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17 August 2017

Mail Time: What if I have to miss the first week or two of school?

A reader will be unavoidably absent until the second week of school.  The question to me: What would I do with an AP Physics 1 class, knowing that the sub is a random adult rather than a physics teacher?

Of all the times you'd have to miss.  Guh, not the first week(s) of school.  This is when physics students most need your guidance.

I don't have an easy answer for this one.  I'd really rather just start school two weeks late than have a random sub for a week.  It's too easy for bad habits to get ingrained, for them to get a false sense of what physics is. 

But I'll bet your school isn't about to cancel physics class for two weeks.  My only suggestion would be to do unrelated enrichment work for a week or two, and then start the course as normal when you return.  See, I think many of us take time out during the school year, or perhaps after the AP exam, to do some one-off activities: a bridge building contest, research about the history of science, etc.

It's not ideal... but you could move some one-off activities to the start of the year.  It's important to choose activities that are not directly related to physics content, I think, so you don't ingrain misconceptions.  So those projects about motion picture physics, or making a video about a physics concept - I don't recommend at year's beginning.  I offer two options that I've done in the past that might be useful here.  I suspect this post's comment section might provide even better ideas.

Option 1: Here is a link to a "pseudoscience" activity I've often done at year's end... they take it as a quiz, then they choose one or two things they marked true to investigate.  In science, all claims are false unless clear evidence is presented to convince an audience they are true. This activity can be done with minimal supervision; the students will discuss well with each other.  Give a few suggested sites to jump-start student research, such as snopes, the straight dope, and the skeptical inquirer.  

Option 2: Pose some astronomy questions, and have students investigate them using some online tools.  I like the Regents earth science astronomy questions, paired with a University of Nebraska set of simulations.   Just pick some of the questions from recent tests about observational astronomy, the motion of the sun and moon, or the phases of the moon; then ask students to teach themselves the underlying geometry using the simulations.  If you email me, I can forward you a few assignments I've given using this method.

Other than that, I don't know.  I seriously don't recommend lab work or math review.  I don't recommend any physics content at all until you're there.  I'd love to hear good ideas in the comments.  Best of luck.


26 July 2017

Methods of in-class collaborative work: 421

This weekend I attended a workshop given by Kelly O'Shea and Danny Doucette.  They showed us their outstanding approach to lab practicals, which they assign as group tests.  

The discussion in the room at several points turned to balancing the group / individual dynamic in the classroom.  On one hand, physics is a collaborative endeavor. Cooperation and communication are skills which we must teach and assess.  On the other hand, we are teaching result-obsessed teenagers, who default to letting the (perceived) smart kids do all the work, probably while making fun of them behind their back.  

If we're going to encourage, let alone require, cooperative work in physics class, we must incentivize appropriate collaboration.  Remember, incentives can and should take forms other than mere grades.  Although others have found success in assigning a direct grade for the quality of participation in group work, I have not; I find students spend more time gaming the grade than actually collaborating.

My personal approach to encouraging effective collaboration is enforcement of the five foot rule.  As always, my way is not the only way.  Another workshop attendee -- I dearly wish I remember who -- mentioned an extraordinarily clever approach to evaluated group work, one that I'd like to try.

He called it the 421 method.  The laboratory exercise or problem to be solved is presented to the class, and then the class is divided randomly into groups of four.  Then, work proceeds in three stages, with clear time limits assigned to each.  (Yes, stages are numbered strangely.  You'll see.)

Stage 4: Discussion.  Each assigned group of four may discuss the problem together; but they may not write anything down.  No pen, no whiteboard, nothing.

Stage 2: Representation.  The groups are subdivided into pairs.  Each pair may communicate orally and using a whiteboard.  However, they may only write representations - no numbers or words.  This means they can use equations, free-body diagrams, energy bar charts, etc.  

Stage 1: Solution.  Now students separate to use pen and paper.  They are assigned to write a thorough response, including representations, numbers and words.  This is turned in for evaluation.

People in the workshop asked, do you evaluate the group work?  Thing is, by evaluating the individual solution in this case, you are evaluating the group work!  If the students were effectively working together, communicating clearly with one another, pooling their talents well, then necessarily the product should be that each individual student can communicate by him or her self.  The student who held back from the group, who didn't actively participate, won't have the benefit of the four folks working together.  

This method does require that you assign lab exercises or problems that are beyond the simplistic.  AP-level questions are good here, or a simpler version of Kelly's group test-style lab practicals could work in this style.  If the whole approach to the problem is immediately obvious to more than one or two students in your class, there's little incentive for high level students to converse in stages 1 or 2.

I'll need to experiment to figure out the precise level of difficulty for this approach.  Nevertheless, I love the idea.  Let me know if/how it works for you.

24 July 2017

Ask for an answer LAST

Greetings from the American Association of Physics Teachers meeting in Cincinnati.  The exhibit hall opened last night with self-serve all-you-can-eat Skyline chili.  I have ascended to my eternal reward.

Since I arrived, I've done a wee bit more than eat chili and tour Great American Ballpark.  Yesterday I attended the High School Teacher Camp, organized by Kelly O'Shea and Martha Lietz.  We spent the day talking shop, meeting colleagues from around the country.  The keynote address was from Kathy Harper, discussing student perceptions of "mistakes" in physics class and how to channel those perceptions in a positive direction.

I've got a *lot* of notes on my phone which will inspire future posts.  For now, I'm going to relay an idea from Martha about her revised approach to AP Physics 1 justification problems.

I've written before about issues teaching students to articulate their reasoning on semi-quantitative or conceptual questions.  In sum: English class, history class, geometry class, and Fox News have taught students to begin arguments by picking a conclusion; then, to construct quasi-logical arguments twisting evidence to support that result, truth be danged.  Students are not used to the idea of beginning with the logical evidence, and then dispassionately asking what conclusion should be drawn from that evidence.

Of course, getting students even to articulate a quasi-logical chain of evidence is a tough challenge in physics class.  Come on, teacher, you know the answer, (think I) know the answer, if I'm right why do I need to say any more?  To break this first barrier, Martha had been an advocate of the "Claim-Evidence-Reasoning" approach to justifications.  For each problem, she would give space for the student's claim, i.e. their answer; for the student to write evidence from the problem statement or experiment; and then for the student to link the evidence to the claim through verbal and mathematical reasoning.  She required every student to address every element on every problem set.  

