Buy that special someone an AP Physics prep book, now with 180 five-minute quizzes aligned with the exam: 5 Steps to a 5 AP Physics 1

Visit Burrito Girl's handmade ceramics shop, The Muddy Rabbit: Yarn bowls, tea sets, dinner ware...

22 July 2009

Mailbag: Writing in physics class

The Nachograndpa, a.k.a. mathematics professor Dr. Barton Jacobs, writes:

"I now have a question that came up with your Mother regarding her English 101 class, where they're supposed to learn how to write for a variety of categories of writing. I said that beyond Freshman Calculus, all math courses require putting together an argument that entails as much in the way of English words as it does symbols. I had vaguely remembered that as early as Freshman Physics this is also true. Is that right?"

Yes, but not in the same ritualized manner as, say, a geometric proof. We do enormous amounts of writing in my class, and I don't mean just lab reports. A typical question on an assignment might be,

"A ball is dropped from a table. The height of the table is now doubled. What happens to the time it takes for the ball to hit the ground?

___ the time increases, but does not double
___ the time doubles
___ the time more than doubles

Now justify your answer."

The "Justify your answer" questions generally require verbal discussions with reference to formulae. I have the dangdest time getting my students not to BS, but to use no more than two sentences with reference to the relevant equation.

The correct answer is that the time increases, but does not double. The justification: "The kinematics equation can be solved for time to get t=root (2x/g). Since we doubled the quantity under the square root, the time to fall increases by a factor of root 2 rather than 2." That's a fully sufficient explanation! But most folks want to go on for paragraphs... why?

Usually because they're not quite sure, and their experience in other classes is that if you write enough with sophisticated-sounding vocabulary and phrasology, the instructor will essentially throw up her hands in despair and award credit. Problem is, on the AP exam at least, incorrect statements interspersed with correct statements will always lose credit; correct but irrelevant statements cannot earn credit. The logic has to flow properly; getting it to do so is as much a writing skill as a physics skill.

09 July 2009

Come to my AP Physics Summer Institute

Folks, I will be at Manhattan College in New York City from July 27-31 for an AP Physics Summer Institute. We spend a week going through the AP curriculum, doing quantitative demonstrations, and generally sharing our ideas about teaching AP physics. Next to the AP reading, the Summer Institutes are the best professional development available for physics teachers, whether at the AP or sub-AP level.

Thing is, we only have a couple of folks signed up right now. I'd really like to get a few more people involved. Please contact Pam Kerrigan at to get sign-up information. The option to stay in the (very nice) on-campus dorms is available. The institute runs Monday - Friday, in the neighborhood of 8:00-5:00, with class ending at noon on Friday. If you're not from the New York area, I still recommend attending. Stay in the dorms, and experience New York city in the evenings!

The nicest part about the Manhattan College institute is that you can earn graduate credit for your attendance. If you have questions about the logistics, please contact Pam at the address above. If you have questions about what will be covered at the institute itself, please email me: I'll give you a call if you'd like.

04 July 2009

Revising my general physics laboratory program

Over the years, my AP physics labs have meshed into a true PROGRAM. That means not only are the individual experiments worthwhile, but that the year’s set of laboratory exercises serves to develop and reinforce a set of worthwhile skills. When the students start to roll their eyes at me as if to say, “yeah, yeah, we know what to do now, we’ve done it a dozen times,” I know that the lab program has been successful.

My AP lab program is designed to teach the lab skills necessary for the AP exam. Most prominent among these is the process of linearizing a graph, and using the best-fit line to determine an unknown quantity. That’s an important and useful skill…but one that is above the heads of my GENERAL physics students, at least at year’s beginning. For general physics, I have an entirely different set of experiments, each one solid, but without any guiding theme. In other words, my general physics labs in no way constitute a PROGRAM.

My summer mission is to begin to change this shortcoming: I want to revise my laboratory exercises so that they all mesh together. Specifically, I am going to try to use the “recurrent” lab model, as presented by Mikhail Agrest in The Physics Teacher, and as discussed earlier on this blog. My goal is for most of my experiments to ask students to use the result of their measurement to make a testable, high-stakes prediction.

[As a brief aside, note that no one is standing over me demanding that my lab exercises meet any particular “inquiry objectives” put together by “learning specialists.” No, I just have some time to invest for next year’s classes, so I’m working on general physics lab. I often see relatively new physics teachers become intimidated by the sheer volume of background work necessary to put together a strong physics course. The fact is, it takes many years before every aspect of your course will be successful. Partly this is because much trial and error is involved – I’ve wasted a couple of years here and there discovering why various physics teaching methods do NOT work for me. More to the point, no one has the time or experience to do everything perfectly right away. I’ve been teaching physics for fourteen years, and I’m only now coalescing my general physics lab program into something above the level of “adequate.” That’s fine – the labs have been adequate, and I’ve spent enormous amounts of time making other aspects of my course really good. Work on one thing at a time, and don't let anyone tell you you stink just because, say, your labs aren't perfect.]

The first topic of the year in general physics is position-time graphs. After nearly a week of lectures, demonstrations, and problems, the first lab exercise is to make a position-time graph for a “constant speed vehicle” using a stopwatch and metersticks. One goal of the experiment is to introduce my expectations for graphs in physics class, including proper labeling, as well as how to take a slope properly. Another goal is to give students kinesthetic experience with position-time graphs. They see that the slope of the position-time graph is the velocity of the vehicle, just as they’ve learned on homework.

