27 June 2010

Encouraging *everyone* to participate in class

We all want to elicit responses from all of our students in class, not merely the very smart or very loud.  And, we all have different methods for encouraging or requiring participation. 

My ex-colleague Jacob Sargent took a page out of elementary school methods by writing each student's name on a tongue depressor.  He drew a stick at random, and that student would be called upon.  (As a side note, my wife and sidekick Burrito Girl points out to me all the time how similar high school teaching methods can be to raising-little-kid methods.)

While I have occasionally done the same thing using random.org, calling on students randomly is not a magic bullet to encourage active participation from all.  When a randomly chosen slug says, "I don't know" and is not embarrassed, calling on him or her has been counterproductive.

I can raise the stakes a bit with the familiar "check your neighbor" question.  I ask everyone to WRITE the answer to the question, then discuss it with their neighbors.  The class votes on the right answer, sometimes for credit.  My colleague Paul Vickers used to give "group quizzes" in this manner.

Michael Ungureanu from my University of Georgia summer institute was particularly enamored of an even more powerful, even higher-stakes approach to class participation which combines many of the above methods:  I ask a check-your-neighbor style question, and require students to write an answer in their notebooks individually.  Then, I allow several minutes for discussion.  But I tell everyone before the discussion:  I will select a student from random.org to explain the answer to me and the class.  (Not just to state the answer, but to come to the front of the room and EXPLAIN.)  If the randomly selected student is correct, I promise to cancel the next day's quiz.

As you might imagine, discussion in this case is generally active and loud.  And, I'd guess that the quiz gets canceled upwards of 80% of the time I use this approach.  That's fine with me, because the class discussion and peer instruction that goes on has a teaching power equivalent to at least three quizzes. 

[Image at the top from harvard.edu.]

22 June 2010

Torque problems involving an extended object

When an extended object -- think a meterstick, a bridge, or a plank -- is in equilibrium, a problem will often require setting counterclockwise torques equal to clockwise torques.  The question that often bugs first-year students is how to deal with the weight of the meterstick, bridge, or plank itself.

When the fulcrum is at the extended object's center of mass, no problem -- just ignore the object's weight.

But imagine a meterstick that's pivoted about the 70-cm mark.  Now 70 cm hang to the left, and 30 cm hang to the right of the fulcrum.  We need to include a torque produced by the weight of the meterstick itself when calculating, say, where to apply a known force to balance the stick.  Torque is force times distance from a fulcrum.  What force do we use, and what distance?

The correct answer is that we use the ENTIRE weight of the meterstick, and we use the distance from the meterstick's center of mass (i.e. the 50 cm mark) to the fulcrum.  If the pivot is at the 70-cm mark, then that distance would be 20 cm.  That torque would be acting in the direction pivoting toward the center of mass.

Students don't initially like that answer.  "But 30 cm is hanging on the other side!  Shouldn't I take that portion of the meterstick's weight into account?"  Well, you did.  Since you used the entire weight of the meterstick, and you used the position of the entire stick's center of mass, no further work is neccessary.

(Sure, you could treat the left and right sides separately, apportioning the correct percentage of the stick's weight on each side and then indicating that weight acts at the center of mass of each side.  Whew, I don't think that even I understand that last sentence.  Better stick [ha!] with the simple method.)

Today Jim Brice, a member of my group at the University of Georgia AP Summer Institute, shared a clever method of assuaging the students' worries about treating the whole meterstick as a unit.  He puts a meterstick on an electronic balance.  Of course, part of the stick hangs off of each side of the balance's platform.  Nevertheless, he asks his class:  Does the scale read the ENTIRE weight of the meterstick, or just the part actually touching the platform?

20 June 2010

Graph presentation and the Duverney Standard

We all ask our class to label their experimental graphs clearly. That seems like such an easy-to-follow instruction. But remember, students come into our class with widely varying lab experiences. Some of their previous teachers have probably not cared about whether axes have correct units, let alone labels. Other teachers might have been almost obsessive-compulsive about the format of titles and labels.

