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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.  


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...


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. 


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.


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.