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26 May 2013

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

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

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

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

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

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

Yes, but the justification is wrong. 

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


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


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

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

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


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


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

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



22 May 2013

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

Given today on a review quiz:  

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

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

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

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

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

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

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

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


17 May 2013

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

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

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

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

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

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

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

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

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

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

The right side is at a higher electric potential.

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

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

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

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

GCJ


16 May 2013

2013 AP Physics B -- my solutions, and comments

I spent some of Burrito Girl's birthday* and then much of this morning writing solutions to the 2013 AP physics B exam.  (Scroll down to find them, or do a control-F on my name.  Chris Becke beat me to posting solutions by a few hours; don't be shy about using his solutions.)  You can access my solutions at the link, but it's through "pretty good physics secure."  You must have an account with PGP. 

* Burriro Girl is my wife and sidekick.  My work yesterday was interrupted with a directive that physics time had ended, and Burrito Time had arrived.

Teachers can get an account with PGP by following the instructions at the PGP-secure homepage.  Students should ask their teacher to sign up.  PGP-secure is a wonderful resource, one that teachers will often use throughout the year.  It's worth signing up.

My comments:  The first question is similar to a famous old Hewitt question, in which he asks about the water level of a lake when a freighter full with iron cannonballs throws its merchandise overboard.  Excellent question.  Questions 2-5 and 7 are straightforward and interesting.  You can add the experiment described in #3 to your repertoire of laboratory exercises.

I'll note that question 5, the thermodynamics question, uses language in anticipation of AP physics 2.  No longer will the exam say "heat added to a gas."  Instead, you'll see phrases like "energy transferred into the gas by heating."

Question 6 is difficult but excellent.  A student really has to know about magnetic fields and forces in order to handle this one.  The last part, about the emf induced in a straight wire, is something I've never before seen on AP physics B; usually we discussed emf induced in a loop of wire.  But it's straightforward enough to use BLv to get the emf.  The side of the wire with higher electric potential can be determined by applying the right-hand-rule for magnetic force to the positive charges in the wire.

Please holler at me in the comments if you find a mistake, which you surely will.  I guarantee I got a 5, but not that I got 100%.

GCJ

13 May 2013

Minimize Drama -- set the tone in your reaction to today's AP Physics exams

As I write this, students are making last minute preparations for the AP Physics exams.  They start around noon today, getting out by 4:00.  This means that tomorrow in class you'll hear your students' reactions to the test.

If you've been giving authentic AP-style tests all year, then today's examination should not surprise anyone.  If anyone is truly put off his or her game by the format or general level of the exam, then do more authentic practice starting right at the beginning of next school year. 

But even the best-prepared students will often emerge from the exam just waiting to tattle on those mean AP exam writers.  I mean, how could they ask that question 3 on the free response?  It's way too difficult for Physics B, or it's not a topic that's on the fringes of the course description, or it's not anything like the problems on the last five exams, or it's about an experiment we've never done, or...  

[figurative SLAP]

What's your reaction?  Whether the student has a reasonable point or not, your job should be to minimize drama.

Switch places with the College Board for a moment.  What do you want your students' reaction to your in-class test to be?  Tests should be authentic measurements of a student's ability.  You want your class to accept and learn from the test as an evaluative tool; then you want them to correct their mistakes in anticipation of improved performance on the next test.  You do NOT want people searching for individual questions to nitpick, hoping to improve the grade by one percent.  That's not productive to the ultimate goal of learning physics.

There's a sports analogy here:  when a team loses, it's undignified to kvetch about the officiating.  Whenever a team complains that they were "screwed" by one bad call that cost them the game, they're generally ignoring the fifteen other plays they themselves failed to make that likewise would have won the game.  It's a team's responsibility to play well enough such that one or two bad calls can't possibly have an effect on the game.  Otherwise, you take your chances.

So when your students come back from the College Board's assessment, steer the conversation away from the "OMG, unfair question!" sort of comment.  Ask instead what they did right, what they saw that they expected and could answer well.  Ask what they struggled on; use that information to improve your class for next year, and tell your students now that you will do so.  

But when they complain, even if the complaint is somewhat reasonable, don't engage.  You are setting a tone.  If you don't want students complaining about the tests that YOU administer in class, what kind of example does it set if you complain about the people administering this national exam?  That's the pot calling the kettle black, and don't think your students won't notice.