Remember that high school students generally view laboratory work as merely a set of steps to be followed, at least at first. They don’t understand why clear communication of results is necessary. After all, they think, the teacher told us exactly what procedure to use, what to graph, and possibly even what our graph should look like. So why wouldn’t she understand our labelless, unitless graph?

Such reasoning is rationale #1 for allowing, encouraging, and requiring some freedom and independence in laboratory exercises. Students must see that they are undertaking an active investigation, not just carrying out a rote process. But I digress. That’s a topic for another post.

I’ve noticed that, no matter how intense my instruction, constructive criticism, red ink, and general nagging, my students’ graphs continue to look like crap… except for folks in the research class. They usually have an epiphany after their first informal presentation to their classmates, who ask pointed questions about the experiment. Suddenly, the presenters see the confusion generated by an unlabeled graph. They look at me sheepishly while I give them the “I-told-you-so-for-two-years” look.

Taking a cue from my research course, I’ve had some success improving graph appearance in first-year courses by clearly elucidating the intended audience for the graph. It’s not good enough that they themselves understand what they mean; it’s not even good enough if they think that I should know what they mean. Their graph should be clear enough that another student at the same level of physics should be able to tell exactly what they did just by reading their graph.

I called this the “Duverney Standard,” named after a popular student who graduated the year before. I expect that Mr. Duverney, despite his ignorance of a particular laboratory activity, should be able to read a graph alone and interpret the experimental results for himself. (Of course, this year the standard will acquire a new name. Perhaps I could sell the naming rights to a graduate for a donation.)

Now, I’m not particularly OCD about graph presentation. After all, Mr. Duverney is known to be a smart gentleman. If a student chooses to use a clear title in lieu of verbose axis labels, fine. If the axis labels are clear, then a title isn’t necessary. The standard is simple and non-negotiable. When axes are labeled merely m (kg) and t (s), I take off significant credit, and simply write “Duverney.” If a student asks about the lost points, I ask whether Mr. Duverney, having looked at the graph, could reasonably reproduce the experiment. That’s usually a sufficient explanation to extract the sheepish “oh” from the student.

Now, Defcon II in the argument would be to scan a copy of the offending graph and send it to Mr. Duverney for comment. There is no doubt in my class’s collective minds that I am willing and able to carry out such a threat. Thus, I’ve never had to actually do so.

18 June 2010

The future of AP Physics B

You may have heard rumblings about the AP Physics B redesign.  In sum, the College Board got a huge windfall of government grant money to examine the effectiveness of the algebra-based AP course.  And, in part since it's unwise to accept millions of dollars and then say "thanks, but we're all good," the Physics B course will change.  In several years (no clue exactly how many), Physics B will become two courses:  Physics B1 and Physics B2.

There are two major reasons for the change.  The stupid reason is based on an early 2000s report from a committee of physics professors blasting the AP B course for its predominance of mathematical manipulation and lack of physics reasoning skills.  Problem is, they were looking at outdated exam questions.  Since 1996 or so, the Physics B exam questions have been rather powerful evaluators of reasoning skills.  The negavitve conclusions of that committee about AP physics B were based on incorrect assumptions; thus, the huge grant and the redesign.

The GOOD reason for the redesign is the overly extensive breadth of the Physics B curriculum.  Sure, some teachers get through all of the necessary topics.  Sure, many of us also believe that a broad survey course is the correct way to begin a high school student's physics education.  However, the vast majority of teachers can not get through the full B course.

Consider two students who earn a 3 on the current AP physics B exam... student 1 has a half-arsed understanding of all of the numerous topic areas, and so earns about 1/3-1/2 credit on all of the free response questions due to his shallow partial credit responses.  Student 2 has NOT covered all the material on the exam, but has a serious grasp of what he has covered.  Student 2 might earn most all of the points on several free response questions while leaving the others blank.

Colleges want to differentiate between students 1 and 2.  Student 2 has a potential future as a physics major, and thus is a candidate for a limited amount of advanced placement; even if he doesn't continue on in physics, he has demonstrated a depth of understanding that is at the college level and thus worthy of some college credit.  Student 1, though, can not talk intelligently about physics.  College professors want to reward student 2 (with admission, credit, and placement) but not student 1.

