The Curriculum Framework for the new AP Physics 1 and 2 courses describes a multitude of "science practices" that will be tested across all topics on the exam. Several of these practices call for students to discuss and argue about physics principles. Quoting from the Framework, students are expected to, among many other things:
* refine representations and models
* pose, refine, and evaluate scientific questions
* evaluate data, both the source of data and the evidence provided by data
* evaluate alternative explanations for phenomena, and articulate the reasons explanations are refined or replaced
That's awful highfalutin' language there. What does that mean about the questions on the actual AP exam? Well, check out free response question 3 on the released example physics 1 exam. It presents two different arguments -- each about three sentences long -- about whether bulbs in series or in parallel will be brighter. The question then asks for an evaluation of the correct and incorrect portions of each argument, followed by a determination of which set of equations supports each side of the argument. When physics teachers first encounter this question many say, holy smokes this is tough, how am I going to get my students prepared to answer questions like this?
Have them create, support, reject, and engage with arguments in class, both with you and with their classmates. They must have experience in stating dispassionately and clearly what parts of a statement are correct, and what parts are fallacious, with clear justification that doesn't merely repeat the arguments.
How do you do that? Practice. Start by having students grade each others' problems to a rubric -- and not just problems that call for straightforward calculation. Make them grade verbal justifications.
A very simple rubric for grading a verbal justification might award one point for clearly stating a relevant fact or a relevant equation; one point for connecting the fact or equation to the answer; and one more point for the answer itself. Focus your energy in class on helping the class understand what it means to logically connect a fact or equation to an answer. "Because of ohm's law" doesn't say anything, but "because in ohm's law with constant voltage, current and resistance vary inversely" does explain why a larger resistor might take less current.
Then create situations in which students engage in discussion with each other. Many of these discussions happen naturally when you assign deep problems and encourage students to work together. On homework, I consider it my role not to tutor students, but to referee their disagreements when they have differing views on a problem's solution. When someone comes to me for help, I ask to see what he's written, and I ask with whom he's discussed his issues. The expectation, as stated to the class from day 1, is that students go to each other first for help before they talk to me. Not because I'm cruel or lazy, but because they must develop the skills of posing, refining, and evaluating scientific claims.
Finally, ask students to state their competing claims and debate publicly with classmates. The debate team continually engages in head-to-head evaluation of contentions; similarly, allow class members to present their ideas to the class for evaluation. As you referee a public debate, you have to be very careful to eliminate posturing. While we want to find the right answer, the goal is the search for the truth -- the goal is NOT to thump chests about who was right and who was wrong. Overly enthusiastic displays of emotion should not be allowed, whether that emotion is positive or negative. That is, the student who gets angry at himself or who ribs a classmate for a wrong answer must be chided; but so must the student who pumps his fist and gives a Marv Albert "Yes!" when he's right. Physics is complex enough that over the course of the year virtually everyone will be right sometimes and wrong sometimes. When you're teaching scientific argumentation, make sure that who is right and who is wrong becomes irrelevant. What matters is that everyone can articulate which were right and which were wrong, and why.