And it worked, sort of - Martha's students were willing to articulate their reasoning using words and equations.  Great.

But it was obvious to Martha that many students were merely guessing at the right answer, then cherry-picking evidence and reasoning that could support that original guess.  I've seen this intellectual stubbornness as well.  I don't know why people's brains have so much trouble adapting knowledge to new evidence.  I just know that they do.  Once a student decides that the answer is choice C, it takes an actual invasion by the Red Army to convince him that maybe the evidence points to choice D instead.

So, Martha suggested... why not ask for an answer LAST?

She subverted the paradigm to Evidence-Reasoning-Conclusion.  After the problem statement comes space for students to write evidence: facts, equations, and information relevant to the situation.  Then comes space for the reasoning: use logical connections to explain where the evidence points.  And finally, at the end, the conclusion: that is, the answer.  

Because the answer comes last, because students are not asked to commit to a conclusion before examining the evidence, students actually, well, examine the evidence.  They stop contorting their logic into pretzels to prove themselves right, and they start doing physics like a physicist.  Martha no longer has to suggest that their answers might be more likely to be correct if they'd use physics.

How am I using this?  I intend to rewrite some of my problem sets, especially in conceptual physics, making just one small change.  Problems have previously looked like this:

[Problem statement blah blah blah]

Answer:____

Justification:









But I'm going to take a cue from Martha, and rewrite this way:

[Problem statement blah blah blah]

Justification:








                                                                                                                        Answer:___


Let me know what thoughts you have, including whether this approach does or doesn't work for you.



20 July 2017

Mail Time: Detailed questions about the test correction process, especially in AP.

 Reader Jessica has some questions about test corrections.

1. For in class tests, you give half credit back for each problem missed. However, sometimes you do corrections with extra questions students have to answer in order to get the credit (which I think is a really great idea). Do all students have to answer these correction questions, or just the ones that got it wrong to begin with?

Just the ones they got wrong to begin with.  Since I’ve started teaching AP Physics 1, I often just hand back a blank copy of the test with a card saying which ones they missed -- this prevents the "well, if I just change this word here, would I be right?"

Is this change due to the way the AP 1 free response questions are asked to begin with? 

Yes.  On the old AP Physics B exam I used to ask additional questions in the style of AP 1 -- the idea is, you can't just get the number your friend got as an answer, you have to write out an explanation.  But AP 1 already asks for explanations.  So there's no additional work required, usually.

2. Do you tell the students what the correct final answers are when you give back the tests? 

No.  For multiple choice, they talk to each other and figure it out quickly.  That's fine, though, 'cause they have to TALK to each other, which is part of the point.  For free response, they figure it out too, but it's a more complicated process.

3. Do you do corrections on fundamentals quizzes? 

Generally no, because we do so many quizzes, and because fundamentals questions are straight-up recall.  For the end of year "Big Butt" fundamentals quiz, yes, we do corrections.

4. For your exam corrections (which I'm assuming are like midterm and final?), you said that you treat corrections like a separate 100 point test that students lose points from if they don't answer the corrections questions correctly (that's a tongue twister). Do all students have to do all the corrections questions even if they didn't miss the points for that question on the original exam? 

No, just the ones they missed.

6. For AP style tests with both mc and free response questions, do you have student fill out the multiple choice corrections form, or do you also ask extra questions for those on corrections? 

They just do the mc correction form.  If we're working in class, I'll often ask a question orally to be sure they understand subtle points.  And I read the correction carefully, to make sure they're addressing any misconceptions appropriately.

7. can you please explain how this works with the flow of the class... is this right?

Day 1 - take test  (Time to correct it and makeups of course) 

Day 2 - give back original test and blank for correcting, they work together to start on corrections. Collect back originals after a set amount of time in class.  Finish corrections for hw? 

Day 3 - collect corrections? 

That's pretty much right.  The schedule can change depending on other goings-on; for example, if half the class is on a field trip, the other half may do corrections, and field trip people just catch up for homework.  I'm very strict about regular homework deadlines, but I've often quietly allowed students who have a lot of corrections to do to take an extra day.  The goal is to get corrections right at all costs.

12 July 2017

Teaching AP Physics C to those who've already taken AP Physics 1: Sequencing

AP Physics 1 is designed as a first-time physics course.  While I suspect the majority of the 170,000 students taking the exam are seniors, the course is perfectly appropriate for sophomores or juniors; I even teach one section of 9th graders, and they do quite well.

So, then, what do you do when these underclassmen want to take more physics in future years?

I highly recommend AP Physics 2.  A high school student who does well in both AP Physics 1 and 2 could not be better prepared for college physics courses.  The deep conceptual underpinning provided by AP 1 and AP 2 will make even a calculus-based college course straightforward.  

That said, I know a lot of folks are teaching the calculus-based AP Physics C as a second year course.  Fantastic.  But it seems like a difficult transition: Much of the mechanics portion of Physics C covers the very same concepts mastered in Physics 1, though there's a good bit of calculus overlaid on those concepts.  Other than circuits, students have had zero exposure to electricity and magnetism.

Sequence AP Physics C like this:

September and October: Do algebra-based electricity and magnetism exactly as covered on the old AP Physics B exam.  Emphasize conceptual understanding.

November through mid-January: Go through the Physics C mechanics curriculum, paying primary attention to the calculus applications.

Mid-January through March: Start from scratch with the Physics C - E&M curriculum, reviewing material from the fall in context, and adding calculus applications.

April: Put it all together.


Why this sequence?

Electricity and magnetism are some of the most abstract concepts covered in first-year physics.  They're quite a change from Physics 1, where virtually every problem can be set up easily and quantitatively in the laboratory.  It's worth spending a significant amount of time just defining and using the concepts of electric field, electric potential, capacitors, magnetic field, induced EMF.  Using calculus while these ideas are introduced adds an unnecessary distraction.  Don't start with integrals and derivatives, which are conceptually opaque even to some of the best-performing high school math students.  

Start with the concept of the electric field, and the relationship F = qE.   Get students thoroughly comfortable with the direction of an electric force and field, with putting an electric force on a free-body diagram.  Then deal with electric potential and PE = qV.  Get students relating the existence and direction of an electric force to the difference in electric potential, and using electrical potential energy in energy bar charts.  Introduce capacitors as devices that store charge (according to q = CV) and block current.  Consider electric fields and potentials produced by parallel plates and point charges.  