What am I going to do differently this year? After each group has calculated the slope of their position-time graph, I will ask them to predict the position of the vehicle after it has been moving for, say, 10 s. Each group will give a location with uncertainty – i.e. 300 +/- 10 cm. Finally, *I* will measure the position of the cart after 10 s. Points will be earned for matching my measurement; more points, including some extra credit, can be earned for groups who match my measurement with the smallest uncertainty.

The second experiment uses “tape timers” to make a position-time graph for a cart on an incline. By taking the slope at several points, a velocity-time graph can be made, and used to find the acceleration of the cart. Except that this year, I will use a sonic motion detector to determine the acceleration of each group’s cart. Matching the acceleration that I measure will be a major part of the lab score.

You see? While every experiment won’t match this format, I’ll adjust as many as I can. You got good ideas? I’d love to hear them. Post a comment.


01 July 2009

What are YOUR grading mechanics?

At the AP Physics Reading we grade over 100,000 exams over just seven days. While accuracy is our primary concern, we cannot neglect the importance of speed. We all have plane tickets home at the end of the week. We don’t have a choice: finish, or else.[1]

The most important way we speed the reading is to have each reader grade the same problem throughout the week. After a few days, the rubric for that problem becomes instinctive. We begin to see the correct answers, and the common wrong answers, in our dreams at night. It seems as if our brains become hard-wired – assigning a score to each part takes place without conscious thought.

In one sense, then, grading speed depends most on the talent and motivation of each individual reader. We share our personal tips with each other because we’re all nerdly professionals. Only a physics teacher would be proud of discovering a shortcut that might save two seconds per exam. Competitive drive plays a role in speeding the reading, as well: though it is drilled into our heads that accuracy is paramount, in every room is a chart on which readers tally the packs they grade. Everyone, including table leaders, can see who’s fast and who’s slow. A reader who feels a bit lazy as the afternoon drags on might be tempted to slack; but if he grades half as many tests as everyone else, his slackage will be apparent to all. On the other hand, we can all fete the speedsters and ask them for helpful advice.

With so many readers and problems to grade, seemingly inconsequential time saving methods can be disproportionately effective. If there’s a way to help each reader grade just a few more problems every hour, that could extrapolate to an improvement the order of thousands of exams every day of the reading. So, you ask, what are some reading “mechanics” that save itty-bits of time here and there?

Back when I began grading in 1999, grades were recorded directly on each exam in felt-tip pen. We would pick up a folder containing 25 exams. Aides had already turned each exam to the correct problem. We would place the pack of exams on a cardboard ramp inclined about 5-10 degrees, allowing us to pull down the next exam as soon as we marked the first. When a pack finished, we put it in the “out” box near the door, picked up a new pack, and did it all again.

This grading style was very smooth for the readers, but required intense work from the reading aides. Since we wrote directly on the exam, it was important that the reader from problem 3 couldn’t see the scores from problems 1 and 2. To avoid subconscious prejudice based on a student’s previous performance, aides covered scores with opaque tape. After all problems were graded, aides removed the tape, and entered the scores into a computer. Even once each individual exam was graded, significant work remained before grades could be released to students and colleges.

About five years ago, physics started using a quicker and better scoring system. We still pick up a folder of 25 exams, but these are no longer turned to the correct problem. The exams are carefully kept in order; we turn an exam to our problem, grade it, and move on to the next exam. Rather than writing the scores on the exams, instead we bubble in the scores with a number 2 pencil on a separate sheet of paper. When a pack of tests is graded, aides just take the scoresheet to the “bubble room,” where it’s run through a machine. Thus, data entry is automatic. We can see instantly score distributions, how the free response scores correlate to students’ multiple choice, whether individual readers are too lenient or too strict… all is known as soon as the bubble sheets are scanned.

What relevance, you ask, might the AP reading process have to a regular high school classroom? After all, we don’t have a sophisticated data entry system, nor do we have an army of aides. Nevertheless, I have tried to take some ideas from the reading to move my nightly grading along. Primarily, I’ve tried ditching a traditional gradebook entirely. Instead, I print sheets of paper with a class list, and put these sheets inside the pockets of the folders that hold my assignments. When it’s time to grade, I just take out the folder. I write the grades directly on the sheet of paper. When I return the assignment to the students, I leave the completed grading sheet in the folder. By the end of the marking period, I have accumulated a stack of these sheets, which I enter into the computer.

What time does that save? I don’t have to carry or organize a grade book. When I sit down to grade, I have to grab one fewer thing. A silly benefit is that it becomes easier to convince myself to sit down to grade a stack of homework: since I don’t have to futz with the gradebook, since I can just start assigning and recording scores, I tend to get started more willingly.

Now, I’m not suggesting that using loose paper rather than a gradebook is going to work for you. I’m merely offering the idea that some thought towards logistics might allow you to save little bits of time this coming school year. And, those little bits add up.


[1] Or else what? It’s never been entirely clear. Fortunately, in the ten years I’ve read we’ve only once even had to work all the way to the end of the last day. The nine other readings have concluded with hours or even days to spare.