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29 April 2015

Mail Time: What do I assign for homework the weekend and night before the AP Physics 1 exam?

My friend Eric sends in a FAQ:

What will you assign for homework the weekend before the exam?
The exam is on Wednesday, so we'll probably do one old AP problem for homework as the assignment for Monday.  Or possibly I'll scour the released official AP Physics 1 multiple choice questions for some we haven't done; I'd assign about three or four with a mandate to "justify your answer."  

What about the assignment the night before the exam?
I assign nothing.  There's no point.  I forbid studying after 5:00 pm Tuesday night.  At a day school, I used to have a cookout; this year on Tuesday I have to take my kid to a baseball game, so the physics party at my house will have to start when I get home after the game.  But we will have a party.  And we will not do physics at the party.

I warn the class a week ahead of time about not studying the night before the exam.  I explain the rationale in terms of sports: would you run a marathon as practice the night before the marathon?  Would you spend the evening in the weight room the night before the state championship football game?  No?  Then don't study the night before the AP Physics exam.  The preparation -- or lack thereof -- they've done all year cannot be changed by one night's work.

Perhaps you'll tell me that one student might have reviewed something he didn't remember and gotten it right on the exam.  I respond that he could have done that review on Sunday night.  More to the point, consider not one student but the overall class.  The number of questions we'll get right just because we go in rested with a confident demeanor, unafraid to mess up, ready to show what we know will make up for any mythical effective review on Tuesday night.

Do you recommend weekend review sessions?  I have NOT scheduled any.  But several other AP teachers are requiring review nights and afternoons.  

I don't at all recommend weekend review sessions.  The resentment they breed and the stress they create are huge costs which I think aren't outweighed by the benefits of the extra study.  

Now, you're you, and you have to find out how you fit in your school's AP ecosystem.  For me, I've found a lot of success letting other teachers be the cattle drivers and holders of whips, while this time of year I am more relaxed than anyone.  That attitude has paid off over the years, both in terms of AP scores and in terms of positive political capital among my students.  (I hold the whip and cattle prod in October and November, but somehow I misplace them when the weather turns warm.)

Good luck.  Not much more you can do -- taper your class like you'd taper your swim team.  You put in your practice time over the course of the season.  No point in doing a big workout the day before the state meet.

24 April 2015

Is AP Physics 1 "Too Difficult?" No.

I received an email this afternoon that I think is important to address on the eve of the first ever AP Physics 1 exam, and into the second year of teaching AP Physics 1.  In sum, is AP Physics 1 "too difficult?"  We've known that the exam would be deeper and tougher, but is it too deep and too tough?

After some very kind words about 5 Steps to a 5: AP Physics 1, which I appreciate, the note asked:

[The] question has to do with the released AP Physics 1 exam free response section.  I have been teaching physics for 20 years and I have never seen my students so frustrated after attempting those FRQs.  I'm usually the last person to say something is too difficult, but has the AP board gone over the top this time?  Just for a reaction I gave this test to my AP Physics C students and they said many of these questions would have been too difficult for them to answer last year.  What are some of the other opinions you have heard from other AP Physics teachers out there?  What is your opinion of this released exam?

My response: The exam is not over the top -- I think it's actually quite wonderful.  The College Board is doing exactly what they said they were going to do:  create a physics test that goes well beyond mere calculation and into deep understanding.  We knew from the beginning that students who think of physics as crunching numbers, doing algebra, and obtaining a right answer would be in trouble.  Such students could always manage a 3 on the AP B exam, but will not likely earn a 1 on the new exam.  

It's been a tough year for me teaching AP 1, primarily because I didn't have an established "physics culture" to help my students through the difficult times:  see the next few April 2015 blog posts.

That said, my upperclassmen are now doing a great job explaining their calculations, describing what they know, etc.  I am not going to have the same ~70% earning 5s as I used to on AP Physics B, but we will do just fine.  It takes months for the students to adapt to expressing physics understanding in words, and to adapt to dealing with difficult problems that don't have a few gimme calculations in them.

The good news is, you've given your students a real test in the style of the AP -- they can complain all they want, but the exam ain't changing.  Let them get the complaints out of their system, and they'll know what to expect on May 6.