Go on to magnetic fields and forces, first teaching F = qvB and F = ILB and their associated right-hand rule.  Consider how a current can produce a magnetic field.  Finally, explain induced EMF, and how to find the magnitude and direction of an induced current in a wire.

This is all AP Physics B stuff.  You can find a wealth of released exam questions on these topics, both free response and multiple choice.  Use them.

In about November, you can move on to mechanics.  You're at a significant advantage by waiting this long to start true Physics C material.  A number of your students will be taking calculus concurrently.  I used to have to teach them how to evaluate basic integrals, while my colleagues in the math department cringed and gnashed their teeth.  It's likely, though, that by November calculus classes have begun teaching integration, at least conceptually.  Physics can follow and reinforce calculus class, rather than the other way around.  And since your students are so well versed in mechanics concepts from their Physics 1 experience, they can focus on how calculus serves as a language expressing those concepts.

(Waiting until November for mechanics also solves a political problem.  If you start with mechanics, you give the impression that Physics C will be nothing but boring review, more of the same stuff from the first-year course.  Then when you bring on the electricity, you'll face a rather hostile audience who's already settled into a cozy senior year routine.  Start with the tough new stuff while your seniors are fresh and motivated.)

Finally, when you come back to electricity and magnetism, those concepts have had time to percolate in your students' brains.  Physics isn't mastered the first time students see it; it's mastered after the same ideas are seen in multiple contexts.  The full-on Physics C E&M unit doubly reinforces previous work: students revisit the concepts of field, potential, etc. that you introduced in the fall, but they also revisit the calculus language that you introduced with the mechanics unit.

I haven't had the opportunity to teach this course.  However, I've heard good reviews from those who have followed the approach I describe.  Try it.  Let me know how it goes.

04 July 2017

5 Steps to a 5 AP Physics: so many choices... here's a rundown.

Okay, obviously this post is a bit of advertising, but I've been asked enough questions that it's worth posting.  In early August, the 2018 versions of the 5 Steps to a 5: AP Physics 1 book will be published.

And I do mean "versions," plural.  It's hard enough that the College Board offers four different current AP Physics exams.  To add to the confusion, there are five different physics books under McGraw-Hill's 5 Steps imprint.  I wrote three of them.  I'd recommend four of them to you and your students.  Here's a rundown.

5 Steps to a 5: AP Physics 1, 2018 edition - by Greg Jacobs
This is the updated version of the top-selling AP Physics prep book that's been in print since the 2015 edition.  It includes two practice tests, both written by me, with complete explanations for all questions.  In the 2018 edition I've updated the text and fixed some errors.  Most importantly, I've rewritten one of the "about the exam" chapters to include the fact sheet that I use in my own class, and that my students carry around like a bible.  

5 Steps to a 5: AP Physics 1, "For the Elite Student" 2018 edition - by Greg Jacobs
I didn't come up with this title - McGraw-Hill marketing did.  I highly recommend it for your classes, though, because it contains some special and new material.  Jeff Steele - an AP Physics reader, head of the Virginia Instructors of Physics - has written a third practice test.

Most importantly, Jeff and I collaborated on a new section called "5 Minutes to a 5."  This includes 180 questions in the vein of TIPERS, but aligned to the AP Physics 1 exam, and all doable in five minutes each.  Each question could easily be the basis for an AP Physics 1 free response item, identical in style and physics content to the authentic exam.  These items would make excellent parts of homework assignments, or quizzes, or in-class worksheets to be followed up with experiments.  

5 Steps to a 5: AP Physics C, 2018 edition - by Greg Jacobs
This has been revised and updated.  The most important revision is that I rewrote some of the free response items in the practice exam to reflect the more intense use of calculus that we've seen over the past years.  In general, this is substantially similar to previous editions.

5 Steps to a 5: AP Physics 2 - by Chris Bruhn - not by me, but I recommend
I reviewed several of Chris's chapters.  He knows his business.  Some of the material is adapted from the out of print 5 Steps Physics B book that I wrote.  But it's Chris's book, and it is excellent.

Do NOT buy 500 Questions to Know by Test Day.
This monstrosity is under the 5 Steps imprint, so people think I wrote it.  I did not.  It is terrible.  It includes an enormous number of computational questions not even good enough for the old physics B exam.  I'm disappointed that this book exists.  I never had anything to do with it, I didn't even know it was coming out.  To me it is shameful that people have bought it expecting the same quality that the other books provide.  

But do buy the other four books, and even 5 Steps to a 5: AP Physics B.  
The out-of-print Physics B book is fantastic as preparation for a typical undergraduate course.  I'm trying to get McGraw-Hill to rebrand this book as a college physics prep book, because I still have alumni asking for it.  

Other questions?  Ask in the comment section, or by email.

29 June 2017

AP Physics 1 2016 problem 2 - bumps on an incline

The question in question asked about a cart on a long, bumpy track.  Specifically, it demanded a sample velocity-time graph for the cart as it crossed several bumps; then it asked what should happen to the cart's speed in between bumps if the angle of the track or the distance between bumps changed.  

I heard from and about a number of teachers who complained.  What kind of crazy-arse experiment is this?  No one does this in their class.  Ridiculous.  The AP Physics 1 exam has jumped the shark already.

My reaction to this question was, "Cool, what a great experiment, I wonder if I could set this up in the laboratory?"  And this week, Zach Widbin did set it up.  

Zach's teaching in Phoenix, but he's from New York, so he attended my summer institute in Mahopac, NY.  On the last day of the institute, teachers spend a couple of hours playing in lab, setting up experiments that they can share and use in their own classes.  

He inclined a PASCO two-meter track by two degrees.  He wrapped rubber bands around the track at 40 cm intervals, providing the bumps -- see the picture at the top.  The cart was a PASCO smart cart, which sends velocity-time data to an ipad via bluetooth.  The velocity-time graph is to the right.  

The original question asked what the graph should look like... but also, what should happen with a steeper incline?  With a larger space between bumps?

Well, Zach checked those things out, too.  The steeper incline gave a faster max speed.  He did smaller bump spacing, and got a smaller max speed.

The AP question itself postulated a very long track, with 100 bumps.  Zach only had a few bumps.  But there's no reason we couldn't tie together several of these two-meter tracks and try this again.  In fact, PASCO makes modular 50-cm plastic track pieces which can fit together in as long or short a string as you'd like.  Someone who has access to a wood shop (or, for those who prefer sexy terminology, a "maker space") could get a long plank, and then drill bumps into the surface.  Zach's approach isn't the only way to go - it's the one-morning-at-an-institute version.  I'd love to see pictures of your own setup.

