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20 May 2018

Did you or your students get a nonstandard form for an AP Physics exam?

The vast majority of students who take an AP Physics exam get the “operational” version of the exam.  This is the one whose multiple choice questions are never released, but whose free response is available publicly 2 days after the exam date.

A small fraction of students take a different form of the exam.  In particular, students outside North America usually get this different form - otherwise, it’s possible for a particularly dirty player to reconstruct the exam and text details to a student in a different time zone.  Within the continental US, that’s difficult - the west coast has entered the exam before the east coast is finished.  In the fall the College Board releases the majority of this “international” version of the exam to teachers who have passed an official course audit.

For statistical purposes, occasional randomly-selected students stateside take the exam form that’s otherwise earmarked for the international audience.  Invariably, when a few students find out that they took a different test than their classmates did, they claim that (a) they took the harder version, and (b) no fair.

Claim (a) has a 50-50 shot of being true.  Claim (b) is utter, ignorant baloney.

Is the international version harder?
Sometimes, sometimes not.  Is this year’s exam harder than last year’s?  It depends on your perspective, what you’re personally good at, and random chance.  The development committee attempts to construct exams of similar difficulty in each year, in each form. If you have access to the released international exams, take a look through.  Some questions are harder, some easier, most about the same.  Once many years ago I was in the super secret room where the international and alternate forms of the exam are graded - on pain of pain, I’m not allowed to tell you any details about the exams I saw.  That’s just as well, because I don’t remember details.  Yet, I can tell you that the questions read exactly like the operational exams - I perceived no difference in difficulty, no difference in the range of student responses.

Are the AP scores lower for the international version?
No.  The difficulty of exam questions does not matter when AP scores are compiled!

On each exam, a subset of multiple choice questions are repeated from previous years.  These serve as a control on exam difficulty.  Even when an exam consists of demonstrably more difficult questions, the meaning of a 5, 4, 3 is identical.  This subset of multiple choice ensures that desired outcome.

Let’s say that all students taking the international version got 40% of the available points, while all students taking the operational version got 50% of the available points.  There are two possible explanations for this discrepancy:

(1) The international exam consists of more difficult questions than the operational exam.
(2) The population of students taking the international exam is weaker.

Performance on the subset of repeated multiple choice questions can differentiate between the two hypotheses.  

If there was no statistical difference in performance on the repeated subset, then explanation (1) applies.  The same level of student getting fewer points means the exam is harder.  Thus, the cutoff percentages for 5, 4, 3 would be dropped.

However, if the students on the international exam scored similarly worse on the subset of repeated questions, then hypothesis (2) is confirmed.  Even on the exact same questions, these students performed to a lower standard.  The cutoff percentages would reflect that lower level of performance.

The overarching goal is that a 5, 4, or 3 means the same level of performance from year to year.  Yes, it is true that for AP Physics 1, about 70% generally works out to a 5, 55% to a 4; those numbers are variable to serve the overarching goal.  Similarly, it is true that about 5% of the student population gets a 5 on AP Physics 1.  But that's also variable, dependent on the performance of this year's students.  There's nothing preventing teachers and students getting better year to year, such that suddenly half the country earns a 5.  I mean, that's unlikely... but the College Board would be utterly thrilled if that happened.

Scores are not manipulated for political reasons.  In the very first year the AP Physics 1 and 2 exams were administered, a diverse committee was assembled and teachers/professors were surveyed to determine the standard of performance appropriate to each score.  While statistics were used copiously, this original score setting included some subjective as well as objective input.  

Once those original scores were set, though, the standard of performance for each score was also set in stone.  The scores this year will be what they will be... if more people get high scores (as has been happening very gradually across the last few years), that represents authentic improvement in the student population taking the exam.  No one is pulling strings behind a curtain trying to depress or increase scores.  Those who promulgate conspiracy theories do so out of malicious ignorance.  The numerous ETS statisticians, the chief readers, the College Board representatives, the development committees... all these people would have to be part of a such a grand conspiracy.  I know many of these folks - I know them to be people of considerable integrity.  They wouldn't stand for fudging scores.  It's not happening.

Similarly, there's no conspiracy to give half of your school a harder exam with worse scores.  If you happened to get a nonstandard form, well, everything will come out in the wash.  Your exam score will reflect your knowledge of physics.  As it's supposed to.  :-)

16 May 2018

Chimera: The skill of learning from a textbook

I was asked whether I still use just fact sheets, or whether I encourage the use of a textbook like OpenStax.

I'm still using fact sheets, and nothing but fact sheets. 