Then, next year, you can think about preparing your students for this level of question a bit earlier on.  Try giving some of the released questions on the January semester exam, or on a major February test.  They're going to have to come to terms with the more difficult nature of the new course; it's going to be a learning process for all of us as to how best to do that.  I know I haven't figured it out yet.  

Good luck -- to you, and to your students next week.  :-)

23 April 2015

Lessons from a year of teaching AP Physics 1, part 3: Culture Matters

And now for a series of posts reflecting on a year of teaching AP Physics 1.  I've already posted a bunch of stuff that worked well; but a good scientist publishes the results of all experiments.  

Part 3: The "physics culture" that you build over years matters.  A lot.

Twice in my career, I've moved to a new school and created a rigorous, successful algebra-based AP Physics program from scratch.  In both cases, it took three years to establish the program -- three years before student expectations matched what I delivered for them.  The pattern in both cases was the same.  I discuss this three-year pattern with participants at my AP Summer Institutes.  I share these as my personal experiences, but I've had numerous physics teachers discuss similar phenomena.

Read the pattern.  Then read to the bottom to see the IMPORTANT lesson learned from this year.

Year 1: Culture clash.  AP Physics is more difficult than most honors or AP courses.  The College Board has statistics to prove this.*  Yet, with no physics culture established, students have the expectation that AP Physics will be no different from other AP courses: work hard, do the reading, and you'll probably get an A and a 4 on the exam.

* For example:  Of students with an SAT verbal score of 700, about 80% earned a 4 or a 5 on AP English Lit; however, students with an SAT math score of 700 only earned 4s or 5s 54% of the time on AP Physics B.  Search for College Board Report 98-4.

In my first year at each school, many of my students fought me through much of the year.  The course was too fast-paced, too difficult, impossible, unfair, ridiculous.  They complained to teachers and administrators, telling tales about how mean I was, how I didn't care about them.  What was really going on was that I was teaching directly to level and difficulty of the AP exam without exception.  What made me "mean" in most cases was that I wouldn't give pity points or extra credit.  Folks were shocked by the black-and-white, right-or-wrong nature of physics.

Knowing the difficult position I was in, I reached out to my students.  I provided extra help, made personal connections in every way I could, hosted physics parties... Yet a bunch of my students -- predominantly seniors -- saw nothing but a grade less than an A, and felt that I, personally, was keeping them out of college.  Many came around by year's end, realizing how well they were prepared for the AP exam and for college physics.  Some never forgave me.

It hurt deeply when some of the students I worked hardest to win over made personal attacks.  In 1997, I played tennis with Jonathan, I encouraged him and made him a leader on our robotics team, I wrote him a wonderfully positive recommendation which I shared with his parents... and then mid-year I heard him and his parents telling a crowd how I had screwed up his college process and his quest for valedictorian because I didn't like Jonathan.  In 2000 I received a scathing evaluation from Jake the headmaster's advisee, saying primarily that I was never available to students and refused to help them... even though he was one of a group who was at my apartment two nights a week or more midyear.  And these weren't isolated cases.  The poison spewed by these few angry teenagers permeated the class, and colored their perception of me.

My class became much more positive at year's end, as college acceptances were known, and folks realized how confident about and prepared for the AP exam they were.  The quiet majority who appreciated my work emerged from the woodwork.  But boy, were those tough, draining years.

Year 2: Student leadership emerges

At both new schools, my class size in year 2 was reduced.  Students looking for an easy grade or a pushover teacher didn't bother to sign up; those who did sign up initially did so with some trepidation, but knew clearly the challenge they faced.

And, in the second year, the course was easier, the teacher "nicer", than they ever imagined.  Most of the improvement in the class's tenor can be attributed to the disconnect between expectation and reality.  In the first year, the class and I were more demanding than any student had anticipated, provoking hostility. In the second year, the horrible rumors about me actually helped the class's attitude.  They expected hostility from me, but they got a supportive and encouraging (but still demanding) teacher.

Was I any different?  Of course I was, but not in the way you might think.  I eliminated activities that didn't work, I changed my approach to assignments as fitted the school environment, I did more of the kinds of activities that DID work with the new student population I was facing.  But I did *not* change my fundamental attitude toward the students -- I still tried everything in my arsenal to build relationships outside of class.  I still didn't budge on the rightness or wrongness of my students' physics.  I still took incredible pains to point out when students were RIGHT, not just when they were wrong, so as to build confidence.

