Independent laboratory exercise: direction of force and motion

Two summers ago, Jeff Steele of Amherst, VA described his laboratory "stations" for Newton's First Law experiments.  He set up five or six situations, and send his students to deal with each of them in turn.  

My 9th grade class is working on Newton's Second Law right now.  The big issue, obviously, is understanding and believing that the net force is in the direction of acceleration, not motion.  Beyond busting that misconception, I want to give students plenty of practice and hands-on experience with free body diagrams and (qualitatively) the second law.

Borrowing from Jeff's "stations" approach, I set up three very basic experiments in the back of my classroom:

(1) a force plate connected to a labquest

(2) a PASCO cart on a track, connected by a string over a pulley to a hanging 100 g mass; the string is attached to the cart via a Vernier force probe, as shown in the picture

(3) a fan cart on a track, connected by a string over a pulley to a variable-mass hanger... the fan provides a force of 0.27 N.

Each of these three setups can support a few different predictions:

(1) I can have students jump off OR land on the force plate, predicting whether the reading will be greater or less than their weight.

(2) I can have the hanging 100 g mass moving upward and slowing down, OR moving downward and speeding up.  In each case, students can predict whether the tension in the string will be greater or less than one newton.

(3) I can require that the fan cart move left OR right, while either speeding up OR slowing down.  (That's *four* different predictions!)  Students can predict whether they should use more or less than 0.27 N of hanging weight in order to get the motion I require.

I randomly distribute lab sheets, so everyone is working on a different prediction.  Each sheet describes the relevant motion, and guides students through the prediction.  When they have made the prediction, I have them do the experiment.  The experiments are simple enough -- and we are by now good enough at using the labquest -- that I can sit in front of the room and wait, only troubleshooting where necessary.  Everyone brings me the labquest to show me the results when he's done.

If you'd like to use or modify the lab sheets for your own class, they're available via this google doc link.  

Quiz question: What is the electric potential at points A and B?

An electron gains kinetic energy in moving 7 cm from point A to point B.  The potential difference between A and B is 21,000 V.  

(a)   What is the electric potential at point A?

(b)  What is the electric potential at point B?

This is in the classic style of my quiz questions -- the student who can recite and use facts of physics jumps straight to an answer, while the student who is "thinking too much" ties his brain in knots.  I frequently ask simple questions in all sorts of different contexts.  Part of Bill Belichick's* genius is running similar plays but out of completely different formations, confusing all but the best-prepared defenses.  I try to do for physics what he does for football offense -- same ideas, different formations.

* Bill Belichick is the head coach of the New England Patriots football team.  He and his public persona provoke a huge variety of emotions in football fans, the positiveness of which usually correlates to a fan's distance from Boston.  No one doubts, though, that he is a football genius, the most successful head coach since Vince Lombardi.  Based on his success, his public demeanor, and his looks, he is often compared to the Galactic Emperor.

After a few days my students can all tell me that a negative charge is forced from low-to-high potential.  But soon we get in to sources of potential, the equation for potential due to a point charge, and don't forget all the stuff with electric fields.  So it's worth asking a basic question in a new context.

The answer to the question above is straightforward from the facts:  Since the electron "gains KE," it experienced a net force toward point B.  Since an electron is forced from low to high potential, point B must be at a higher potential than point A.  Since the potential difference between the two points is 21,000 V, point B must be 21,000 V higher than point A.*  

* Point A can be at any potential, any at all.  All that matters for problems with electric potential is the potential difference between two points.  There's no such thing as an "absolute" potential, only a potential compared to some other reference point.

What's so confusing, then?  The window dressing in the wording is confusing.  How many students will see "7 cm" and immediately convert to meters and try to plug into kQQ/d?  How many will just give up when they can't figure out the electric potential at point A, when that doesn't really matter?

Now, some lawyers would whine that this is not a "fair" question.  "Part (a) is a trick!  And you gave irrelevant information.  That's not nice."  

Oh, baloney.  A student who truly understands electric potential recognizes what "potential difference" means in any context, not just when he's asked to spit back a definition.  A student who becomes confused by these questions does not understand physics as well as a student who sees the correct approach; this question becomes an important mechanism toward internalizing a deeper understanding of "potential difference."  The student who gets this wrong and learns from the mistake will likely do well on the AP exam (or on the class final); the student who just gets pissy about fairness will not likely do well when faced with AP-style problems that similarly pose questions in new contexts.  Physics class isn't about how many answers we can get right to pad our grades, it's about discovering how the universe works.

You have a simple question in an unusual context?  Let me know in the comments or by email.

Is there any point at all in talking in front of the class?

In my 9th grade conceptual classes, I've done less and less talking as time's gone on.  And I've actually seen improvement in test, quiz, and homework performance.

When I started teaching 9th grade last year, I'd spend maybe 20 minutes per class leading a discussion while students highlighted their text.  Certainly this was more effective than when I talked to the class for 40 minutes per class back in 1996, the last time I taught 9th grade.  

Later in the year, I handed out "fact sheets" and went over the facts with the class, again leading a brief discussion.  These discussions would go for 10 minutes or so a few times per week.  I'd also lead an interactive discussion about the previous night's homework sometimes.

This year, I pledged to do NO DISCUSSION, NO LECTURE.  

Each class leads off with a three-minute quiz; students grade the quiz, so I dictate the answers to the quiz.  But everyone focuses carefully on what I say, because they care very deeply about whether they get points on their quizzes.  I make sure to explain the answer to each question first before explicitly stating the answer, so that I maintain attention as long as possible.  This isn't lecture; this is going over a quiz.