Many people have argued with me that they want to promote learning new physics from a textbook.  Reading a textbook is a skill, one that seems to be demanded in college physics courses.  Fact sheets and videos, they argue, eliminate textbook learning.

True.  And I desperately want to eliminate textbook learning.  

Textbooks are interesting and useful and fun to read for those who already have context.  In history, where the vocabulary and general situations (kings, wars, human conflict and leadership, etc) it's easy to learn from a textbook.  Life science texts with outstanding diagrams can lead to good learning.  Even language texts can be strong, especially if they include easily navigated glossaries and grammatical summary tables.

Physics cannot be learned from a textbook, I don't think.  Instead, students can learn facts from a fact sheet by rote. Then they can learn the skills of applying those facts to interesting situations, and experimental skills in lab, and how experiment connects to prediction... and finally, at the end of the year, students can go read a textbook successfully because they have the context for understanding.  

I think that process holds in first year physics as well as advanced physics - everyone I knew struggled to understand our quantum text, or advanced fluids text, or advanced E&M text.  I thought I was stupid because I couldn't read and understand the textbook.  My friends either also thought they were stupid, or thumped their chest and pretended to understand.  

We all figured these subjects out eventually, as we did problems and learned facts from our teacher and practiced with each other.  But the idea that it's a useful or even attainable skill to read a text and understand new physics the first time you read it?  Utter baloney, in my mind.  :-)

14 May 2018

The Jacobs Physics Podcast, ep. 3: 2017 AP Physics 2 problem 2, resistivity experiment

The 2018 AP Physics 1 exam involved an experimental graph to determine resistivity of an unknown material.  Interestingly, so did the 2017 AP Physics 2 exam.  This week's podcast deconstructs the problem and rubric from the 2017 P2 problem, including digressions about:

* writing procedures
* how I feel about formal lab reports (i.e. don't do them unless you're truly dedicated to the process)
* the 1/4 page or 1/2 page rule for graphs
* how I feel about science fairs (i.e. don't do them unless you're truly dedicated to the process)

Click on this link to listen to this week's podcast.

Feel free to send feedback via email.  I've already addressed a couple of peoples' questions on the podcast; I'd love to have more.  If you want me to use your name, say so!


10 May 2018

2018 AP Physics 1 Solutions

What a great exam this year... I particularly loved #3, the quantitative-qualitative translation question in which students have to work through a non-constant torque graphically and conceptually.  The paragraph question which combines energy and momentum concepts is likely to show up at our year-end conceptual physics tournament in the near future.

My solutions can be found at this TEACHERS ONLY link.  Yes, really, teachers only.  If you ask for access as a student or parent, Gardner, who runs that site, will take away your birthday, and for extra measure, he'll lay a little spell on you right there.  He'll turn you into stone, or a dog, or a chair.

Teachers, if you'd like access, please follow the instructions at the site.  Send Gardner a request to join, along with evidence that you are a physics teacher.

The official solutions will be available in the fall on the College Board's AP central website.  I'll be grading problem 3 - woo-hoo!  (No, I've no clue yet what the rubric will look like, just that I'll be grading thousands upon thousands of problems 3.)


07 May 2018

The Jacobs Physics Podcast, episode 2: 2015 AP Physics 1 Problem 3

This week's podcast discusses the first ever paragraph response problem, the one about two identical spheres landing on the ground, and why they land at the same time.

Digressions include:

* the four minute drill
* last-minute AP prep
* how your physics class already supports goals of project based learning
* how AP rubrics are developed
* what constitutes a "paragraph response"?

Click this link to listen. 

Would you like me to discuss a particular issue on the Jacobs Physics Podcast?  Send a note via email...


06 May 2018

How do you get ready for the AP Physics exam in one night?

You don't.  Physics understanding is built over months, not days; new skills cannot be crammed at the last minute any more than just one night's worth of weightlifting right before the big game can benefit a football player.

Nevertheless, you might feel that your students aren't anywhere near ready for the exam coming up in a day or two.  What can you do?  How do you help?

Take a long-term approach.  Instead of performing triage, instead of brutalizing yourself and your students with weekend-long review sessions and late-night tutoring, conduct a tactical retreat.  Evaluate, why are your students feeling so unprepared?  What can you as the teacher do next year to avoid this last-minute panic?

Thing is, thinking about next year is not your instinct.  Your teenage students live in the moment.  This moment says, help me now while my academic life seems to be crumbling.  And you care about your students, so you help... then you and they move on in the catharsis after the exam, cleansing your collective memories of the despair and desperation youall felt this weekend.  (Or worse, normalizing those feelings as a necessary and unavoidable part of academic life.)

