Structure of a class: AP Physics 1 [Formerly AP Physics B]

At Woodberry, the first-year AP class is generally taught to bright seniors.  Now, this includes a wide swath of folks, from the valedictorian all the way to the dedicated student with a 550 SAT math score.  Although everyone in the class is by definition a serious, motivated student, some are more motivated than others.  Even in AP it is still necessary to grade homework daily, hold students accountable for their preparation via quizzes and such, and provide entertainment enough to keep attention as the students trudge through a seven-period academic day.

A typical class starts with a quiz, which begins at the bell.  Daily quizzes in AP are sometimes 3-5 question multiple choice; sometimes a question about the previous night's homework; sometimes pure recall; sometimes even an authentic AP problem.  The purpose is not only to review, but to provoke a good discussion.

I collect the problems from each student's desk during the quiz.  

We take as long as necessary to discuss the quiz.  Then I'll ask the class pointed questions about the previous night's homework, possibly provoking a good discussion.*  Once questions about previous topics have petered out -- or once I decide we've talked long enough -- we move on to the day's lesson.

* Though I never just do the problem on the board for the class.  The dialogue here is more like "So, how did you figure out the mass of the cart, since it wasn't explicitly given?"

The "lesson" is generally an example problem that I pose and work through for the class.  Equipment is set up on the demonstration table so that the answer to the problem can be verified experimentally.  This is the "quantitative demonstration" -- we develop the intellectual habit of placing every problem in a laboratory context.  

This "lesson" is as much performance art as it is classical lecture.  I'm modeling the habits of good problem solving through the way I structure the board work; I'm interrupting frequently to ask "check your neighbor" questions; I'm engaging the class at every opportunity, and using every trick at my disposal, in order to maintain focus.  A typical demo takes anywhere from 10 to 30 minutes to complete.  

I try to time the end of the lecture with some sort of cliffhanger -- "Now we've solved for the cart's speed, will the measurement match the prediction?"  or "So tomorrow, we'll double the mass of the cart, and decide how the cart's speed changes."  

Since AP Physics 1 is less broad than the B course, I frequently have time to end class with 5 minutes of individual problem solving on the night's homework.  Students are expected to get something accomplished in those five minutes: the "ticket out the door" is to show me the written work they've done.  (I don't care whether said work is right or wrong... the only way anyone gets in trouble is if he shows me a blank paper and says "I don't know what to do.")

What about lab days?

The above class structure is used about three days each week.  One double-period is used for laboratory work.  I discuss the specific structure of a lab day in this post.  

And tests?

In AP, I only test once a month or so.  Those tests are entire period tests, usually using the 90-minute lab class.  Sometimes I'll use a class day soon after to do corrections.  

Review for the AP exam in April requires a different structure, too -- but by then I've established the class norms enough that I can use the 9th grade class structure with music and independent work.  

Structure of a class: 9th Grade Conceptual Physics

I believe in establishing a predictable routine for any class that I teach.  At the lower ages, structure is even more critical.  Because my students know generally what to expect from class each day, they are less vulnerable to distraction.  After a couple of months, the class is almost able to carry out the daily functions without me even saying a word; thus, we can focus on learning physics rather than on what specifically to do.

Now, your routine will almost certainly differ from mine, depending on your personality, your class time, class size, age of student, level, all sorts of things.  But I'm asked often enough about what actually happens in class on a day-to-day basis that I think it useful to go through the routine.

In 9th grade, on a typical class day:

We start with a 3 to 4 minute quiz, during which I collect the problem set that was due.  (Since I collect the problems from each student's desk personally, it's nearly impossible for a slacker to skate by without me noticing incomplete homework.)  The students trade and grade the quizzes.

I take just a few minutes to answer questions and to show the class any information necessary for the day's activity.  If we have truly new material, I've already printed and handed out a fact sheet for their reference.

I ask who had the highest quiz score; this student gets to choose the Pandora station that I play during the rest of class.*

* Music is perhaps the most interesting innovation from this past year of teaching freshmen.  Since the last half of class almost always involves students working independently as opposed to me talking at the front of the room, there's no reason NOT to put some music on in the background.  The quiz is taken that much more seriously, knowing that music selection is the reward for performance.  It's amazing to me just how important this reward is to the class.  And, woe to the class when someone doesn't complete his problem set.  We don't listen to music unless everyone turned in the homework.  Peer pressure can be useful...