These students from the SECOND year at each school are now the alumni with whom I am closest.  They heard all the bad things about me, and plunged into my class regardless.  They had a universally positive experience, which they then shared enthusiastically with the next year's class.  They defended me emphatically to the numerous naysayers in the community who still believed rumors about my nastiness.  And they recruited for me, such that in year 3...

Year 3: A self-perpetuating physics culture 

In the third year at both schools, my class size swelled enormously, to a size much bigger than in year 1.  The absolute top students were going to take my class regardless.  But the type of not-quite-top students who had kvetched about me in year 1 and avoided me in year 2 suddenly were not only in the class, but doing extremely well.  Yes, I was better at teaching AP Physics aimed at the right student population based on three years of experience. But also, the students had established a correct perception of who I was, what my class was about.  In year 3 and beyond, students knew through accurate gossip exactly what to expect in AP Physics.

The result in both cases was a well-subscribed program where not just the 780-SAT-Math set was earning 5s on the AP exam.  Students experienced success, intellectual rigor, and real fun, and so they told their friends to jump on board.  Because of the positive team atmosphere in my class, year after year I was recruiting marginal students, teaching them physics, boosting their confidence such that they performed brilliantly.  My classes for years averaged about 63-65 on their PSAT math, yet all but two students passed the AP exam, and more than 70% earned 5s.  And the number of angry, hostile seniors was minimal.  (Non zero, especially in January, but negligible.)

So what is the "lesson" I learned from teaching AP Physics 1 this year?

We had not taught an official AP course for four years prior to this year.  I taught freshmen for the past two years; colleagues taught an honors course, but one without a high-stakes no-excuses AP Physics exam.  When I returned this year to teach AP Physics 1, I missed three important changes in the school's physics culture:

(1) I hadn't taught any of the current seniors, so I had none of the gossip mill matching expectations to reality.  These folks didn't know me, neither personally nor by reputation.
(2) Because of changes to our overall curriculum, the top students were no longer taking AP Physics as juniors and seniors.*  The population in this year's AP class was equivalent to the bottom half of the classes I had taught for decades.
(3) AP Physics 1 is a much more difficult course than AP Physics B, especially for students who are not top of their class.

*They had taken honors physics as freshmen, and were in either AP Physics 2 or a research course as seniors.

In other words, I was back to year 1.  And I didn't realize it, at least not until November.

In my junior-senior AP Physics 1 class, this year has been every bit as difficult as my first year at Woodberry, as my first year at my previous school.  My efforts to build personal relationships with students haven't flagged; but the majority of the class stayed distant and huffy, with several seniors attacking me ridiculously and personally on a mid-year evaluation.

My class has become much more positive now at year's end, as college acceptances are known, and folks realize how confident about and prepared for the AP exam they are.  The quiet majority who appreciate my work are emerging from the woodwork.  But deja vu: this was a tough, draining year.

Moral: Be conscious of your school physics culture.  Cultivate it.  

If you're had a tough year in this new AP Physics 1 program -- and I know many of you have, 'cause I've heard from you by email -- stay the course, and know that you are not alone.  Remain positive and recruit for next year.  Be as self-critical as you can be (more on that in the next post) in order to change what needs to be changed.... but also recognize the things you've done that worked well, the students who have had positive experiences with you and your class.  Know that next year will be better, and two years from now you will have a smoothly running physics teaching machine.  Eventually, students, parents, and administrators will match their expectations to the reality of your AP Physics 1 class.

20 April 2015

AP Physics 1 electrostatics assignment and video: PSSC Coulomb's Law

I've never, ever been able to get a quantitative Coulomb's Law demonstration to work.  The best, and only, quantitative electrostatics demonstration I know comes from Wayne Mullins, and is described here.  But that experiment measures electric potential; how can I directly measure the force between charged objects?

While *I* can't measure that force, the stereotypical 1950s scientists in this PSSC video can.  

Watch them charge and discharge the foil-covered balls.  Watch them move the balls closer together and farther apart, and measure the change in electrostatic force between the balls.  Watch them cut the charge of one ball in half, and show that the force likewise is cut in half.

