I'm home now from the AP physics reading, recovering after two hours of sleep. The physics readers' lounge is a happenin' place, especially when no mental or physical functions are required in the morning beyond sitting on an airplane.
I graded my ninth lab problem in twelve years of AP physics reading, along with my twenty-bazillionth "justify your answer" response. So please trust me when I offer some advice from a reader about responding in words to a physics question.
Every so often, I hear of English teachers who condescendingly demand that science teachers participate in some sort of "writing across the curriculum" initiative. Aside from the haughty superiority implied -- when's the last time an English teacher integrated any quantitative skills into his or her classroom? -- I would argue that physics already requires writing skills that are not well-taught in typical high school English classes. There's no need for any initiative, because we're doing plenty of writing as-is.
A standard part of an AP exam question, as well as a standard part of each of my in-class physics tests, is the requirement to justify an answer or to explain some reasoning. The ability to respond appropriately IN WORDS to physics questions separates those who deeply understand physics from those who are just plugging numbers into equations. I have to coach students throughout the year on their responses.
The absolute most important hint about verbal responses: BE CONCISE. I just read about a thousand exams a day for seven days. How do you think I felt when I opened an exam booklet and saw a wall of text? Sure, I waded through the long essay, because that's my job, but it was not fun; and I might easily have missed the important part of the response amongst the flowery language.
Conciseness must be taught in class, and from the beginning of the year. My own rule is to use two sentences with reference to a relevant equation -- no more. So on a homework problem, when a student writes a beautiful page-long essay, full of nothing but correct information, his answer is marked wrong. Why? Because I'm teaching a writing skill here. I'm testing more than whether you know physics; I'm testing whether you can communicate physics in the manner that I require.
The importance of concise answers goes beyond fighting AP reader fatigue, and even beyond teaching a writing skill for its own sake. A two-sentence answer will often save a student from himself. I can't count how many times a student's answer was beautiful... but then he kept writing, saying something obviously incorrect, and lost credit. Students shouldn't be trying to show off, or trying to game the test by giving multiple answers hoping that one is right. Those strategies ALWAYS backfire on the AP exam.
My strong suspicion, though, is that verbal diarrhea has paid off for such a student in his class. We can prevent such problems through consistent grading all year. Do not give a student "+1" or extra credit for a deeper answer than necessary, even if it's beautiful -- the effect is to create incentive for long and wrong answers. Do not give pity points on the grounds of "oh, he really tried hard on this question, he wrote a lot, so that's better than nothing." Do not give pity points on the grounds of "English teachers always pass a student who writes a lot, even if the content is crappy, because it's just so good that the student is willing to write at all."
Read a student's first couple of sentences. Award credit for right answers, partial credit for partially right answers, and no credit for wrong answers. Whether you're teaching AP or not, you will be developing valuable communications skills. And you'll be saving yourself all kinds of time, as homework justifications will become easy to read and grade.