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27 April 2016

Some practice quizzes to review before the AP Physics 1 exam

In the lead-up to the AP Physics 1 exam, I ask some short fundamentals-style questions on a quiz each day.  These questions are far less detailed than actual AP Physics 1 questions, but are deeper than my fundamentals questions leading up to the old Physics B exam.  The purpose of these quiz questions are to focus my students' review, and to focus their attention in class.  If I just said "listen while I tell you about gravitational mass again, even though I told you two months ago and you probably forgot," I'd get little useful knowledge imparted for the class time spent.  However, "let's answer the questions to this quiz you just took" keeps students invested -- if nothing else, they care whether they got a quiz question right five minutes ago.

How do I write these?  I put my fact sheets into random.org's list randomizer.  Then I just riff on the facts in the order they come up.

Here's one quiz.  I gave them a strict 3 minutes to finish.  Feel free to use in your class.  I'll post another in the next day or two if people like them.

1.      What is the more common word for the “magnitude of the velocity vector”?


2.      One metal sphere has a charge of +3 mC.  A second metal sphere has a charge of -2 mC.  The spheres are touched together.  What is the charge residing on the two spheres while they are in contact?

3.      A planet’s orbit about a sun is elliptical.  Consider a system consisting of just the planet.  Is the planet’s angular momentum about the sun conserved?


4.      An object is hung from a spring scale, which reads 2.0 N.  Dividing by 10, it’s determined that the object’s mass is 0.2 kg.  Which kind of mass was determined?


5.      An object is pulled at constant speed to the right by a rope, which is angled 30o above horizontal.  The tension in the rope is 5.0 N.  Is the force of friction greater than, less than, or equal to 5.0 N.


6.    An object is attached to a horizontal spring, compressing the spring by 0.15 m.  A second object, twice as massive as the first, compresses the same spring by 0.15 m.  By how much has the potential energy of the spring-object system changed? 

26 April 2016

Just the facts: all of AP Physics 1

You may recall my post last year at this time, of a brief topic list for AP Physics 1.  That's been a useful document, as I've referred to it throughout the year each time we review for a major test or exam.

But with a week to go before the actual AP exam, I'm bombarded with more requests for a more detailed summary.  So here you go.  

I don't use a textbook in my classes.  Instead, I hand out fact sheets giving just the basics of each topic.  I expect my students to memorize the information on the fact sheets over the course of the year; importantly, my students know that they should NOT memorize anything else!  Success in physics, and on the AP exam, consists of applying these facts to new and interesting problems.

Anyone may use my fact sheet -- I'd love it if you'd cite the source, but that's not even that big a deal.  After all, these are simply facts of physics.  If facts of physics aren't public domain, I don't know what is.  :-)

Fact sheet link via google docs here:  AP PHYSICS 1 FACT SHEET

(Anyone, teacher or student, who can't open this for some reason: just email me, and I'll send a copy.)

If I've left something out, or if you'd like to argue about phraseology, please post in the comments.

GCJ

24 April 2016

Rule 1 of teaching, and how it applies to AP exam review

This summer as you're preparing for your physics courses, I'd highly recommend reading the Teacher's Manual for 5 Steps to a 5: AP Physics 1.  It's a free download from McGraw-Hill.  The framing device for the manual is "5 Steps for You to Help Your Students Get 5s."  It discusses many of the specific approaches I take to my classes, all in the context of the AP Physics exam.  Of course, these approaches are equally applicable to teaching any level of physics.

Integrated throughout the text are what I'd consider the Three Commandments of teaching... not just teaching physics, but of teaching high school at all.  In the Teacher's Manual, I discuss these commandments with reference to beginning the course.  But they apply equally to the AP exam review that many of us are deeply engaged with this time of year.  

In case you're interested, Rule 2 is "Trust, but verify."  Rule 3 is "Your students don't listen to you.  (That's okay.  They don't listen to me, either.)"

Rule 1: Never condescend.