But you cannot live in the moment.  Your career teaching physics will span years if not decades.  Chances are that if your class is panicked, this isn't the first year they've felt this way.  By definition, it's insanity to do things the same way again and again expecting different results.  So consider how you can change... starting with the first day of school.

Your students certainly feel an urgency now, with the exam looming, to pay attention to physics, to figure out difficult concepts even at the cost of significant brain energy.  It is our job as teachers to create that urgency throughout the school year.  Use every trick in your book, and learn new tricks: give frequent quizzes, don't let students get away with half-arsed work, do test corrections, don't answer questions during tests and quizzes, enforce the five-foot rule so each student is always writing her or his own understanding, don't slow the pace of the course for anything less than the apocalypse, give less-frequent cumulative tests rather than unit tests...

Spend the next few weeks asking your students what they might suggest youall could have done to reduce the pressure at year's end.  Spend the summer building your toolbox, by reading this blog, by reading everything available on the College Board's AP Central website, by finding other physics teachers with whom to talk shop, by attending a summer institute.  Come to school in August with a well-formed plan so your students spread their panic in itty bitty, barely noticeable doses throughout the year.

Be prepared, though.  Teenagers live in the moment.  You will get pushback as you demand more effort and engagement in the fall.  It is, in fact, our job to weather that storm, to keep our students focused on long-term goals.  That means some difficult conversations early on, that means some political damage control with parents, colleagues, and administrators.  It will all pay off in May, when your students approach the impending AP exam with calm confidence.  Then the NEXT year, the positive feedback from your well-prepared students will mean less pushback in the fall.  And so on, until you have a well-understood culture of learning physics intensely but calmly, without drama or last-minute fear.

30 April 2018

The Jacobs Physics Podcast: AP Physics 1 2017 problem 3.

I recorded a 30 minute episode in which I discuss - with digressions, of course - the third problem on the 2017 AP Physics 1 exam.

You can see the problem via this link to the official College Board website.

Then, click here to listen to the podcast!

I'd love to hear your feedback; in particular, I'd love for you to tell me what you'd like me to discuss on a weekly podcast if I continue it.  You can find my email through the Woodberry Forest School website.


23 April 2018

Do I need to know about pressure/displacement nodes in AP Physics 1?

I was asked about standing waves in pipes on the AP Physics 1 exam.  Specifically, is it important for students to understand the difference between a variation in air pressure, and a variation in the amplitude of particle displacement?

On one hand, it is certainly important to understand nodes and antinodes in closed and open pipes.  When the boundary conditions are the same (closed at both ends, open at both ends) the fundamental frequency is v/2L and all multiples of the fundamental frequency can be played; when the boundary conditions are different (open at one end and closed at the other), the fundamental frequency is v/4L with only odd multiples available.  

Then, we have to understand the WHY behind these facts.  Students must be able to draw pictures of standing waves, must be able to identify the wavelength as it relates to a picture of standing waves and as it relates to a pipe length, understand why the speed of waves is constant, how the wave speed relates to the speed of particles in the material, how the particles actually move in a transverse wave and how that relates to the pictures and to the wave's amplitude.  (Whew.)

But, when drawing standing waves, are we drawing a representation of the air pressure or the particle displacement?  And, does it matter?

Any time you're wondering about what will be tested on an AP exam, be as a biblical fundamentalist: ignore peoples' pronouncements and go straight to the source text.

I've just been through four released AP Physics 1 exams.  Not a single question addressed the difference between air pressure and particle displacement.  Now, that doesn't mean a question next year or the year after couldn't do so, because the AP Physics 1 exam is only a few years old.  

Nevertheless, I'm taking a Bayesian approach.  The difference between particle displacement- and air pressure- representation is extraordinarily abstract, and difficult to understand for a student who hasn't studied fluids, anyway.  The mathematics and representations of standing waves work fine even if students don't know what exactly they're representing.  Therefore, I don't address this issue of pressure vs. particle displacement.  

If that means my students have to guess on one multiple choice question every half-decade, that's a price I'm definitely willing to pay for simplifying their understanding of standing waves in a pipe.

22 April 2018

Why is the net force on a car greater than on the driver?

A reader of my 5 Steps to a 5: AP Physics 1 book sends some kind words about the book, and then asks:

Number 2 in the Forces and Newton's Laws review chapter is about the net force on a driver vs. a drag racer.  To calculate the net force on the drag racer you omit the mass of the driver.  I was wondering about the logistics of that.  How can you only use the mass of the car and not include the driver?  I do understand why the force on the driver only includes the mass of the driver.  The seat exerts the force on the driver's mass.  But that seat force would also be backward on the car, and so to accelerate the car at the acceleration determined would require the additional force on the driver, correct?