I've already had my students read up a bit on Coulomb's Law.  I don't see the point in lecturing on it -- what can I say that isn't in a standard treatment, since I can't do a live experiment?  But I can show this video.

Tonight I've assigned my students to watch the first 16:00 of the video as homework.  Then, in class tomorrow, we will work on this problem set.  The problem starts with a situation like in AP Physics B 2009 problem 2, where two charged balls hang from two strings.  Instead of asking about electric field lines and electric potential, I go straight to the equilibrium conditions.  Then I add a conceptual piece: I double the charge on one of the hanging objects; describe any changes.

Try the video and the assignment.  Let me know if you have other non-Van de Graff suggestions for a quick AP 1 level Coulomb's law treatment.  (Sorry, I gave up on Van de Graffs after seven years of never getting them to work right.)

17 April 2015

Lessons from a year of teaching AP Physics 1, part 1: "diving in"

And now for a series of posts reflecting on a year of teaching AP Physics 1.  I've already posted a bunch of stuff that worked well; but a good scientist publishes the results of all experiments.  

Part 1: Should you "dive in" to the AP Physics 1 expectations for verbal response?

When I wrote the 5 Steps to a 5 Teacher's Manual, my intent was to begin the year with essentially my well-received and well-practiced AP Physics B course.  I had always covered mechanics before the first trimester exam in November. I intended to cover waves and circuits in December.  This coverage would be equivalent to what I did in physics B, with plenty of calculation, but also some "justify your answer" questions and laboratory work.

Then, in January, I intended to start over from the beginning of the course, with deeper discussions of all topics.  This is when I would introduce multiple representations of energy and momentum, when I would ask essay-style open-ended questions, when I would do unguided laboratory work.  Rotation would come in February and March, as a review of all of the mechanics topics previously covered twice.  I know from long experience that the best way to learn physics is to see the same material several times, separated by months; so that's how I set up my class.

But I got some bad advice.  And I took the bad advice.  D'oh.

At an AP consultant meeting in April of 2014, I mentioned my plans during an open discussion of teaching ideas for the new course.  I was set upon by this room of physics teachers: "You shouldn't ever do that, let alone recommend it."  "You must teach the deeper expectations of the new course from the beginning."  "This course absolutely must be inquiry-based from day 1."

Now, these people who were attacking my idea were people I know and trust: excellent physics teachers with significant experience, and with serious understanding of the new AP Physics 1 curriculum.  I had twenty respected physics teachers telling me, in so many words, that I was being an idiot and setting my students up for failure.  I'm used to ignoring well-meaning but dumb advice from teachers who aren't familiar with my subject or with me; however, when some of the most-skilled people in my profession were telling me I'm wrong, I'd have been arrogant to dismiss their concerns out of hand.  

The argument that finally carried the day with me threw my own advice back in my face:  "Start the course with AP expectations, don't ease into AP expectations.  If you start with calculational physics, and if students have success and good grades with calculational physics, then most will not be willing or able to adapt later to the deeper and harder expectations for conceptual and descriptive physics."  I've always told teachers to dive straight in to physics when teaching upperclassmen, and not to pussyfoot around difficult topics.  Students need to know from day 1 the level of the course.  By day 45 they, invariably, have adapted to the expectations, such that the rest of the year and the AP exam are just more days at the office.  

So I did what my colleagues suggested, against my own better judgment.  I dived in, with every problem set requiring significant verbal response, with all test questions going deeper than mere calculation.

And it didn't work.  I was right with my first idea.  I set a poor tone for the class, and I paid the price.

What were these AP consultants on about, then?  What did they miss?

Primarily, I think my colleagues weren't familiar enough with what I mean by calculational physics.  I have never, ever taught physics as a math course, in which students hunt for the right numbers to plug in to the right equations.  My students were so successful in AP Physics B precisely because I integrated conceptual understanding, verbal justification, and an understanding of the physical meaning of numerical answers into a calculational course.  They didn't realize how close my AP Physics B course was already to a good AP Physics 1 course.

In the first few months of this school year, I and my students struggled mightily.  The weaker students felt that everything was over their heads.  The stronger students argued with me about whether their writing communicated what they thought it did.  Problems and activities took way longer than I had budgeted, because writing about anything takes a long time; writing about physics without a solid calculational background takes even longer.