Trust in the breadth of the examinations.  One part of one question has an insignificant effect on a student's overall grade.  If he did well on the exam as a whole, then there's no point in whining about one poor question.  It only takes about 115 out of 180 points to get a 5 on the AP Physics B exam.  In the highly, highly unlikely event that your student's confusion on one particular question made the difference between a 4 and a 5, then it seems far more rational to look carefully at why he missed the other 65 points.

Good luck today... I'll post my solutions shortly.

06 May 2013

Book Review: Gigante's UPCO Regents Physics Review

I first met Kris Gigante in person at the 2012 USIYPT.  We had made contact online, and he took me up on my offer to serve on the physics fight juries.  Kris enjoyed the tournament so much that he brought his own team last year, coming one point short of winning the Clifford Swartz Trophy.

So Kris knows his business as a physicist, and as a physics teacher.  When he told me he was writing a Regents review book, I agreed to review it for my site.  That's the disclosure.

Kris's book is outstanding.  It's standard in some ways: For example, he includes a content review chapter for each topic on the Regents exam; some practice items at the end of each content chapter; and then a few of the authentic open-source Regents exams at the end of the book.  The content review is well written and to the point, as is ideal for a review book.*  The standard material is quite good, and not that much different from other available review books.

*Or any book.  Why the #&$@ don't publishers give us textbooks our students can read and use, but instead decide to cram as much tangential crapola as ed professors and state curriculum directors and marketers can possibly demand?  Oy.

Where Kris's book stands out is in a couple of unusual and extremely helpful nuggets.  

At the beginning, he asks, "How well do you know your physics course?"  Then he asks 75 questions that I would consider "fundamentals" of physics.  For example...

If an object has a positive velocity and a negative acceleration, describe the motion of the object.  

Name seven kinds of EM waves in order of increasing wavelength.  

What quantity stays constant in a series circuit?  In a parallel [circuit]?

These questions -- questions that you as a teacher can use verbatim for your fundamentals quizzes -- go on and on for four pages of dense text.  What a treasure for a student.  It's the perfect diagnostic, to decide where to focus studying; it's also a perfect last minute review tool, to remind oneself of some facts to know.

Then in the back are fact lists in a similar style to the "just the facts" posts that have been so popular.    Each unit includes 1-2 pages of facts to know, along with a blank checkbox.  I get it: once the student knows the fact, he checks the box.  Then over the course of the year, the checkboxes slowly fill in, and voila, he's ready for the exam.

Certainly this book is perfect for a Regents student.  But also for any teacher of general level physics, it's worth grabbing a copy.  If you teach Regents, use the fact lists as a guide to preparing tests, lessons, or exam review.  If you don't teach Regents, the fact lists are still a useful guide.  Check the boxes yourself to record which facts you expect your own students to know.  Then you can find Regents problems that test these facts, and put them on your own in-class tests.  

GCJ

03 May 2013

Which are the most difficult AP Physics B questions, and the Pictorial Index

Drew Austen poses this intriguing thought experiment: Which AP Physics B problems have been the toughest?

My first instinct is to remind everyone that the AP Physics B exam has been of remarkably consistent difficulty over the years.  While sure, some questions have been tougher than others, the variation has been minuscule compared with the variation in textbook problems, most college or high school physics class test problems, or with even AP Physics C problems.

Used to be, back when I was teaching three sections of AP Physics B every year, I could perform a party trick: name a year and a problem number, and I could reliably describe the AP Physics B problem from that exam.  I can't do that off the top of my head any more.  

But I do have a cheat sheet: the Pictorial Index of Previous Exams.  This file includes an inch-square box for each problem on each exam stretching back to 1979.  Each box includes a thumbnail picture of the diagram in the problem stem, and a few words of summary about the problem. 

I first received a copy of this index on CD from Gardner Friedlander, who administers the PGP-secure site.  If you teach AP, get yourself a password to this site.  I can't link you a copy of the Pictorial Index, but look on PGP-secure for B FRpictindex.  There Martha Lietz, an outstanding physics teacher from Evanston, has posted her version.  You can pick up my version by attending an AP Summer Institute.