Students are released to work at their own pace on a set of problems and experiments.  Generally, the students solve a problem or a series of problems, as they would on a nightly set.  As they finish each problem, they show me their work.  If they're wrong, I send them back to their seat to do it right.  If they're right, they proceed to the next step.

Usually the "next step" after solving a problem is to head to the back of the room, where the problem is set up as an experiment.  The students are asked to perform the experiment to measure whatever they predicted on the in-class problem.  For example, the in-class problem might include questions about the motion represented by a velocity-time graph; the experiment would then be to produce the graph with a cart on a track and a motion detector.  They show me a printout or a picture of the experimental results... and then they get a new problem to do.

We end class in time to straighten up quickly, and for me to hand out the next day's problem set.

This format has variations... sometimes we do a more traditional laboratory exercise in groups instead of the individually-focused problems and experiments.  Sometimes we do test corrections, or problem set review.  But the typical quiz - brief talk - independent work model is almost always used.

No "Mercy" -- train students to treat exams dispassionately

Saturday we finished grading all 100,000+ AP physics exams.  I was assigned throughout the week to two problems – B7, the one about atomic energy levels; and B4, the one about two blocks connected over a pulley that create projectile motion.  I feel like I’ve seen every available correct approach to these problems, as well as every possible misconception that our students have.

As you can probably imagine, a non-negligible number of exams were taken by students who were woefully unprepared.  We saw blank papers, and not just on question 7.  We read clever doodles, poems, messages, “Mr. Lipshutz should be fired,” “Thank you for grading my exam, hope you have a great day,” and, of course, the classic “Kick Mr. Kirby* in the butt for me.”

* Observed multiple times over the years at the reading:  “Hi, you’re Martin Kirby?”  “Yes, I’m Martin, nice to meet you.”  “Hey, could you turn around?  I have a message to deliver that one of your students wrote on his exam.”

I don’t mind all these.  If a student didn’t study appropriately all year, his punishment – or at least the natural consequence – is to sit still for three hours taking an exam he has no chance of passing.  If he wants to spend his time entertaining me, more power to him.

One of the things students write that most bothers me, though, is the serious or semi-serious plea for mercy.  “Please, I’ve worked hard all year, my teacher is new, I really want credit in college, have some mercy!  Give me some points, please!  We didn’t do a problem just like this in class, but I’m answering the best I can, be nice and give me a 3.”

What makes the plea for “mercy” bothersome, while I laugh respectfully at the animal drawings? 

Perhaps the principal battle that physics teachers must fight involves those students who don't adjust well to the unique nature of a physics course.  Students who have earned As throughout their school career because they can memorize and they have a big vocabulary often become frustrated by creative problem solving.  As they see they might not earn an A in physics, as they (think they) see their visions of being valedictorian of their Harvard Medical School class going up in smoke, they -- and their parents -- fight.  Such a student tends to use "compassion" and "mercy" as a weapon.  They attempt to portray the physics teacher as a mean person, hoping to pressure him or her to back off the course expectations.

This sort of smear campaign would be comical if it weren't so effective.  All it takes is one or two physics-ignorant teachers to champion such a student's cause; then even if the teacher stands her ground, a subset of students feels validated enough to persist in their hostile attitude, spreading their incompetence and despondence throughout the class.  And if the principal doesn't give an emphatic smack-down to the first whiny parent, the teacher is up a creek.  

I want to change the conversation everywhere, not just in physics.  An exam should be viewed as a dispassionate, objective evaluation of a student's skills.  Teachers do not "give" grades, students "earn" grades.  A score, good or bad, on a test doesn't reflect on the character of the teacher: a teacher is not "kind" if the class does well, a teacher is not "mean" if the class does poorly.  

Poor performance on an exam should be viewed like a loss at an athletic contest: it doesn't necessarily reflect on the character of the test taker, it's just an evaluation that "your team was not as good as the other team today."  A loss should not be attributed to the fact that the coach is mean; a win doesn't mean the coach is compassionate.