Making appropriate simplifying assumptions is a physics skill that we all must teach.

The comment section of my post about the 2017 AP Physics 1 exam is full of interesting discussions about the recent test.  One of the reasons I write this blog is to provide a venue for intelligent, professional teachers to share ideas and find advice.  I appreciate the good-faith questions, the reasoned comments.  I don't read posts in other venues about recent AP tests because they are too often full of us-vs.-them bile, full of ignorant complaints about the exam that really boil down to "I and my students were unprepared for this exam, and now I'm angry about my own failure."  Those who post to this blog display far more class than that, and I appreciate it.

The purpose of this post is to respond to and rebut some of the comments about the 2017 exam.  I'm going to say some of the posters are wrong; or at least that they hold some incorrect basic assumptions about AP physics.  I want to be clear -- I have total respect for those who have posted.  I mean no criticism of their motives.  It is entirely appropriate to discuss, or even criticize, AP exam questions.  The College Board does not and should not demand Trump-style loyalty; as much as I support those who create the AP Physics 1 exam, I will not fall into the role of Fox News.

Nevertheless, I see a continual theme in these comments, and in questions from teachers at my summer institutes, that betrays a fundamental misunderstanding about teaching first-year physics.

It is our job to teach students to make appropriate simplifying assumptions about physics problems.

A lot of discussion back in May concerned problem 4 on the 2017 exam.  A light disk collides with a heavy, pivoted rod.  The question asked, essentially, where should the disk hit the rod in order to induce the largest angular speed in the rod - close to the pivot, or far away from the pivot?  The straightforward solution required referencing either torque or angular momentum.  The best answer explains that the angular momentum of the disk mvr is larger when r, the distance from the pivot, is larger, giving the disk more angular momentum to transfer to the rod.

But wait, some folks said.  The problem never said the rod was uniform.  What is the rod weren't uniform?

As far as I can see, the answer still stands.  No matter the shape of the rod, the disk still transfers its angular momentum to the rod, and thus farther from the pivot means more angular momentum transferred.

Commenters made several arguments.  Let's start with:  (1) If we look at some possible mass distributions for the rod and disk using computer simulations, it's possible in some cases to get bigger speeds closer to the pivot.

I don't see it myself, but I'm not ruling out the possibility.  I know the problem stated that the rod was much more massive than the disk; I strongly suspect that the only way to get bigger speeds closer to the pivot is to make the disk itself big.  That said, I could easily be wrong here.

The big, friendly point, though, is that it doesn't matter.

The first step in solving any physics problem, at any level, is to make appropriate simplifying assumptions.  For example, when we calculate the normal force on me when I stand still, we ignore the buoyant force of the air on me, we ignore the force of Jupiter on me.  We just say the normal force in this case equals my weight.  It's not useful physics to say, "Well, what about the case when the wind is blowing 50 mph and your right foot has trouble staying planted?  And that buoyant force can in fact be calculated, it's nonzero.  Oh, and the force of Jupiter exists, it's just very small compared to your weight."

These statements are entirely correct... and they're entirely irrelevant to the problem as stated in an AP Physics 1 class.  We always make the simplifying assumptions that allow us to solve the problem.  THEN, if it turns out that our solution doesn't match a measurement, we can consider if other issues must be taken into account.  This is not just an introductory physics thing - this is how I was taught to do research.  Solve the basic problem first, then add complexity.

Once you have to concoct far-out situations to perhaps find a situation that could cause an argument, that argument is about semantics, not physics.  If you sound like a lawyer, you're wrong.

(2) But what if a student was confused, and dealt with a non-uniform rod?

Firstly and most importantly, no one did.  At all.  I was table leader for this problem.  I asked everyone I knew to watch for anyone who treated the rod as non-uniform, or for any student who indicated that she might be confused by the lack of specificity.  Not one of the 170,000 students that we looked at was, in fact, confused by the uniformity or non-uniformity of the rod.  Not one.  

I didn't ask development committee members, but I suspect they didn't consider the structure of the rod, either.  Set up the experiment in your classroom - who's going to use a non-uniform rod?  And if they do, either (a) the problem still works the same way, or (b) you have a problem way, way beyond the scope of AP physics 1.  

It is our job as physics teachers NOT to lawyer up in response to a question that might, maybe, in a professional physicist's mind have a deeper layer.  Rather, it is our job to show our students how even seemingly complex problems can have simple solutions which can be verified by experiment.  It's our job to teach students to answer the simple question rather than become paralyzed by the complex details.  We must show how "eh, the population of the US is something like 300 million people" rather than "we cannot make any statement about the US population because someone is born every 7 seconds, rendering any estimate immediately obselete.  

Excessive precision and detail is not to be lauded.  Excessive precision and detail is the enemy of understanding in introductory physics. 

10 June 2017

Making students - not parents - self-select for an honors or AP course

An oft-repeated refrain: due to the ignorance of counselors and pressure from helicopter parents, too many students who aren't ready tend to be placed in advanced physics courses.

Understand that I believe in physics for all, not just for the best and brightest.  However, in order for a physics class to be successful, the participants have to be ready academically for the level of the course.  When students reach to take a level of physics that's beyond them, they generally have a miserable time... and they drag down the class such that everyone has a sub-optimal experience.  

Why reach?  When there's a general-level physics course available, the only people who should be placed in a higher course are those who would be bored by the simplicity of the general course.  Honors placement isn't a prize to be won, it's a match to be made.  Epidemic at the high school level is the student who is inappropriately pushed to take honors-level science and math courses, then successfully whines and argues for grades, just to be placed in an even higher-level course then next year... then crashes and burns in college, where the student-emperor is revealed to have no intellectual clothes.

I've talked to more than one high school teacher who adapted in ridiculous but practical fashion by cutting out unweighted, general-level courses altogether.  These folks label their basic course "honors physics," and their advanced course "AP physics."  They teach "honors physics" sort of the same way I teach my ninth grade conceptual physics.  Whatever works, I suppose.

