first the pedagogical purpose and effect of this requirement. Without careful instruction, students do not easily or correctly differentiate between the object experiencing a force and the object applying a force. Then when they're asked about the third law, they're up a creek. By requiring these written phrases on every diagram, I'm forcing** everyone to think about interactions on every single problem. When it came time to formally teach Newton's third law, it took only a day: just switch the applicant and the applicantee. For the first time this year, my students truly understood the third law, and hardly ever missed a question about it.
But now put yourself in a (smart) student's shoes and consider his reaction to my requirement. "Gawd, this is stupid. I know that tension is the force of a rope. I just didn't write it 'cause that's dumb busywork. Then he took off points... Mr. Jacobs is an arse."
And consider your own personal reaction to requirements of this type when you were a seventeen year old high school student. If you're reading this blog, you were probably a very good student, one who enjoyed difficult classes. You yourself probably rebelled against this sort of requirement. Perhaps you might have said,*** "Well, Mr. Jacobs is making us do that because the dumb kids need the endless repetition. I certainly don't need it, though, so I'll just do things my way and not waste time writing all those words." Then when your teacher took off points, you got snitty, you argued, you were sullen or sarcastic for a few days. (You also missed a few more Newton's third law questions than you needed to, but you forgot about that.)
*** In so many words, anyway, 'cause by 17 years old you were, unlike me, probably bright enough not to say this out loud.
Fast-forward to today. Teachers have a tendency to aim their teaching at students who were like them.
Imagine that you ask the class, "Now, what do I write for the normal force?" The bright guy says, "force of the ground on the block." You say, "good," and move on.
Ten minutes later, you're on another problem with a normal force. When you were in physics class, you dreaded this moment. You just knew that the teacher would pick the stupidest guy in class and ask him, "What do I write this time for the normal force?" And you just knew that he would say something ridiculous like "um............................... um........................ the force of the normal block on the string?"
So now that you're the teacher, you've pledged not to torture your good students like this. Perhaps you write "Fn = force of ground on block" and ask, "Any questions? Everyone understand? Good." Or perhaps you even just stop writing the interactions in words, assuming that you've shown the class how to do this already. And sure enough, your bright students never get that exasperated-with-their-classmates look.
Nevertheless, if you DID ask a random student to describe the normal force, what are the chances he would get it right? It's your job to teach him, too, not just the students who are predisposed to becoming physics majors. Teaching the lower-middle class successfully, while not entirely boring the top students, can be very difficult, but is essential to keeping the class positive and motivated all year.
My own technique is to review in context. I do ask the question again. "Billy, what do I write for the normal force?" However, I'm always asking this kind of question in the context of new problems. The most-talented students may have to wait a moment for someone else to answer a silly question. But then we're right back to a new and interesting problem. The lower-level students realize they can't just tune out, because I'm relentless about expecting them to demonstrate some basic skills. Thus, while they'll always be a bit behind the leaders, they aren't nearly as far behind as they might have been. They might need to review difficult topics three or four times over the course of the year, but they'll catch up in the end... because they've been continually drilled on fundamentals.
So when the bright kid says, "Mr. Lipshutz, I know this, why are you making me waste my time writing silly extra words on my homework," how do I react? I start by wondering why it's such a big deal -- if he knows what to write, why is it so horrible to take a moment to prove that to me? I might appeal to the idea that I want all of our problems to look similar, so that the class can help each other more easily. I might be transparent about my pedagogy, giving an impromptu Newton's third law lecture to show the benefit.
In the end, if the student pushes my patience, the answer is, "Because you'll lose points if you don't. You may drop the class if you think this requirement is overly onerous."
I'm sorry, 17-year-old self. I know I hated that answer. I also hated it when mom said, "Eat your veggies because I said so." But haven't you told your own kid that on occasion?