Perhaps my favorite of Peter Bohacek's "Direct Measurement Videos" shows a collision between a marble and a wooden block. The marble rebounds; the block both translates and rotates after the collision.

In my classes (and in my AP summer institutes) I pose three questions:

(1) Is linear momentum in the horizontal direction conserved?

(2) Is angular momentum about the origin [marked in the video] conserved?

(3) Is mechanical energy conserved?

Each of these questions is quite deep, requiring evidence from the video, calculation, and clear explanation. It's one thing to say "yes, the linear momentum is always 0.19 Ns," supported by two pages worth of chicken scratch. But can a student coherently present his calculation so another physics student can follow it? Can he explain not just the equations used to produced the 0.19 Ns value, but why those equations are valid, the meaning of the values used in the equations, and how those values were obtained from the video?

We want students to explain "The marble is moving at constant speed, so

*d = vt*is valid. The marble goes 23 cm in 5 frames; using the 960 fps frame rate gives a speed of 44 m/s."
We

*don't*want "I used the ruler to get 23 cm, divided by 5, multiplied by 960 because there are 960 frames in one second, and that's 4.6 times 960 which equals 4416, but with sig figs and dividing by 100 to convert from cm/s to m/s I get 44 m/s."
Describing a calculation is a difficult skill. Students tend to want to show haw they did some algebra; and they don't understand me when I tell them that the algebra isn't important. I'm still struggling to teach this skill. I'm no expert, but I can offer a few hints.

**A time or space limit**provides necessary structure. When I ask for a written discussion of one of the questions above, I give a strict one side of one page limit. Then there's no room for digressions. In an oral presentation, I give three minutes max. Students have to decide for themselves what information is important enough to spend precious time on. And with practice, they learn to state results rather than to belabor arithmetic.

**Questions and evaluation from outsiders**can focus students quite well. This is the universal trick to teaching writing or presentation skills: creating a clear and authentic audience. When students know they will be questioned -- or "examined" -- by a classmate, alumnus of the course, or even by another teacher, they work differently. To me, students think they merely have to convince me that they know the answer. To outsiders, students recognize that they have to explain their methods as well; and they intuitively meet a higher bar for convincing those outsiders of their own competence.

**A ritualized physics fight combines a time limit with external evaluation.**I'm training my 9th grade honors class to give three minute oral "reports" in answer to each of these questions. Eventually, I'll have an "examiner" from outside the class -- a senior in our research course -- ask questions of the reporter. I'll even plant a "challenge question" with the examiner, so that each student can expect to get questions at both a basic and advanced level.

Here's the rubric to which the examiner will evaluate the reporter. Note that I've indicated to the examiner some features of the reporter's explanation that I expect to see -- for example, if the reporter doesn't indicate somehow that the marble's linear momentum after collision subtracts from the rod's momentum to get the total, then there's some missing physics. But most of the rubric is non-specific to the problem at hand. The examiner is asked to evaluate the reporter's competence at explaining the video evidence and how the evidence leads to an answer.

What guidance do I give the students as they prepare? At first, very little, just a demand for practice. I show everyone the rubric (with the problem-specific elements and the challenge question redacted). I give time in class to practice; the evening's assignment is to practice twice in front of other students. Then I serve as examiner for some practice fights against randomly chosen students. After a few of these, I demonstrate how I might present a similar problem in just three minutes. I require more practice. It's the upcoming physics fight against an

*external*examiner which keeps everyone motivated to improve.**Want to have a physics fight with me?**Physics fights are the basis for the US Invitational Young Physicists Tournament, where each team serves as reporter and examiner for three rounds each. But the USIYPT problems (the link goes to the problems for the 2017 tournament) could each result in an undergraduate thesis. The USIYPT is designed for the top high school physics students in the world. What about your AP Physics 1 students in their first year of study? What about your really talented honors freshmen? Shouldn't they be able to try physics fights too?

While there's not a formal, national-level tournament (yet) pitting teams in physics fights over high school level problems, I see no reason we can't arrange "friendlies". I've posed three problems above, each appropriate for a three minute presentation and five minute examination. Do you want to challenge me and my students to a physics fight? We could do this by skype or the equivalent. Your student gives a report, my student does the examination. Then one of my students reports with one of yours examining. For now, I am inviting people for "friendlies" only, where we can fill out the rubric and make comments, but without true competition. Eventually I'll figure out a way to have scoring, a miniature competition, and a "winner." But really, we're all winners if we're getting our students to communicate about physics, right?