I never want to shut a student out from ever taking an advanced physics course.  When physics teachers kvetch about unprepared students in their classes, the classic riposte is that "tracking" permenantly marginalizes those who don't have resources at home to push themselves academically.  This is a legitimate and important point.  Many not-so-great students could, in fact, handle advanced physics if they had been introduced to high-level quantitative skills early in their academic careers.  One of our goals as high school physics teachers should be to cast a wide net to catch everyone who can possibly learn our subject at some level.  

And right there is why I love teaching conceptual physics.  Any college-bound student - and probably a significant fraction of non-college bound students, too - can handle rigorous physics with no calculator use.  And a large fraction of conceptual students are then fully capable of success in AP Physics 1 as a second year course.

The trick, then, is to identify students ready for AP Physics 1 as a first-year course, while at the same time teaching an outstanding lower-level course to those who don't meet that bar.  How to do that in a political environment in which parents whose special children aren't selected for AP Physics feel personally slighted and storm your boss's office demanding retribution?

Your answer must depend on your particular school environment.  It likely begins with relationship building among the science faculty and administrative decision makers.  Make sure your counselors and principals and deans not only understand your placement procedures, but also the reasons behind your procedures.  Reassure them that you are serving all students, that those pitchfork-wielding parents' children will still be well-served by your program.

An elegant solution that has worked for us has removed placement decisions entirely from parents and administrators.  We place all 9th graders in a general-population conceptual physics course at year's beginning.  Three weeks in we resection, creating one AP Physics 1 section (labeled as "honors physics.")  Below is the procedure, as it's described to interested faculty and parents.

Students who are interested in honors placement have been asked to do two things.  We have been very clear with all sections, both orally and in writing, about the process.

(1) Honors practice problems.  We are posting one to two extra problems each week which are at the level expected of honors students.  Those who are considering honors are asked to solve these and turn them in.  We encourage the students to discuss their solutions with us before they are turned in if they have questions.  

(2) Honors quiz/test questions.  Each of our first three weekly assessments includes an honors-level question similar to the honors practice problems.  We ask the honors candidates to attempt these problems -- this gives us a gauge of how well they understood the practice questions.

After three weeks of class, the physics teachers will choose the honors section based on a holistic evaluation of all interested students.  We look at their performance and effort on the honors practice problems; at their performance on the honors assessment questions; at their effort and performance on the regularly assigned work, including laboratory work; and at whatever progress they do or do not make in the first three weeks.  We've found that the class is nearly self-selecting, in that those who attempt the honors problems figure out within a week or so whether they can -- or whether they want to --  handle that level of work. 

One important point about honors selection is that students must themselves want to take on this level of work.  We’ve had a number of students over the years who could possibly handle the material in honors, but they chose not to do the test questions, and thus to remain in regular conceptual.  That was a good choice universally for those students – they earned high grades, then had the opportunity to take the honors course in their senior year.  We are purposely trying to divorce the honors decision from the parents, advisors, and even physics faculty -- the students are the ones who are in large part deciding whether they can or want to do the work.

04 June 2017

Deriving expressions in AP Physics 1

Reporting from the AP reading here in Kansas City, where I've discovered that Jack Stack barbecue is excellent, but still no match for Gates.  And, where I've been immersed for days now training people on the rubric for the 2017 AP Physics 1 exam problem 3.  

Based on my experience here, I think it's worth a reminder to teachers about the expectations for "deriving" an equation on an exam.

Introductory physics is all about communication of ideas, and not as much about getting the One True Answer to a problem.  Physics is not a math class.  

Students in my class may whine (early on, at least) about not getting full credit for a poorly presented problem that nonetheless includes the correct answer.  Okay, so your English teacher requests an essay with textual evidence analyzing Shakespeare's characterization of the Romeo/Juliet relationship.  Your entire essay: "He loves her."  You earn a failing grade, of course.  How effective or intellectually honest do you think it would be to whine that your essay deserves an A because the answer is right?  I mean, the answer is in fact right...

A derivation, like any physics problem, is an exercise in communication -- but a derivation requires communication primarily in mathematics.  Just because the answer is right, just because a student knows in her head what mathematical steps she intends to take, that doesn't mean the derivation has served its purpose.

So what SHOULD we expect from students on derivations?


1. Start from first principles, and explain what first principles you're using. That means something from a "facts of physics" list: Newton's laws, Kirchoff's laws, conservation principles, the definition of acceleration or impulse or power... most anything on the AP equation sheet or on my fact sheet will work.


2. Communicate the reasoning for each step.  I think words are best here -- an annotated derivation can hardly fail to earn credit where correct.  Try circling terms and explaining what they mean.  Try telling the reader why you've substituted various terms into the equation you began with.

3. Show enough detail that a strong physics student at another school can understand without asking for clarification.  The audience should NOT be the expert physicist.  I personally don't need to derive an expression for the acceleration of a three-body system connected over a pulley, because I've done so many of those problems that I can write the answer based on memory and instinct.  My students, though... they need to start with Newton's second law for the system, explaining what expression is used for each term and why that expression is relevant.  

4. Use algebra to communicate, not to solve.  I often see students take three steps merely to rearrange terms in an expression, using annotations like "commutative property" and "divide both sides by m."  Assume the audience knows how to do math.  Use the way the math is laid out to highlight reasoning.  For example, if you have energy terms before and after a collision, write all terms clearly in a single line, with before the collision left of the = sign.  Label each term with a circle and a couple of words.

I'm sure readers - both blog readers and AP Readers - may have some further thoughts.  Please post in the comments.  

20 May 2017

Conceptual Physics Tournament Sunday May 21 2017

[The following is a letter to my school's community describing our project-in-lieu-of-exam that will happen tomorrow.  If any blog reader is interested in creating something similar at her or his school, please let me know.  I'd love to help out!]

Folks, tomorrow is the first ever WFS Conceptual Physics Tournament.  

Instead of preparing for an exam, our students have been preparing to present and discuss the solutions to some rather deep problems, which are attached to this email.  Mentors from the AP Physics classes have helped the 3rd formers conduct experiments, to understand the underlying theory, and to deliver a two-minute talk.

Tomorrow, each student will be assigned to report in two "physics fights."  Think of the physics fight as a thesis defense.  The reporter presents his/her two-minute talk; and then the examiners engage in conversation with the reporter for five more minutes.  The examiners are probing how deeply the students truly understand the problem, and how clearly the students can articulate their understanding.