So when I see students pleading for mercy on their AP exams, I despair.  Such pleas are too often learned behavior.  They see whining, begging, smearing as effective in their own school, so they try it on the AP exam itself.  Now, I don't know what makes anyone think that an AP reader, who is generally a consummate professional bound to grade precisely to a rubric, has any ability or desire to raise or lower a particular student's score.  That's as crazy as suggesting that NFL officials are out to get Seattle, or Cleveland, or whoever.  

Strike down the language of "mercy" early in the year, so you can focus on learning physics.  If you ever need help on that account, I am more than happy for you to show this post to parents, colleagues, or administrators; or I'll even add an amicus curiae on your behalf.  None of us anywhere can teach properly if our students are gaming schools' social structure rather than practicing their problem solving.

Trick to writing paragraph-length comments: Sort the roster first.

Every marking period -- that is, six times each year -- teachers at my school are asked to write a paragraph-length comment on each of our students.

Now, most schools don't require their teachers to write narratives about students.  But those teachers who have a similar requirement, even if it's only twice a year, are asked to take on a daunting writing task.  That's four to five thousand words each marking period.  How is it possible to streamline the process?

A colleague of mine, Matthew Keating, shared the key secret with me several years ago.  See, I used to run my grades, print out the spreadsheet, and then go in order:  "Okay, Adams.  What can I say about him... now Baker.  Now Cabrera.  Now Davis...."  While alphabetical order is the way my spreadsheet and my school's computer system sort the students, it's not a *meaningful* order.  You'll be jumping around mentally, writing about dissimilar students in quick succession.

Matthew pointed out the one easy trick:  Sort the class from lowest grade to highest.  Then write.  This sorting places similar students in quick succession.  Phrases and descriptions can be reused or adapted in the very next comment, not from the comments you wrote half an hour ago.  

More importantly, such a sorting improves the quality of the comments as well as the speed of writing.  Comment writing is a long, intellectually draining process.  Sometimes I'm running on fumes by the end; sometimes I'm pushing up against a deadline by the end.  

So why not get the tough comments written first?  The comments for students with the lowest grades are the ones that will be most scrutinized.  These are the comments most likely to get a teacher into trouble if they are not carefully phrased with politically correct but nevertheless clear and direct language.  These are NOT the comments I want to bang out hurriedly in a desperate attempt to just be done.

This way, when the brain is beginning to fade, what's left to do?  The A students, whose parents are more likely to skim over your praise as just a small part of a hagiographic report card.  For these folks, a two-sentence note with phrases like "I very much appreciate his strong efforts this marking period" will be sufficient.  I can write those kinds of comments in my sleep, which is sometimes essentially what I have to do.

Acceleration doesn't "move." Acceleration "is."

The big misconceptions about acceleration boil down to thinking that "acceleration" means the same thing as "velocity."  Other than repetition and drill, I think the best way to bust the misconceptions is to make students write.

I might ask, "What is the direction of the car's acceleration?  Justify your answer."

For years, I would accept for full or almost-full credit an answer like "The problem says the car is going west, but the car is slowing down.  Slowing down means acceleration must move opposite the direction the car is going, so acceleration is east."

This year, I put my foot down.  That answer earned zero or almost-zero credit, to my students' considerable protest and dismay:

"My answer's right," they said.  "The acceleration is east."

Yes, but the justification is wrong. 

"No it's not.  You have a fact right here in the notes that says slowing down means acceleration and velocity move opposite each other."

That's emphatically NOT what I said.  Acceleration does not "move."  Acceleration "is."  The correct justification says that "the acceleration IS opposite the direction of motion."  When you say "acceleration moves east," you're implying that acceleration and motion are the same thing.

"Oh, come on, now.  The car's moving, isn't it?"

Sure, but the direction of the motion is the direction of velocity.  Acceleration does not have to be in the direction of motion.

"So you're going to take off all those points 'cause I got one word wrong?"

Conversation over -- I don't discuss points, only physics.  The correct response here, student, is to pledge not to ever again refer to acceleration as "moving.\

Was it worth fighting with whiny freshmen who thought they knew physics better than I?  Absolutely.  We just finished three weeks of review for the cumulative final exam.  Of all the mechanics topics we covered, motion was the one they handled best.  Mistakes about the amount or direction of acceleration were rarer than ever.