So, B1 will be *easily* short enough to teach in one year, as will B2.  However, the combination of B1 and B2 will probably be too much to be taught as a single first-year course.  That is the goal.

As for the content of each course:  In July, the curriculum development committee is scheduled to release the topics and curriculum goals for B1 and B2.  It's not as simple as splitting Physics B down the middle, because of state standards, or at least state standards in large, influential states.  For example, the New York Regents exam covers simple circuits.  If AP B1 did not include circuits, then presumably New York schools would be hesitant to teach B1 as a first year course.  The committee has navigated all kinds of political minefields in deciding upon its final course content.*

* Which, therefore, will likely draw complaints from EVERYONE.

More importantly, the committee is committed to testing conceptual understanding throughout the test.  The style of questions will be changed such that verbal justification is not an afterthought to a calculation, but rather is the primary focus of each problem.  For example, consider this year's AP physics B2, about a fluid mechanics experiment similar to one I do in class.  The test asked for a derivation, followed by graphical analysis and interpretation of experimental data.  A source close to the committee suggested that the style of the redesigned exam questions would likely BEGIN with the experimental data, verbal analysis and interpretation... and only then require derivation and calculation.

What should you, personally, do about this?  Well, nothing, for now.  The Physics B test will not change for a few more years.  Over the next few years, you might consider how a B1 and B2 course might fit into your overall curriculum.  I'm myself considering teaching B1 as our honors freshman course; then, B2 would be a junior-senior course in the style of my current B course, but open only to those who have passed the B1 exam.  Others are considering dropping Physics B and teaching Physics C as a second year course, or C mechanics as a first-year.  Most who already teach an honors first-year course will just combine B1 and B2 into a single, second year course.

And finally, do consider ramping up your emphasis on written justification and conceptual understanding.  Sure, my students will always memorize equations.  But your calculus student with the 750 math SAT who is obstinate about not showing his work will not survive in Physics B1 and B2.  If you're not already doing so, step up the fight against the student who thinks physics is all about calculating the right answer.  That's what good physics teaching is, anyway; and that's the style of teaching that will promote success on the redesigned exams.


16 June 2010

Projectile lab with a marble: use a photogate!

Greetings from the AP reading in Fort Collins, Colorado.  I'd say 3/4 of my teaching ideas have germinated in  this enclave of friendly and professional physics teachers.  Today's thought comes courtesy of David Moore, who is part of the team grading this year's fluids experiment problem. 

A common  laboratory exercise asks students to predict the landing spot for a marble projected off of a table top.  Usually the marble is rolled down a ramp from the same height every time to ensure a consistent initial horizontal speed.

Measuring that horizontal speed is tricky.  Motion detectors don't pick up objects as small as marbles very well.  I suppose video analysis would work, but that's too intricate for a general physics class, I think.  In the past, I've had the class use stopwatches.  If the marble rolls across a flat tabletop, then the distance of the flat region divided by the time to travel that region gives the marble's horizontal speed.  However, my lab groups have made consistently incorrect predictions using this method.  Just a small reaction time issue can cause the marble to miss the target by 30% or worse. 

David says he uses a photogate placed near the end of the table!  Knowing the marble's diameter and the time during which the gate is interrupted, the marble's speed can be calculated.  Even better, use two photogates near each other:  the speed is the distance between beams divided by the time between the beams' activation times.  Reaction time or stopwatch clumsiness is not an issue when photogates are involved.

09 June 2010

Textbook? What textbook?

I spent many years in search of an AP textbook that students could and would read, and that had good end-of-chapter problems in it.  Several books meet the latter criterion -- often by posing so many problems that sheer probablilty ensures a few good ones.  But none of the mainstream texts truly are readable by a typical AP student.  They cover far more than is on the AP exam, which one might call a strength; but a newbie physics student doesn't want breadth, he wants concise, to-the-point prose.  My students gave me heck about the $150 cost of a new book that they didn't find particularly helpful. 

My response:  I eliminated the textbook altogether for AP physics.