The net force on the car is its mass times its acceleration. Sure, the driver may be pushing backward on the car; the road is also pushing forward on the car.  That's all true, but all the question asks for is the net force on the car, which does mean the force of the road minus the force of the driver.  I don't know the value of either of those forces.  I just know about the car's mass and its acceleration.

Similarly, you're right that the only horizontal force on the driver is the force of the car.  That's the net force on the driver.  Yet, that net force is still equal to the driver's mass times her acceleration.

Since the car and driver move together - when one speeds up, the other does too, by the same amount - they have the same acceleration.  Thus ma must be bigger for the more massive car.

16 April 2018

Students: you don't need more AP practice problems.

This is the time of year when physics teachers tend to be approached by hyperconcerned AP students worried about the upcoming exam.  "Can you find me more practice problems?  I want to do as many as possible so I'm prepared." 

The teacher's answer should almost always be "no." 

Huh?  Um, Greg, why not provide more practice problems?  Why shouldn't students be encouraged to study more, especially when it's they who are taking the initiative? 

I'll give you two reasons.

For one: It's the quality of preparation that is useful, not the quantity.  I've assigned an enormous number of AP practice problems as homework, tests, quizzes, and in-class exercises.  For each of these, I've helped students understand what they've done right and wrong, and how to do better on the next exercise.  We don't just do a problem and forget about it. I check homework and make students redo problems that they substantially didn't understand.  We grade each others' quizzes in class.  We do test corrections.  In-class exercises lead to discussion, or sometimes experimental verification.  

In contrast, handing students a thick pack of review problems encourages the mindset of just getting the answer right, without the deep engagement required when working in conjunction with a formal class.

And a thick pack of review problems can't possibly be useful unless the problems are themselves high quality.  I just got an email from a prominent supplier of inferior physics lab equipment saying something like "With the AP exam approaching, try these practice questions for $99!"  Stupid, stupid, stupid.

Authentic old AP free response questions with authentic rubrics are available to students through the College Board's official site.  The five steps book has an enormous number of questions, each with a thorough solution, not just an answer.  These are publicly available to students - and the 5 Steps book costs a fraction of $99.

I've seen some of the material that's peddled by others as AP exam preparation.  It's uniformly terrible.  Pick up a random other prep book and look at the content and style of questions.  The GOOD ones are repackaged AP Physics B material, heavy on the calculation, nowhere near the style and depth of the true AP Physics 1 exam.  Folks, there aren't that many people in the country qualified to write AP Physics 1 questions.  The vast majority of those are already in the employ of ETS and the College Board.

For two: I refuse to feed the test anxiety beast.

At this point in the school year, with three weeks to go before the AP exam, I'm tapering my AP class.  We are winding down, not up, in our preparation.  I have a lot of reasons for relaxing my demands in April, but a primary one is to emphasize with my actions that "test anxiety" can be limited by building a positive culture.

We've done the necessary deep practice all throughout the year.  I've ensured every day that students have not just done the homework, but they've paid careful attention to it; if they don't, they come in for consultation.  For each incorrect answer on a test, my students write a clear correction.  We've built the habit that practice doesn't make perfect; perfect practice makes perfect.  

By year's end, all of my students know what they can and can't do.  They understand their strengths and weaknesses, and how to play to those strengths on the AP exam (because they've done so on eight AP-style practice tests already).  They know how to handle the adversity that is sure to come on an exam on which 70% is a top score.  They've already done their practice problems on tests and quizzes and homework - that's how they know all of the above.

What message would I be sending, then, if I kept handing ever-larger sets of practice questions in the lead-up to the exam?  I'd be destroying the confidence I've struggled so hard to build.  There's no need to scare students into studying more.

Top students tend to work themselves into a state right before an exam.  Or, they do more and more practice problems as a way to show off to their peers... it's the secular equivalent of the holier-than-thou churchgoer.  But we're talking about top students!  They don't need to study more.  What are they going to do, turn their 5 into a 6?  Make them relax, both for their own mental well-being and for the sake of their classmates.

But what about students who will struggle to get a 3?  They will do better with a low-key approach, too.  Rather than shame them for not knowing everything - which is what we do, like it or not, when we shove a stack of practice problems at them - focus these folks on just a few topics that they can improve upon.  They're not going to get a 5 because they studied for days.  But, a few judicious hours here and there might well secure that 3.  

So, don't encourage more practice.  Encourage good, targeted practice in the weeks before the exam.  Encourage a relaxed, confident attitude in the days before the exam.  When students recognize intellectually and emotionally that AP exam day is just another day at the office, then your class is ready to rock.  No extra practice questions required.