What if I had gone with the calculational approach at the beginning?  The fact is, students come to me answer-focused.  It is my job to make them process-focused by year's end, but I can't change their mindset instantly.  One advantage of calculational physics is that a calculation is right or wrong -- it makes a correct, experimentally testable prediction, or it does not.  My class used to stop arguing with me about points and grades very quickly, because it's hard to argue with "do the experiment, and if it gives the value you predicted I'll give you credit."  

A second advantage of starting with calculational physics is that weaker students -- folks who can pass the AP exam at year's end, but who will be significantly challenged the whole way -- can find some early success.  No, the AP exam will not award much credit at all for getting the numerical answer to a kinematics problem right.  But please don't underestimate how difficult even algebraic kinematics is for many students.  I used to hear regular feedback during our April review in the style of, "wow, remember when a problem like this was so, so difficult?  I wonder why I found it so hard, I get it now."  A good number of students need to play with equations and numbers, to work on the skill of plugging into equations until they're comfortable recognizing known and unknown variables in a relevant equation.  I denied my class the opportunity to experience success and earn credit for their learning process; and so my weaker students because hostile as they felt more and more overwhelmed.

Could I have avoided the hostility?  Not entirely...  physics ain't easy, and every year I've had at least some seniors initially angry at me*.  But with a more appropriate start to my course, weak students could have felt like they were getting somewhere, because they would have gotten some right answers.   Then they would have gradually come to the same understanding of physics that they've obtained anyway.  Ideally, the grousing fades into background noise as the class realizes, student by student, how well they're understanding physics, and how intrinsically exciting physics is.  That process took seven months this year; had I started the year MY WAY, I suspect that I could have reduced that to one or two months, just like I did for years in AP Physics B.

* Why angry?  Angry because they're working hard and not getting A's, angry because they perceive that I'm reducing their chances of going to Harvard, angry because I collect and grade their homework every night (even though I don't assign a lot compared to most physics teachers), angry because my class is the first time they've ever NOT understood something instantly, angry because they're 17 year old boys... take your pick.  They almost always get over their anger by year's end.

Lessons from a year of teaching AP Physics 1, part 2: Mindset

And now for a series of posts reflecting on a year of teaching AP Physics 1.  I've already posted a bunch of stuff that worked well; but a good scientist publishes the results of all experiments.  

Part 2: "Inquiry" only works if your students are in a mindset that allows them to inquire.

Last summer our faculty read a pop psychology book about "fixed" and "growth" mindsets.  In short, a student in a fixed mindset thinks of school as a set of tasks to be completed, some of which he's intrinsically good at, some of which he's not.  A student in a growth mindset considers school to be a series of challenges to be conquered, some of which will lead to success, some of which will not, but all of which will be valuable experiences.  I wasn't impressed with the text of the book itself, but the concept of the two mindsets -- and the common language that our faculty now have to describe our students and our work with them -- has been valuable to me.

If you read this blog, you know that I am as far from a fluffy educator as it is possible to be.  I'm a physics teacher -- I do what works, not what's popular. I evaluate my work not in reference to the buzzword of the month but in reference to physics learned, relationships built, attitudes toward science adjusted, and scientific misconceptions squashed.

Nevertheless, as I was preparing to teach AP Physics 1 last summer, I got excited about the opportunity to use some different approaches with my class.  I had significant success with my ninth graders the past couple of years moving away from the front of the classroom, letting them work through problems themselves rather than showing them directly what to do.  I saw that the new AP exam didn't cover as much material, leaving more time in class to allow for experimental investigation on the students' own.  And the AP Physics world was buzzing with ideas that sounded fun.  The quantitative demonstrations that I've done in lecture for years have proven effective; but I was excited to try new, adventurous things for a new course that seemed to require more active participation from students than quantitative demonstrations allowed for.

In other words, I pledged to have more of a "growth mindset" than ever this year.

But it's not about me.  I didn't take into account the mindset of my students.