The other major differences in my approach to kinematics this year was using units of "m/s per second" for acceleration rather than m/s².  I know that helped get students understanding what the magnitude of an acceleration meant.  But I think the key to getting the direction right was fighting to eliminate the phrase "acceleration moves."

The friction force is NOT the "force of friction on the object."

Given today on a review quiz:  

A 0.5 kg puck sliding on a horizontal shuffleboard court is slowed to rest by a friction force of 1.2 N.  

(a) On the dot below, draw a free body diagram of the puck.

(b) For each force, indicate the objects applying and experiencing the force.

(c) Determine the amount of the normal force on the puck.

I've answered part (a) in the picture.  Only three of my forty students got this wrong: one forgot the friction force, two forgot the normal force.  To their immense credit, not a soul put some bull honkey such as "force of motion."

Also to their immense credit, not a soul misidentified the object applying the force marked "weight."  Every one of the class listed weight as the "force of earth on the puck."  And, everyone got the normal force right: it's the force of the shuffleboard court (or of the "surface") on the puck.  So I'm proud of my class for avoiding two of the more common problems introductory students would have with this question.

So why did at least a third of the class say "force of friction on the puck?"  No, no, no, "friction" isn't an object!  Only objects can apply forces!  The correct statement is that Ff is the "force of the shuffleboard court on the puck."  

I expect this mistake (and many, many others) in the winter when we first cover forces and free-body diagrams.  Anyone know why I managed to teach about weight and normal force successfully, but not friction?  :-)

EMF induced in a straight wire -- 2013 AP Physics B problem 6(e)

I feel a disturbance in the force... problem 6 on the 2013 AP Physics B exam seems to be generating significant discussion of emf induced in a straight wire, rather than in a loop of wire.

First of all, take a look at the question, to be found in this file at collegeboard.com.  I can't post the question here for lawyerly reasons.    

In the last part, we have a wire carrying a current to the right.  A second wire is placed above the first wire, and is moved upward at constant speed.  

Finding the amount of induced emf is easy: ε = BLv.  Although the magnetic field is different at different distances from the current-carrying wire, the question asks about induced emf at one specific location.  So calculate the magnetic field B produced there, use the length of the wire, and the speed the wire is moving.  Fine.

But which side of the wire is at a higher electric potential?  Lenz's law is the usual approach to such questions.  Without a loop of wire, though, I don't know how to apply my usual approach to Lenz's law.

The way I understand this situation is to consider the effect of the magnetic field on positive charge carriers in the wire.*  The force on these positive charge carriers is given by the right hand rule associated with F = qvB, the magnetic force on a moving charge.  Which direction is this magnetic force?

*Or on the electrons, or both, or whatever.  When you've got a conductor, you can equivalently discuss the motion of electrons or the "holes in the electron sea" which are essentially "positive charge carriers."  Who cares.  Just don't call 'em "protons" and we're all copacetic.

The positive charge carriers are moving with the wire, or up the page.  The magnetic field produced by the rightward current in the bottom wire is out of the page (by a different right hand rule).  Thus, the force on these positive charge carriers is to the right.  Similarly, the force on negative charge carriers in the moving wire is to the left.

So as the wire moves, the right end of the wire becomes more positively charged, the left end becomes more negatively charged.  Which is at the higher electric potential?

The right side is at a higher electric potential.

"Whoa there, Hoss," you say.  "By your own definition, positive charges are forced from high to low electric potential.  These positive charges were forced to the right... so why is the right side the HIGHER electric potential?"

Because the positive charge carriers were forced to the right by a magnetic force, not an electric force.

To find the side with higher electric potential, consider what would happen ELECTRICALLY to a positive test charge placed in the wire, ignoring magnetic effects.  This positive test charge would be repelled electrostatically by the positive right side, and attracted to the negative left side.  Positive test charges are forced from high to low electric potential; the right side must be at higher electric potential.

A more conceptual approach:  Again, separate magnetic and electric effects.  The right side becomes positively charge, the left side negative.  Now if the magnetic effects were removed, which way would the current -- flow of positive charge -- be?  Obviously from + to -, or from right to left.  Current flows out of the positive side of a battery, or from the high voltage to low voltage.  So the right end is at higher "voltage."

GCJ