When setting the tone for your course in September, it's important that your students perceive that they are being treated like adults.  Yes, I understand that we are NOT officially teaching adults, and that some of our students will need intervention because their actions are not adult-like.  Nevertheless, the assumption of good faith on your part will go an enormous distance toward earning cooperation from your students throughout the year.  The majority of teenagers are, in fact, intellectually and emotionally ready to behave as adults.  But this majority can be hypersensitive to perceived disrespect or condescension.   

In the context of AP review:  It can be quite disheartening during review time to see our students making the same danged mistakes that we've worked on eliminating -- especially when such mistakes are made by the particular students who spent part of the year hostile, or lazy, or arrogantly overconfident.

Nevertheless... there's little point in reminding students about their personal shortcomings right now.  It's so tempting to say, "No wonder you're struggling.  Remember all those poor homework assignments?" or "Now, you would remember the definitions of wave properties if you had paid appropriate attention in class."  But do you really want to sound like a frustrated, nagging parent?  Your student will tune you out the same way he tunes out his mom when she complains about how he never helps out around the house.  

Just help the student, patiently.  Or don't help -- it's reasonable to politely and respectfully point to the correct fact sheet or old homework problem: "John, before  you try correcting this problem set, take a look at the wave definitions at the end of chapter 12.  I think that'll put you on the right track."  It's not your job to re-teach course material from scratch, but you should expect that even diligent students need reminders of things you studied earlier in the year.

It's not worth revisiting past failures in the runup to the AP exam.  Just be glad that your student is putting forth some kind of effort now.  Be respectful.  Lazy students know they're lazy without you rubbing it in their face.  And they're only going to change their future behaviour in response to a personal, internal decision to do so -- certainly not in response to a nagging physics teacher.


20 April 2016

Reviewing for the AP Physics 1 exam - three general approaches

Several folks have asked about reviewing -- "tapering" -- for the AP Physics 1 exam in two weeks.  Should we be doing anything different for Physics 1 than we did for Physics B?

First, my general notes about the leadup to the AP Physics exams:

* The last two weeks are the time to do LESS work, not more.  Remember, most of your students are taking other AP exams, and the teachers in those courses are pushing hard.  You'll get more out of your students if you assign, say, one free response problem per night, or maybe three multiple choice with justification required.

* On that note, it doesn't take much to remind your students of concepts you've discussed earlier in the year.  Sure, it's frustrating for half your class to say that the bigger force must be in the direction of movement, especially since you went over and over and over that issue back in October.  But this time, most of your students just need the brief reminder that comes from screwing up (again).  Make sure they have the chance to make all the canonical mistakes one last time before the exam.

* Don't teach your students to game the test!  This means don't try to predict what topics will be on the free response, don't attempt to find patterns in answer choices or in past free response rubrics.  Students who show a solid knowledge of physics will do well; those without solid physics knowledge cannot do better with One Weird Trick.

* Generally, folks are better off knowing how to do a few things well then how to do everything kinda okay.  Especially with students who are unlikely to earn 5s, help them truly master a few topics.

Those of you who were in the trenches for AP Physics B remember the enormous breadth of the course.  Especially for my top students, the last two weeks were all about quick reminders of seemingly hundreds of topics.  Some of the techniques I used for this broad review are still applicable.  

1. I still use fundamentals quizzes extensively 

These are quizzes where answers are straight-up, memorized facts.  I still do the 4-minute drill.  Start with basic recall... all the work you did this year means that your students probably have perfectly good problem solving skills.*  So be sure they know the facts from which they can solve problems.

* And if they don't, not much you can do about it now -- problem solving is an art form that is learned over months.

That said, the facts that I've asked students to learn are a bit more complicated now.  In the old days, I was often asking for recall of equations, or of problem solving techniques.  Now, my questions are a bit more conceptual.  Not "write the three kinematics equations," but "when are the kinematics equations valid?"  Not, "write the work-energy theorem," but "when no external forces act on a system, what quantity is conserved?"  If you'd like some sample fundamentals quizzes, email me; if you'd like the whole lot of these that I used this year, come to my summer institutes, and you can have a ginormous CD-ROM.

2. In class, I'm still doing creative lab work.

In Physics B, the last weeks were spent working problem after problem.  But since the topics in Physics 1 are so limited, and since deep understanding of physical situations is so prized, I've changed that approach.