One of the two main reasons that Woodberry teaches ninth-grade physics is, in fact, student mindset.  Freshmen in their first months at boarding school have a wonderful growth mindset.  It takes a huge step for them merely to commit to attending a difficult academic school away from home.  So when our 14 year olds are presented with a non-traditional class -- one with near-daily lab work, one where the teacher doesn't merely lecture in front of the room, one where they can't always just memorize the correct answer -- they adapt quickly and enthusiastically.  The ninth grade consistently names physics as one of their favorite courses, and I never lecture from the front of the room for more than five consecutive minutes.

Seniors work differently.  They expect a certain sort of class, in which attention to the teacher and completion of homework leads to academic success.  They are, for the most part, poster children for the fixed mindset.  After two years of teaching the puppy-like freshmen who adapted willingly to whatever I suggested, I didn't anticipate the hostile reaction I'd get from seniors when I made plans NOT to stand at the front of the room.

For example:  For freshmen, an amazingly successful approach has been to have them, individually in class, solve a problem step-by-step.  Each student shows me his work after each step, where I either say "correct -- move on" or I explain what's wrong, and have him do that part again.  Once all steps are completed, the situation described in the problem is set up experimentally in the back of the room.  The student does the experiment to verify (or not verify) the answer to the problem.  The ninth graders love this approach, because they don't have to sit still, because they get instant feedback, and because they get to manipulate equipment themselves, seeing viscerally the connection between abstract problem solving and physical reality.

The seniors hated this approach, for the most part.  In particular, they took it personally when they got part of a problem wrong.  I would explain politely where they had expressed a misconception... and they would get mad at me for one reason or another.  "You never told us that."  (Well, I did, just I didn't scream it from in front of the white board.)  "How was I supposed to figure that out?"  (By either having good insight, or by developing good insight trying some things that don't work)  "When are you going to let me move on?" (When you get this first part right... which you might very quickly if you'd quit arguing and whining and just try it again the way I suggested.)*

* I rarely articulated these responses.  One of my colleagues suggests that I should have been more aggressive about sharing my point of view with my seniors, in order to nip these complaints in the bud.  I'm still not sure.

I was flabbergasted by the seniors' resistance.  Why were they reacting so differently than I had expected?  In retrospect, it was clearly because I had underestimated the fixed-ness of their mindset.  They came to me ready to be told what to learn, with a low tolerance for failure.  I had spent most of my career adapting my AP-level course to students who need careful guidance along the path to true inquiry.  And then I threw all those adaptations out the window in the name of trying new approaches that had worked with freshmen.  Oops.  It sounds utterly stupid now that I articulate that statement.

Every single new idea that I tried for AP Physics 1 was met with hostility from my seniors.  They didn't begin to settle down until late October, when I went straight back to teaching from the front of the room.  Now that we're at the end of the year, now that I've done a good amount of lecturing, I have been able to bring out some "open-inquiry" style activities with success.  My mistake wasn't in adapting new ideas to my course; my mistake was expecting that my seniors would have enough of a growth mindset to try those new activities right away.

How am I going to change what I do?  Well, for one, I'm teaching all freshmen next year.  One of my colleagues, who is far better than I at dealing with seniors, will teach the upperclass AP-equivalent course.

But were I teaching the seniors again, I'd go back to what worked -- quantitative demonstrations, presented from the front of the room while the class listens and asks questions.  That atmosphere is what seniors know and expect.  I would, as I have for decades, gradually introduce crazier elements of physics inquiry... but I've learned my lesson that the upperclassmen aren't ready to inquire at the beginning of the year.

Is it evidence of a growth mindset to structure a course to match students' (initially) fixed mindset?

And, is that Zen, or just gobbledygook?

08 April 2015

Don't be shocked by AP Physics 1 questions. Justify your answer; Annotate your calculations.

Most of us who teach AP Physics courses are starting cumulative review for the exam.  I've written previously about sources for exam questions in the style of the new AP Physics 1 or 2 exams:  5 Steps to a 5, AP Central, the Big Amazing Resource at PGP-secure.  

Point is, if you haven't gotten the message yet, please hear it now:  Students who can calculate to get right answers, but who can not explain physics clearly in words with reference to facts and equations, shall not pass.  It used to be, on AP Physics B, that clever number crunchers could earn a 3 by knowing which equations to use in which situations.  Such folks will fail on AP Physics 1.  Unless, that is, they practice and get good at explaining in words WHY they chose the equations they did, WHY they chose the values for each variable, and what the fundamental principles are behind the relevant equations.