First, a couple of caveats.  The College Board's audit requires that each student have access in- and out of- school to his own copy of a text.  I meet that requirement because I now have an enormous set of books left by graduating students, donated by friends who didn't need them anymore, salvaged from lost-and-founds.  I take these text to our library and put them on reserve.  (I teach at a boarding school -- students have access to these books in the library day and night, and can even check them out.  Only one student ever did so.)

Secondly, I wouldn't recommend eliminating the textbook to a relatively new AP physics teacher.  Even though my students only used the texts occasionally, in my first few years *I* used the text regularly.  It was important that I figure out for myself which were good problems, how the text advises approaching each kind of problem, how the text presents material.  In those first few years, I needed to ask students to read some things out of necessity -- I didn't yet have the skills or the class time to teach every topic properly. 

Now, though, I have a generous bank of questions that I can ask for homework, virtually all of which are written by or at least substantially modified by me.  I don't need the end-of-chapter problems any more.  I also have taught each unit enough times that I actually don't *want* students reading a text.  (If I use slightly different language or procedure from a textbook, the students get crazy confused.  In the first few years, my correct approach was to adjust my language or procedure to match the text.  Now, I want the freedom to do things my way.)

Sure, a textbook offers more than just a how-to guide to approaching AP physics questions.  But I ask you:  is the extra enrichment privided by a textbook -- at least, provided to those who bother to read something beyond the immediate scope of the class -- worth $150 per student per year? 

My answer was "no".  Although I continued to require the $15 5 Steps to a 5 book, this year I did not ask students to buy a text.  I received no complaints.  One student did make use of the library reserve books, especially to read more details than I gave in class about atomic physics; that's consistent with the average number of students who did the same reading in the past few years.  An unintended consequence of eliminating the text was that more students read the 5 Steps book more frequently.  Since that book is carefully focused on only the information necessary for the AP physics exam, my students got far more significant benefits from what reading they were willing to do. 

In sum:  eliminating the textbook was an unmitigated success.

Now, all that remains is to find a similar approach for the general physics course.  Even though I've never found a decent textbook for that level, I'm not comfortable asking lower-level students to rely exclusively on their class notes for studying.  I'm not convinced that the textbook -- I use the Glencoe-Merrill monstrosity seemingly written by education majors for education majors -- does much good.  But in general physics I can't justify the lack of a text to a parent or administrator.  For AP, the justification is easy.

05 June 2010

More multiple choice: New York Regents exams

When I first started teaching, my college roommate and Yankees fan Dai Duong sent me a New York Regents exam.  Of course, in those days the exam was hard-copy only, not online.  I remember liking the exam, and using some questions from it on my own tests, but since I lived nowhere close to New York* the Regents exam strayed from my mind.

*I was in Boca Raton, Florida, which is close to New York only in terms of culture.

Frank Noschese's comments on my previous post about multiple choice appropriate for general physics inspired me to check out recent Regents exams.  Frank is right -- the National Science League tests are essentially a rich man's version** of the Regents.

** "Rich man's" version not because NSL questions are any better, but because it costs money to purchase each year's test.  The Regents tests are available free online.

The level of the Regents is dead-on to what I teach in my general physics course.  My choice of topics does not exactly mimic that of the Regents exam, but is pretty close.***  In any case, now that Regents tests are online, I've now found another solid souce of physics questions.  Thanks, Frank!

*** More on topic selection in General Physics soon.

02 June 2010

Breaking news: No more penalty for guessing on AP multiple choice

Just released in a letter to College Board consultants:  Beginning next year (on 2011 exams), the multiple choice score will not include the "guessing penalty."  The multiple choice raw score will be the total number of correct answers, period.  Expect a formal announcement to all AP teachers in August; the course descriptions on AP central supposedly now reflect this change.

What does this change mean for you?  Well, it should mean almost nothing... students should always guess on multiple choice questions that they read.  However, it now becomes critical that students make random guesses if they are running out of time at the end.  This is certainly a substantive scoring change, but I believe that its effects will be way overblown on message boards and in conversations.  It's not a big deal.