I use the pictorial index all year long when I teach AP.  I print out a master copy at the beginning of the year. Then when I assign an old AP problem on a test or as homework, I mark off that problem on the index.  I can quickly scan to find a problem on a particular topic; or I can narrow down my search if I want "oh, that magnetism question from 1998, or maybe it was 1999, I don't remember."

Looking at the pictorial index now, I can recall the more difficult questions over the years... these are my own perceptions, based not on the intrinsic difficulty of the topic.  Rather, I call a problem "difficult" if it challenges even students who have a firm grasp of the underlying concepts.

2012 B2 -- two spheres colliding as they hit the ground; symbolic manipulation of conservation of energy and momentum

2011 B2 -- electric field and potential produced by a charged sphere with a metal sphere placed nearby

2008 B3 -- wire-and-magnet experiment on a scale

2006 B2 -- determining when a sprinter stops accelerating

2002 B3 -- 1993 B3, electromagnetic induction including graphs of current and voltage through a wire loop

1995 B3 -- hanging mass inside a loopy roller coaster

You have a different list?  Of course you do... we're doing the AP Physics equivalent of discussing "which was the best Boston Red Sox team ever?"*  There's no right answer, just a pub argument.  Go ahead and throw your choices in the comments section.

* Two possible answers include (a) 1917, and (b) the team with the best steroid provider

01 May 2013

Mail Time: I'm teaching AP Physics 1 a year early. Should I come to a 2013 summer institute?


Ravi Lall, from Woodstock School in India, writes:


I am planning to start the new Physics 1 course in my school from August and for this purpose I wish to attend your summer institute. I have already attended one summer institute last year in Loyola university in Physics B and C Combined.

Do you think this year’s summer institute will be helpful for me? I am basically looking for some info and help for the new course structure.

How much portion of your workshop will cover the new course structure?

Please advise whether I should attend this year or in the summer of 2014?


Hi, Ravi.  I'm going to post this to the blog because it's a question that a bunch of people will have.

You are starting to teach AP Physics 1 in the 2013-14 school year.  Great idea, and not unusual; I've heard from plenty of folks who want to get the head start.  The thought is, get them in AP Physics 1 this coming school year, then they can take BOTH the Physics 1 and Physics 2 exams in 2015.  Since AP Physics 1 (unlike physics B) is actually designed to be a student's first-time introduction to physics, it makes perfect sense to make your first-year course for advanced students a Physics 1 course.

So, what about summer institutes in 2013 or 2014. 

First, there's the official word from the College Board:  Consultants are instructed to direct at least 20% of this summer's institutes to the new exams, while up to 80% of the institute can focus on Physics B material.

In practice, and speaking for myself only, my summer institutes as I've done them for years are well suited to AP Physics 1 and 2.  I certainly plan to spend some significant time discussing the details of the new exam -- number of questions, style of questions, how to prepare students to give more verbal responses than the Physics B exam required.  But the way to prepare students for this new exam, I think, should not be nearly as revolutionary as some would have you think.  The exam is revolutionary; the effective teaching methods are those that many of us have been using and advocating for decades.

The new exams will expose those students who think physics is about plugging numbers into the correct equation.  The Physics B exam already exposes such students; but these folks will be getting 1s, not 3s, on the new exams.  The new exams will expose the excellent students who can solve problems well, but who can't exactly explain in words why they solved the problems the way they did.  Such students will be getting 3s and 4s, not 5s.  At my summer institutes, we always have discussed, and will continue to discuss, ways of guiding students to a deep understanding of the topics at hand.  I'll show numerous quantitative demonstrations; I'll share class activitites; you'll have a chance to discuss physics teaching with other physics teachers (and you'll learn as much from them as you do from me).

The only major difference between this summer's instittute and next summer's institute for me will be the demos and topics we choose to discuss.  I'm not going to do much, if anything, with rotation this summer; rotation is the biggest topical change from Physics B to Physics 1-2.  I'm going to teach to topics that are common to the B and 1-2 exams.  But in summer 2014, I plan to bring out some rotation demonstrations, and to spend a full day of the institute on rotation.  

I hope you can join me this year.  Dates are July 15-19 at North Carolina State University in Raleigh; and August 5-9 at Manhattan College in New York City.

greg