We've been doing released AP 1 and AP B problems for homework each night.  In class, though, we've not just "gone over" the problems... we've set up the situations in lab.  So far we've performed the experiment suggested by each of the 2015 released AP 1 free response questions.  

This approach has sort of replaced my "exam corrections".  Instead of asking pointed questions about the mistakes my students made on these problems, we're actually doing the experiment, and then I've followed up with a problem in laboratory format.  Again, if you'd like a set of these laboratory-format AP 1 questions, email me, or come to my summer institutes.

3. And I use these cool simulations

Take a look at this post, where I discuss Taft School's simulation labs in preparation for the AP Physics 1 exam.  As regular readers know, simulations do not replace or replicate real experimentation.  However, at this point in the year when students should be more than comfortable with laboratory investigations, playing with good simulations (like the ones at the linked site) can be useful and fun.  Best of all, they can be done at home OR in class.  

These Taft simulations are perfectly set up for AP 1.  They truly simulate experiments that can in principle be performed.  They allow for students to control multiple variables.  You can use them to quantitatively verify quick calculational predictions; you can use them to predict qualitative trends and answer "what happens if" questions; or you can use them to make full-on experimental graphs which can be linearized, and the slope used to measure a quantity.  How versatile.

Do you have a different approach to AP review?  

Please post in the comments.  The best physics teachers adapt multiple approaches learned from others, and make the combinations of approaches their own.  You can read probably 30 different approaches of mine to exam review on this blog.  Let's hear other thoughts, too...

GCJ

10 April 2016

Mail time: What's the difference between (qV) and (q*delta-V) in electrostatics?

A reader discovered these two quiz items among the materials I gave him at a summer institute:

The electric potential at point A is -30 V; the electric potential at position B is 0 V.  

1. What is the electric potential energy experienced by an object with charge +1 C when it is placed at point A?

2. What is the electric potential energy experienced by an object with charge +1 C when it is placed at point B?

(Read carefully.  I answer #1 and #2 at the bottom of the post.  But a reader asked something different.)

The question from the reader asked:

"I know the relevant equation is PE = W = q(deltaV).  My issue is with the delta...It seems the answer to #1 is -30 V since deltaV is (-30V). Does this mean for #2 the answer is choice A since deltaV is (+30V) or zero? Are we even using deltaV or just the electric potential for that point, in which case it is zero for #2?"

The delta in the equation referenced is part of the work-energy theorem -- work done by an external force is equal to the change in kinetic plus change in potential energy.  Here, the potential energy experienced by a charge q is qV, where V is the electric potential at a position.  

The equation referenced -- W = PE = q(deltaV) -- answers not this, but a different question! 

The question that I think you mean to answer is, "How much work is necessary to move the object from A to B (with no change in kinetic energy)?"

From simplifying the work-energy theorem, the work necessary to move the charge is qdeltaV = (+1 C)(+30 V) = +30 J.  The value of the electric potential itself is irrelevant... the term that appears in the work-energy theorem is the CHANGE in potential energy.  Work done by an external force CHANGES the object's potential energy.  And to change potential energy, the object has to move from one position to another where the electric potential has changed.

The charged object moved from point A to point B.  Call the electric potential zero at point B-- fine.  There's no electric potential energy at B.  That's actually not relevant to the problem.

What's relevant is how much greater or less the electric potential at B is.  A positively charged particle moving from V = 0 to v = 30 V gained electric potential energy, exactly the same amount as if it had moved from V =100 V to V = 130 V or from V = -30 V to V = 0 V.

And that's why there's a delta in the equation you referenced.

But now to the original quiz question, from the top of the post:

What's the potential energy of the +1 C charge at point A?  When using PE = qV, the negative signs are important.  So PE = (+1 C)(-30 V) = -30 J.  It does have potential energy, even though it's never moved.  But that potential energy is kinda meaningless unless the charge does move.  Kinda like I have potential energy relative to the ground when I stand on top of the Sears tower, but that's rather meaningless unless I jump off.  :-)


The charge has no potential energy at B because PE = qV = (+1 C)(0 V) = 0 J.