Once a week or so, I have students grade each others' problem sets to a rubric.  I stand in front of the class and dictate the rubric.  Students pay very careful attention, because they are responsible for grading a peer's paper correctly.  I find that 15 year olds are unabashed about writing total bull honkey for my reading pleasure, but they get quite embarrassed when their friends have to take off points for silly answers.  Again, this isn't lecture at all, even though I'm talking.  The class hangs on every word because they are immediately accountable for what I am saying.

Beyond grading items and (briefly) answering specific questions brought forward by students, I don't talk to the class.  Important information -- definitions, equations, etc. -- is communicated exclusively in writing, via fact sheets. I turn on music and let them get on with an activity of some sort.  Usually this activity involves both problem solving and experimentation; sometimes this is test corrections, or collaborative review of previous topics.   In any case, the vast majority of the class is spent with students working on their own,* and showing me their work at regular intervals.  

*Collaboration is allowed using the five-foot rule -- they can talk to each other as much as they want, but when they are writing anything to be turned in, they must sitting a minimum of five feet from any other student.  I'm right there to enforce the rule if necessary.

Whatever we work on, students must show me every single part of every single problem before they move on.  If their work is wrong, I explain the issue, and make them redo the work right then and there.  If I don't see someone at my desk for more than about five minutes, I have a word with him -- either he's distracted and needs a (figurative) kick in the arse, or he needs to plow through some misunderstanding, which cannot be done by staring at a blank page.  I make such a student write something to show me right away.  We're modeling how to approach complicated homework problems, so that such a student knows what to do at night when he's flummoxed.  

I see a lot of the same mistakes from multiple people.  But it's amazing how well information flows through the class.  I don't even need to prompt people much of the time -- they explain to each other how to approach problems, or what mistakes not to make.

The point is, the no-talking-by-me approach has produced better performance than ever, and by a good margin.  If there's something I'm just dying to explain to the class, I don't say it; instead, I design a quiz question to make my point in the context of grading the quiz.  If everyone is screwing something up, I make sure that several of the homework or in-class exercises address the issue.

So is there any point in me talking to the class at all?  

Would you like to be a juror for the 2014 US Invitational Young Physicists Tournament in San Jose?

On behalf of the US Association for Young Physicists Tournaments, I would like to extend to you an invitation to serve as a juror for our 2014 Young Physicists Tournament on Friday, January 31 and Saturday, February 1.  There, you’ll high school students from around world engaged in presenting and discussing physics research they had done in their classrooms under the guidance of their teachers.  The USIYPT is far more like a physics debate tournament than a science fair – we ask jurors to evaluate students’ ability to ask and answer questions, to engage in discussion of their undergraduate-level research.

This year, the Harker School in San Jose will again host the tournament. We are expecting as many as nine teams, from as far away as New York, China, and Tunisia, to participate. We’d like to get as many local jurors as possible for the event – physics professors, graduate students, chemists and engineers, high school physics teachers, industrial researchers; in short, anyone who knows enough physics to judge presentations by high school physics students well enough to evaluate experimental techniques and reasoning skills. This is a great opportunity to network with other local physical scientists and engineers in a fun setting. If you know anyone  in the bay area who would make a good juror -- or if you know anyone who would be willing to travel to San Jose (at their own expense) to judge -- please have that person contact me. The time commitment would be from roughly 9:00 AM to 2:30 PM on Friday and 9:00 AM until 5:00 PM on Saturday. Lunch will be provided.

If you would like to see more about this event before accepting this invitation to participate as a juror please go to our web site: http://www.usaypt.org/.

Thank you for your consideration!

Greg Jacobs, President, USAYPT

Woodberry Forest School, Virgina

greg_jacobs@woodberry.org

Mail time: is the sign on electric potential energy important?

Joseph Rao, from my 2013 Manhattan College AP Summer Institute, writes in:

For AP Physics do we pay attention to the sign on Electric Potential energy?  I know we do not assign a sign to Electric field and do assign one to potential difference when calculating potential some distance from a source charge or charged plate. 

Hey, Joseph... the sign on electric potential energy is important, because objects are forced from high to low potential energy.  A charge released from rest with -30 J of PE will speed up to a place where it has -80 J of PE.  

Note this is different from electric potential (i.e. voltage).  A positive charge is forced high to low voltage, while a negative charge is forces low to high voltage.

GCJ

Quiz: How to read a labquest's motion graphs

It's all well and good to teach students to interpret motion graphs.  We should, of course, go beyond simply asking whether a, v, and x are positive or negative -- we should demand verbal descriptions of motion.  We should ask what kind of object is represented by the graph.  We should expect students to use normal language, such as "traveling east" and "speeding up" rather than, for example, "going positive" and "acceleration congruent with motion."

I find that my students, even my freshmen,* get pretty good at interpreting motion graphs on homework.  However, in the laboratory, their skills sometimes vanish.  They use a motion detector on a cart on a track, then tell me with a straight face that it started at 50 m/s, slowed down, and stopped after 0.05 s.  Or, they assume that the vertical axis reading on a position-time graph tells how far an object moved, not its distance from the detector.  These are mistakes I've ground out of the class on sterile homework problems, but not when confronted with a labquest reading.

*ESPECIALLY my freshmen

I'm not going to wax poetic about why misconceptions persist in new contexts despite a physics teacher's best efforts.  Rather, I'm going to keep my efforts coming.

Today we have an (announced) labquest quiz.  I've taken four photographs of actual labquest readings.  For three of them, I ask students whether it's a reasonable graph, and why.  For the fourth, I ask for a calculation of distance traveled, and for the duration of the cart's travel.  These are all questions I've asked repeatedly in class when a student shows me data.  However, now I'm checking to see who really understands, and who just nods his head in the hope that I'll shut up.  :-)

Please feel free to use the linked quiz in your class.