Physics fighting is a spectator sport.  We encourage all members of the community (including parents of 3rd formers!) to come out to watch a few physics fights.  These will take place in Manning and Kenan.  We will run approximately eight rounds, beginning at 1:00 and ending around 3:00.  The specific fight schedule will be posted in the dining hall and to the news folder around 12:30 Sunday.

Alex Tisch and Colin Manning have done tremendous work preparing their students, not just in the past couple of weeks, but all year.  The mentors have taken to their task with relish.  The 3rd formers have worked very hard, and are ready to demonstrate their knowledge.  Come see the fruits of their labor.  

GCJ

11:30: Examiner arrival, lunch in Terry Dining Room
12:00: Examiner training in Terry Dining Room
12:30: Posting of fight card
12:50: Examiners and students move to fight rooms

1:00: Round 1
1:12: Round 2
1:24: Round 3
1:36: Round 4

1:48: Break; examiners switch partners

2:00: Round 5
2:12: Round 6
2:24: Round 7
2:36: Round 8

19 May 2017

AP Summer Institute: June 26-29, Mahopac, NY

Folks, a quick bit of advertising here... we're still looking for a few teachers to fill up my AP Summer Institute in Mahopac, NY from June 26-29.

At this and all of my summer institutes, I'll take you through all aspects of my AP Physics 1 classes.  We'll do some of the different kinds of experiments I've discussed on this blog.  We'll share ideas with each other - I leave every institute with new ideas to try out.  And I can discuss physics teaching beyond AP Physics 1, including conceptual physics as well as AP Physics 2 or C.

For those looking to meet various certification requirements, this particular institute allows you to earn graduate credit.

Please contact Mark Langella, the institute director and a pretty dang impressive AP chemistry teacher, for signup details.  You can contact him via the Putnam-Westchester Industry and Science Teacher Alliance.  

I hope to see you in New York...

GCJ

04 May 2017

2017 AP Physics 1 solutions

I enjoyed writing my solutions to the 2017 AP Physics 1 free response questions.  You can find the questions linked via the official College Board exam site, here.

I very much like the direction the "quantitative-qualitative translation" question has gone.  Twice, students were given an equation, and asked why it does or does not make physical sense.  That's such a great skill to develop, and to test.  And I loved the experimental question... the experiment itself was quite straightforward.  But the "based on this data, how do you feel about this conclusion?" question was amazing.  It gets at the heart of evaluating quantitative evidence, at basic numeracy.  If every journalist and politician in America could answer this question accurately, the world would be a better place.

Okay, now I'm going to link the solutions.  But, please note that due to College Board copyright rules, only teachers can access them.  The PGP-secure website requires verification that you are a teacher in order to sign up.  

One of my favorite people, Gardner Friedlander, runs this teachers-only wiki.  He became quite frustrated last year because so many students asked for access -- many pretended to be teachers. Folks, Gardner isn't stupid.  He verifies that you're a teacher.  Please don't make him come after you for impersonating a teacher -- he will take away your birthday.

So, students, do you want access?  Ask your teacher to join PGP-secure.  Your teacher may share the solutions "for face-to-face teaching purposes."  

As always, I guarantee that I've earned a 5, but not that I get every detail right.  Please note my mistakes in the comment section.

My solutions can be found via this link, at PGP-secure.  This is a wiki for physics teachers only.  If you are a teacher but don't have access yet, follow the instructions at the linked page; you should be approved in a few days. 

GCJ

01 May 2017

How long should my answers be on the AP Physics 1 exam?

Quick answer: probably shorter than you think.

Below are a variety of AP Physics 1 prompts, and how I would suggest structuring your answer.  Please note that, while I do grade the exams every year, I am writing here in the role of independent observer.  I am not a representative of the College Board.  These are my own simplified instructions to my own students, which may not be perfect in all situations.

Yet I think teaching is much better done by simple guidelines rather than legalisms.  If you want the legalisms, go to the College Board's course and exam description; and look at the AP Central page where they go in to great detail about the requirements for a paragraph response.  I don't think a student wants to see such detail.  I think, in fact, that we should actively discourage students from a rules-bound approach to any answers.  Encourage your students to simply answer each question, and move on.

On my last day of class before the exam, I will remind students of the types of prompts below, and my guidelines for the length of the response required.  And then I will let go, and wish them the best.


"Briefly Explain:" or "Briefly justify"  Answer in one sentence.

"Derive an expression:"  Use variables only, start with an equation from the equation sheet or a fundamental principle.  It will help to annotate your work with words, but complete sentences are not necessary.  Full credit can usually be earned without words at all as long as the mathematics is communicated clearly. 

"Describe a procedure:"  Two to three sentences, never more.  Say what you will measure, and what equipment you'll use to measure it.  And stop writing.

"Explain" or "Justify your answer:" About two sentences, or perhaps one sentence with reference to an equation.

"Answer in a clear, coherent, paragraph-length response:"  Five sentences, four is often enough.  Do not repeat the question in the answer. Get to the point.

For all responses except mathematical derivations:  Use sentences with subjects and verbs, but without fluff.  

What if I need more space than the question provides?  Then you are writing too much.  The amount of space provided is deliberate, and reflects the length of response expected.  Yes, I know you are allowed to use scratch paper and staple it into the book.  You're also allowed to publish your bank account information online.  Just don't.  

Don't fear for the lost point.  Students tend to write page-long essays because they fear that the grader will "take off" if they miss one small detail.  But those students miss the bigger picture.  Running out of time on question 5 could cost them seven points, while writing an extra page may in their imaginations earn one point.  And writing that extra page is far more likely to lose credit for an incorrect statement than to gain credit.  

Please don't be afraid.  Answer each question briefly and confidently.  If you have to guess, guess briefly.  Believe it or not, we readers know when you're just writing random crap because you have no idea how to approach a problem.  And rubrics are written such that they are unlikely to award credit for baloney.

Kick arse tomorrow.  Let me know how it goes.  I'll post my solutions when I can.

GCJ






28 April 2017

Mail Time: Last-minute questions before the AP Physics exams

The AP Physics 1 exam is next Tuesday.  Remember, no studying after Monday's class!  Lots of last-minute questions coming in.  Here are a few quick ones, with responses that sometimes are merely links to other posts.

From Nikki:  I'm confused on the AP Physics 1 Practice Exam Question 2. I understand the explanation in scoring guidelines, but it seems like they have ignored the PE due to the gravitational pull of the Earth and I don't understand why. Any chance you have some insight into this?