Ideally your students have been practicing their verbal justifications for all or much of the year.  If not, there's still time.  Assign the authentic practice exam questions, then make students correct everything they got wrong until you approve thoroughly of their solution.  

What if you need some free-response practice beyond the few resources linked above?  It's okay to use old AP Physics B questions or textbook questions, as long as you demand verbal explanations.  That's easy enough to do: in every question that says "Calculate," add the sentence to the end, "Annotate your calculation with a description of your approach."  Then hold students accountable for this description by marking the answer wrong and requiring a redo if there's not clear, verbal evidence of why they chose the relevant equation, and why they chose the values to use in the relevant equation.  Don't even look at whether the numerical answer is right; instead, pick one or two tricky aspects of the solution, and see if the student's explanation addressed those issues.  

In terms of multiple choice, there's no shortage of good questions available at the links above.  Physics B multiple choice from the last couple of released exams are also very good questions to use, as long as you eliminate the questions that are out of scope for the new course.  For all multiple choice questions, they become excellent verbal response practice if you simply add "justify your answer" to the end of them.

In my class the past week, each night's assignment has been three or four multiple choice questions that my students already saw on the trimester exam they took in March.  They are allowed to collaborate to determine the correct answer; but, they must justify that answer in clear, coherent prose.  This writing practice will serve my students well when they face the authentic AP Physics 1 exam on May 6.

02 April 2015

Bar Chart exercise: x and y momentum, angular momentum, and energy

I first became explicitly aware of the qualitative bar chart representation for energy conservation TIPERS book.  I know some of the newer textbooks, such as Etkina, use it as well.  Qualitative bar charts are a great way to cover energy conservation rigorously yet conceptually, without equations getting in the way of understanding.  They are simple enough to use... One bar per type of energy in the system, with three major columns:  Initial state, final state, and "work done by external forces".  The bars in the "initial" column plus the bars in the "work done by external forces" column must add to the bars in the "final" column.

I like the bar chart approach enough that I tried it for impulse-momentum problems, too.  Instead of one bar per type of energy, the approach becomes one bar per object within the system.  The columns become initial state, final state, and "impulse provided by external forces".  Again we can add bars: initial plus impulse equals final.  

In particular, the bar chart approach helps tremendously with the meaning of a negative sign.  Students can see quickly whether or not the bars add together properly; they can see why an impulse must be negative, and translate that to the direction that the external force is pushing.  Or, they can see why, mathematically, the work done on a system must be negative, and relate that to the direction of force and motion.  When I use equations only, negative signs tend to slip through the cracks.

(As an aside, I certainly didn't use qualitative bar charts EXCLUSIVELY.  I covered impulse and energy the same way I always have, but once we were comfortable with the basic concepts and ideas, I introduced the bar charts as a way of reviewing, and as a way of justifying conceptual questions without a bunch of equations.)

Now that we're at the end of the year, it's time to put everything together.  All those bar charts blend together in students' minds.  They know that everything in the universe is conserved, of course -- ask them, and they'll tell you that "it's the same before and after."  But ask them what "it" is, or why "it" is conserved, or how you can show that "it" is the same, and you get confused folks.

In order to review all of the conservation laws, I created a set of bar chart exercises.  For each, I posed a situation, with a clear definition of a system, an initial state, and a final state.  Then I asked for four separate bar charts: one for horizontal momentum, one for vertical momentum, one for angular momentum, and one for energy.  An example is at this google docs link to the first bar chart exercise .  

For each chart, I asked for a justification.  I did the first exercise in class, with the students coming to me after they finish each chart.  In that justification, I was primarily looking for a discussion of the external forces, external torques, or work done by external forces; they need to tell me what specific force external to the system was acting, and why it did or did not cause a torque or do work.

Note that this is a DIFFICULT exercise.  For all those who pooh-pooh when I talk about rigorous physics without mathematics, try this with your class and see how they do.  Questions like this are the first step toward being able to do mathematical physics.  If you can't set up an equation properly, then all the math skill in the world won't help you get complicated problems right.

Try the problem in the link, which is based on a situation in an old AP Physics C exam.  A future post will likely discuss the solutions, along with some student misconceptions.  Let me know if you have any questions or improvements.