Monday, November 5, 2012

It's just a grade...

So far this year my regular-level physics classes have had three lab assignments.  Those are Inelastic Collisions in One Dimension, Elastic Collisions in One Dimension, and Explosions in One Dimension.  Each of these involved placing Vernier Motion Detectors at opposite ends of a Vernier Dynamics Track and graphing the motion of a pair of carts as they are put through the various situations described by the lab titles.  (To be clear, I have done the same lab just as well with Pasco.  I don't advocate Vernier in particular.)

In our labs, students took multiple data runs with varying amounts of mass added to the two carts.  The whole goal of this is for students to see that momentum is a useful concept in physics, and that its conservation holds true in many different situations.  Hopefully they will then believe it when I tell them that this holds true in any situation where there is no outside force acting on a system.

In each of these labs, the students obviously have to calculate the momentum (mass times velocity) of both carts, both before and after the collision (or explosion, but let's just say "collision" to make it simpler).  Students therefore had to have a way to find the velocity of each cart.  Earlier this year students learned that the slope of a position vs time graph actually is the velocity of the object being graphed, so that's what I expected they would do.  Sure enough, each lab group graphed x vs t for the carts and used the software to determine the linear fit for the time before the collision, and for the time after the collision.  They printed out these graphs and stapled them into their notebooks, with the slope clearly labelled.

And then they wrote that the momentum is conserved.

There are no calculations in many of the notebooks.  No equations to show whether or not momentum had been conserved in these collisions, and to what degree.  There is simply the statement that momentum is conserved.  Clearly we've got a disconnect here between my expectations and student performance.  It's amazing how exhausting it is to continually stay on top of these things.  Students by and large do not want to do this kind of work--it's just a grade.  It's very hard to stay excited as an educator when students so clearly see no connection between the work they are asked to do and conclusions they are asked to make, let alone questions on an assessment.  To actually connect this with what they think their futures will be?

Today I assigned a step-by-step exercise  for the classes in question. For each of the three labs, students were asked to calculate the initial momentum of cart A, and then of cart B, and then the sum of these.  And then the final momentum of cart A, and then of cart B, and then the sum of those.  And then the percent difference.  And then the question: does your data demonstrate that momentum was conserved?  It may not have been fun, but it clarified things.  Many times students showed me that the data in their notebooks was not sufficient to calculate these values, which means that they could not conclude one way or another about the conservation of momentum.  What should we do?  Well, frankly, the fact that you realized that you documented the wrong data is more important than the number crunching.

I hope this serves us well in our next lab: Impulse.

Saturday, October 20, 2012

The Order of Concepts and Chapters

After a decade teaching high school physics, I'm still working on what order the material should come in.  At this point I am pretty happy with the order for my regular-level course, where I start with momentum and collisions.  Those classes will have their second test of the year later this week and after that we will use impulse to transition to our first mention of Newton's Laws and acceleration.  I think it works really well in developing a strong student knowledge of a very important fundamental concept in physics: Conservation of Linear Momentum.  It really bothers me that so many textbooks seem to just tuck that topic as an afterthought in a chapter somewhere after energy.

The course where I am really unhappy is AP Physics B.  Perhaps it is only natural, since College Board themselves are unhappy with AP Physics B, so much so that they are splitting the course apart soon (some excellent details here).  In training with VASS and NMSI I have been encouraged not to start with Mechanics, specifically to avoid introducing students to the physics course with some of the most complicated math and concepts (vectors and acceleration are traditional problem areas).

I have tried to start the year with Thermodynamics and Fluids before, and I think that went well.  Most of the students remember their chemistry, so PV=nRT and some of the other topics actually make Thermo a good place to start.  But there is a lot of hand-waving when talking about work, as in the work on or by a contained gas.  And I admit I struggled to explain fluid physics to students who hadn't yet learned about kinematics, forces, or energy.

This year I started the AP course with geometric optics, and I think it went swimmingly.  Reflection, refraction, lenses and mirrors are mathematically easy and conceptually clear.  The lab equipment is uncomplicated.  And to be honest, lenses and mirrors are fun.  I should write a followup at some point about the fun we have with those labs.

What did not go well was the obvious followup chapter in waves, interference, and diffraction.  Student learning was much weaker, the material less intuitive, the labs less clear and helpful.  Student confidence was damaged and I don't want to repeat that next year.  As this year's class moves on to kinematics, part of my thoughts will still be with how to better teach waves (in a very limited period of time) next year.

Tuesday, October 16, 2012

"Next Year I Will..."

So I set up this blog years ago to help me keep track of what I do well and (more importantly) what I do poorly as a science teacher.  I haven't made much use of it, because I just don't feel as though I have the time.  Clearly, time management is something that I do poorly.  I have got to find the time to reflect on how each lesson goes, or else I simply won't be able to improve my teaching in the way that I should be able to.

The one issue that is prompting me to finally write is lab notebooks.  I don't know how many science courses require lab notebooks at the high school or college level, and I am a bit agnostic on their value.  Lab assignments themselves are of paramount importance, but I am open to the argument that the composition notebook is not the best format for documenting the purpose, procedure, data, and conclusions of the students during the assignment.  If somebody would make a compelling argument to that end, I would stop using composition books as lab notebooks and I would move on to some other format (digital or otherwise).

Until then I will continue using the composition book.  This brings me to the title of this post.  Next year I will have a composition notebook from each student by the end of the first week of school.  That simply has to happen.  I will have them securely locked in the cabinet in my room and they will not leave the classroom--that part I already manage to do most of the time.  But in grading some of the notebooks I am finding all kinds of notes from the students in first couple of weeks of this school year, and many assignments are missing altogether.  Too many notebooks simply start at the second lab of the year.  Between my falling behind in grading the lab notebooks and my trusting that students would bring note books just because they needed them, I have left a gap where nobody was properly responsible, and the result is that assignments were not properly completed and documented.  That simply isn't an acceptable practice.