Nikki, the PE is for the earth-spring-object system, and is 1/2(k)(x^2) measured from the equilibrium position of the spring/object. This post gives further details.


From Paul:  If a student uses the language of calculus to justify or explain an idea on the AP exam, will that be accepted by the AP reader?  This came up because it is an algebra-based course.

Paul, please take a look at the second answer in this post.


From Matthew, four questions:

1) If you give practice exams ... What percentage on the multiple choice do your classes typically average? I am just curious as a gauge for myself and my classes moving forward.

Probably 60-65% on average with authentic physics 1 questions.


2) On the first FRQ part b is says a student would receive a point "for recognizing that the force causes a change in momentum or a change in velocity." Would the student have to state that? The question asks to calculate the magnitude of the external force on the system.

State or imply, mathematically or verbally.  Just writing "F=ma" doesn't cut it, but writing that and then trying to calculate a change in speed for a would probably earn the point.


3) If a student answer a part of the FRQ correctly and then adds something that is incorrect would they receive credit for the correct response?

No.  When there are multiple responses, we pick the one that earns the fewest points.  They can't game the test.  :-)


4) Is there any type of final reminder you cover with them before the exam?

Try this post here, about "BOUX" day.


From Michele: On the MC questions that are multi-select (two correct answers), how are they scored? Do students need to get both choices correct in order to receive credit?

They need both choices right to get credit.  No half points.


From Michael: For AP grading, are points taken of for not simplifying expressions?

Michael, as long as the solution is as required -- "solve for a in terms of given variables and fundamental constants" -- then any form of the solution is acceptable.

Got others?  Go ahead and post a comment or email me.  I'll see what I can do.

21 April 2017

Reviewing for the AP Physics 1 exam: No big practice exam, but Big Butt Fundamentals Quiz

I take an approach to exam review that's consciously different from what other teachers do.  I am doing no tests at all this month, no practice AP exams.  We're solving one authentic AP Physics 1 free response in each assignment; we're practicing a couple of multiple choice questions each day.  We're doing corrections on anything we miss.  I'm getting students to grade other students' work to an AP rubric wherever possible.

Why am I not doing practice exams?  Because every test we've taken all year has been in (or close to) AP format and style.  My students know how to pace themselves so as not to run out of time.  They know how to communicate enough to get credit, but not so much that they waste time and ink.  They know the level of difficulty they will face on multiple choice and free response problems... because we are doing some each day.

Importantly, while I'm giving some questions for homework, I'm doing others as brief in-class quizzes.  It is critical that students have practice working on AP level problems without a safety net, without the ability to ask friends or teachers clarifying questions.  But we do that all year, on every test and quiz!  Since I never allow students to ask questions on tests or quizzes, I feel no pressure now to give any further authentic AP practice.

One type of major assessment that I do use is the "Big Butt Fundamentals" quiz.  I give students 30 minutes to answer 30 questions that are, for the most part, straight off the fact sheet.  The first twenty questions are pure recall; the last ten require some processing, but are still testing misconceptions or ideas that are fundamental to students' knowledge of physics.  Feel free to use this quiz in your own class.  I create it by randomizing the fact sheet, and then just riffing off each fact.

The purpose of the Big Butt Fundamentals quiz isn't to play "gotcha".  It's to get students' noses into their fact sheet.  It's to show the students what they know well, building confidence; it's to show students what they might have forgotten, leading the students themselves to look up the correct answer or to discuss the question with friends.

I ask students to correct the Big Butt quiz by writing a complete sentence stating the reasoning or fact behind each answer.  Rather than just writing "kx", they'd write "the force of a spring is kx."  They are putting their answers in context.  

I don't ask for complete sentences as a punishment, or because my ed school training or my teacher's edition told me to... I'm making the students write so that they have a better chance of remembering a fact that they already got wrong once.  My students are generally cooperative with this rationale, because (a) I don't ask them to do much this time of year anyway, and (b) they see by now the connection between correcting what they get wrong the first time, and strong performance on future physics problems.   As we say, practice doesn't make perfect.  Perfect practice makes perfect.


20 April 2017

A large bug on the edge of a DVD

A large bug of mass 5.0 g lands on the outside edge of a DVD*.  The DVD has mass 9.0 g and radius 6.0 cm.

*DVDs are still a thing, right?  Or, at least I expect that most of my 15-18 year old students know what a DVD is without further explanation.  Or, I'm an old man.

I use this setup to introduce newton's second law for rotation, and the additive nature of rotational inertia... and then to discuss conservation of angular momentum.

(a) Does the bug's presence significantly affect the rotational inertia of the DVD?

By itself, the DVD is a disk, with rotational inertia (1/2)MR2.  That gives 160 g*cm2 as the disk's inertia.

The bug adds its rotational inertia algebraically.  The bug should be treated as a point object, whose rotational inertia is MR2.  That gives 180 g*cm2 as the bug's inertia.

The rotational inertia of the bug-DVD system is then 340 g*cm2. The addition of the bug nearly doubles the DVD's rotational inertia; thus the presence of the bug is significant.


(b) Initially the bug and DVD are rotating at a constant angular speed.  Then, the bug moves to a new position 3.0 cm from the DVD's center.  Explain why and how the DVD's rotational speed changes.

No external torques about the center are exerted on the bug-DVD system ('cause no net force at all acts).  Thus, angular momentum is conserved.

Angular momentum is When the bug approaches the center of the disk, the bug's (and thus the system's) rotational inertia decreases because the R term in the inertia formula decreases.  To keep angular momentum from changing, then, the ω term must increase.  The DVD will speed up its angular velocity.


(c) Does the bug exert a torque about the DVD's center as it moves toward the new position?

Tricky.  There's no EXTERNAL torque on the bug-DVD system.  But angular momentum can still be conserved when internal torques act.  The torque of the bug on the DVD would be internal to the bug-DVD system.

Consider the DVD by itself.  It changes its angular speed.  So by Newton's second law of rotation, it must experience a net torque.

What can possibly provide that net torque?  The weight of the DVD and the normal force of the spindle on the DVD both act through the center of the DVD; they provide no lever arm, and thus no torque.

The only other possible provider of torque is the bug.  But how, in terms of torque equaling force times lever arm, can the bug do that?

Since the bug rotates with the DVD, a static friction force must act between the DVD and the bug.  That friction force acts tangent to the rotation of the disk, and thus has a lever arm with respect to the disk's center.

17 April 2017

Mail Time: Why do released AP Physics 1 exams include only 40 multiple choice?

Reader Aaron Shoolroy asks, in the comment section of a separate post:

The Physics 1 exam description says 50 MC questions, but it seems like all of the secure exams available through the course audit page have 40 questions. Does anyone know how many MC questions will be on the actual exam this year? 

Aaron, I'm sure you're not the only person wondering.  The AP Physics 1 and 2 exams will, as stated in the course description, include 50 multiple choice questions.  The last five of these will be "multiple correct", requiring the student to select both of the correct answers for credit.

So then, why do the released exams only give us 40 multiple choice?  Long answer coming.

During the Physics B dynasty, multiple choice exams were only released every five or so years.  See, a subset of the questions on each test are re-used on future tests in order to provide concordance from one exam to another.  For example, if the student population taking the test does better on these re-used questions, then the overall exam scores should go up -- even if performance on the rest of the test doesn't likewise improve.  That repeated subset of questions serves as an experimental control.

In order to keep a statistically significant bank of these re-usable questions, the College Board carefully hoarded them.  By only releasing exams every five years, it was easily possible to keep a secure set of questions in circulation.

During the development of the AP Physics 1 and 2 courses, one of the major points of pointed feedback to the committees said, please stop with the learning objectives, and give us practice questions.  I know I delivered that message more than once, and I wasn't the only one.  

See, people listened.  The College Board pledged to release the international version of the test nearly in its entirety every year, for the purpose of providing materials for use in class.  That's an enormous wealth of material for teachers, to the extent that we're only three years into the course yet I haven't been able to assign all available questions this year.  

(By the way, most of those released exam items are only available to those with an active AP course audit account.  That's to ensure that these items remain secure enough that it's unlikely students can simply google the solutions to them.)

I know the development committee and the ETS physics people have had to work extra hard the past few years in order to meet the demand for all these test items.  I have told them in person, I'll continue to tell them in person, and I'll say it here -- THANK YOU.  By releasing so much authentic exam material, they've allowed teachers and students to get a real sense of the form, content, and degree of difficulty of the exams.  They've allowed me to assign authentic practice in the lead-up to the exam.  They've provided me with practically unlimited laboratory ideas - virtually every question can be investigated experimentally.

Oh, but you asked me a question, and I rambled.  Why are there only 40 questions on the released exams?  Because the College Board removed the 10 questions that will be re-used in future years for statistical purposes.  Losing those ten questions is more than a fair trade for 40 multiple choice and five free response items, which are more valuable than gold to an AP teacher this time of year.


08 April 2017

For April AP Physics 1 classes: Here's a list of experiments, go do them.

At this point in my senior-level AP Physics 1 class, we have learned all necessary fundamental skills.  We have practiced solving problems with forces, motion, energy, momentum, rotation, circuits, and waves.  We have learned the critical laboratory skills, including how various equipment works, how to present data graphically, and how to use the slope of a graph to analyze data.

In the last month of the course, I'm using class time to put all these skills together in practice.

I have two 90 minute classes each week.  In these, I've been starting with 20 minutes or so of preliminaries: a TIPERS-style quiz, discussing the quiz, taking questions on homework.  Then I release the class to play.

What do they play with?

I've given the class a list of seven experiments; I can come up with more as necessary.  The list is below.

A student picks one and begins work.  I am happy to help with equipment questions, but not with "how am I supposed to do this?" questions.  (For those, I ask them to collaborate with a classmate.  That works at this stage of the year.)

How do they report their results?


Very informally.


I ask for a few sentences describing what they measured, and what equipment they used to make the measurements.  I ask for a few sentences describing how the data was analyzed, and how the data answers the question posed.  That's it.  No "formal lab report", no "purpose / procedure / results / conclusion."

Sure, occasionally I get a student who tries to give me a page with a bunch of messy numbers on it.  I simply send him back to his desk to do it right.  But this removal of formality in lab work has worked wonders for years.  It mimics what students will be asked to do on the AP exam -- in just a few minutes, writing by hand, describe an experiment including procedure and analysis.

What if I don't have enough equipment for everyone for these setups?

Part of the beauty of this approach is that I never have more than a couple of folks at a time working on each experiment.  Students are directed to work in any order they desire.  Often they will choose based on which experiment's equipment is available.

If several labquests are on the fritz - as they often are - it doesn't matter.  Because (a) students will have incentive to choose an experiment that doesn't involve the labquest, and (b) students will have incentive to figure out new and interesting methods for measuring what the labquest can measure.  For example, rather than plug in motion detectors to the labquest, they might learn to use video analysis on their phones.

Here's my list.  Each one can take anywhere from 20 minutes to an hour.  You'll recognize some 'cause they're inspired by old AP problems.  One (number 7) was created by a veteran of my class when he needed a project in another class.  I'll probably post some other time with specific notes about each... but for now, these have been a good start to independent lab work in the spring.


1. A transverse wave is traveling on a string.  If the frequency on the wave machine is doubled, what is the new average speed of the point?  Use a high speed camera on slow motion to directly measure the average speed.


2. Use a pipe, a meter stick, and a frequency generator to determine the speed of sound at room temperature.  Find somewhere with a temperature below 50 degrees F, redo your measurement, and see if the speed of sound has changed.


3. A 1 kg object traveling on a frictionless horizontal surface collides head-on with and bounces off of a 0.5 kg object initially at rest.  Give experimental evidence for (a) the percent of total linear momentum that was conserved, and (b) the percent of total mechanical energy that was conserved.


4.  In the circuit shown above, the sum of the resistances of resistors R1 and R2 is 80 kΩ.  Resistor R1 and the 80 kΩ. resistor are now swapped.  A student claims that the current must always increase in the right-hand branch of the circuit, because the total resistance of that branch must decrease.  Test this claim experimentally.
.



5. Create two pendulums: one with 50 g of hanging mass, one with 100 g.  Release both from the same angle.  Predict and give experimental evidence to show how each of the following differ for the two pendulums:
Period
Maximum kinetic energy
Maximum acceleration




6. We’ve learned that the period of a pendulum is independent of the amplitude.  Provide experimental evidence for this claim; present your results graphically.



7. You are given two objects to be placed on either side of a pivot, as shown above.  The total mass of the two objects is known.  You may vary the distances from the pivot at which you place the objects.  Use the slope or intercept of a linear graph to determine the mass of each object experimentally.