Wednesday, August 21, 2013

Why are you spending so much time on kinematics? (The NGSS Edition)

There is a long-standing tradition, in high school physics, of spending the bulk of the first quarter on kinematics. Definitions are slowly pieced together, ticker-tapes are dotted with carbon, motion sensors click away, graphs are drawn, and algebraic expressions are rearranged.

As teachers, we like to assure ourselves of the foundational importance of a deep understanding of position, displacement, speed, velocity, acceleration, and time.

And when one-dimensional kinematics has given its all, we reward ourselves and our students with two-dimensional kinematics. Projectiles are launched, simultaneous equations are wrestled with. And the hunter shoots the monkey.

All this is possible without a hint of Newton's laws of motion. One fourth of the school year sneaks past us while we frolic in 16th-century applied mathematics.

At the end of the year, we lament all the topics that we, again, failed to get to. Darn you, state testing! And snow days! Rainbows? Diffraction? Why the sky is blue? Electricity? Magnetism? Optics? Maybe next year.

At least my kids can solve x = v0t + 1/2 at^2 for t, even when v0 ≠ 0. So... Victory!

Have you seen what the Next Generation Science Standards (NGSS) expect of us in terms of kinematics problem-solving?

Approximately nothing. The first thing NGSS wants us to worry about is Newton's Second Law.

So I pose the query of the post title. And I do so not pretending to be someone who will never go off-script in terms of NGSS. NGSS content is significantly narrower than California 9-12 Physics was.

Where we choose to go "off-roading" in physics content is a value judgment. And I will cover the basics of linear motion. The basics. Not the Complete Robust University Mastery Curriculum. I'll be in and out in two weeks tops. Not six.

And I'll cover other, groovier topics not mentioned in NGSS. Some people naysay forays beyond the realm of the FCI as curriculum that's a mile wide and an inch deep. I disagree.

If NGSS needs essentially nothing in terms of kinematics, how can you justify forfeiting up to 25% of your academic year to it? I honestly don't think you can. But I've been wrong before.

Bear in mind my inquiry regards plain old high school physics. Not AP or IB. But please don't be put off by my polemical tone. That's just how I am. If you think I don't know which way is up, feel free to slap me silly with the power of your arguments. I can take it!


Andy Rundquist said...

This hit me right where I've been thinking for a while. I posted recently about how no "physicist" does kinematics (meaning check the journals, etc) but that it's just one of our many very successful models. I love what you're thinking about doing here, and I'm getting even more excited about the NGSS.

Paul Martenis said...

My question is, how does one really learn Newton's second law without kinematics? If students lack an understanding of acceleration, then F = m a is merely senseless plug and chug.

cskesler said...

Even as I post this, I am working to reduce the kinematics footprint in my academic year.

I tend to use kinematics as the playground in which students learn that equations are more than just formulas for computing answers. You can't find anything more concrete that does so much to connect seemingly lifeless algebra to things going on right in front of you.

And, ideally, they learn to see equations as variable relationships. If they carry it forward to future lessons, it means I can communicate tons of information just by showing them an equation that relates other variables, such as potential curves for universal gravitation.

At the best moments, we barely have to start into learning something before students are deriving relationships and inferring the next steps. As I said, I am applying your advice right now. But don't give short shrift to the benefits of bringing equations to life.

I am also biased by my experience as an AP teacher. It always seemed that time spent in foundational kinematics paid off for students' scores, whereas other areas didn't translate as well. So working on a broad analytical view of the amazing world around us sometimes took a backseat to perfecting projectile motion.

Dean Baird said...


How deep an understanding of acceleration do you think students need to gain an introductory-level understanding of Newton's Second Law?

"Change in velocity." "Speeding up, slowing down, or changing direction."


Coaxing "a" out of v^2 = v0^2 + 2ax? Appreciating that a concave-down plot on an x vs. t graph indicates negative acceleration while not necessarily revealing whether a body is speeding up or slowing down?

Too much!

Paul Martenis said...

Dean, I agree with you almost completely. I'm not one of those folks spending six weeks on quadratics. Like you, I find the algebra to be a great time suck (and not that great at teaching acceleration). Like you, I emphasize recognizing the concept of acceleration.

The disagreement comes when you say "Done." I 'm sure you realize the concept is not that simple.

I find it's one thing to get the kids to say back those three kinds of acceleration. It's another, more involved thing to get students to actually look at a motion or a representation of it and then identify whether it's accelerating, and be able to say why.

I don't want people underestimating how much effort (and time) it takes to really get that concept.

My bottom line: Don't spend *so much* time trying to do all of kinematics. Instead, spend quality time on the kinematics that is important to your purpose.

Dean Baird said...

Given that the human mind wrestled with the concept of acceleration for more than 2000 years before mastering it, teaching the concept in introductory physics presents an open-ended blank check if you let it.

When I say, "Done", I mean I'm going to demonstrate the concepts, discuss the definitions, engage the students in some lab experiences, and move on.

Will they construct a master's understanding of acceleration? No. Will they understand motion enough to proceed into new territory where the concepts will be reinforced and built upon? Absolutely.

I don't think it's reasonable to expect intro learners to go from zero to master in our first-year classes.

Acceleration will occupy as much of the curriculum as you allow it to and still many students will not deeply get it.

Accept that the deep understanding does not come about upon initial introduction and move on. Flawed or incomplete conceptions do not prevent what we called "advanced" performance on state-mandated testing.

As instructors, we well up with pride if a student can correctly interpret the meaning of the slope on a transposed-axes motion graph.

But I think it's nice if they can explain why the sky is blue to their grandmother, too. When I have to pick, I'll take the latter over the former every time.

Dean Baird said...

My experience as an AP Physics teacher has informed me that too much time in kinematics results in students keen to use the equations of Uniform Accelerated Motion to solve problems in Simple Harmonic Motion.

Their mastery of UAM makes it they're go-to solution when they're otherwise unsure how to solve a problem.

It would be better if they fell back on energy conservation when lost.

Marc Braden said...

I've fought with this each of the 9 years I've taught, and I've tried dealing with it differently each time. Last year it was teaching electric circuits first to build cars whose motion we analyzed. This year I started with programming on day 1, having students model first data that they are familiar with ( loops that output a bank balance with interest compounding, number of particles with radioactive decay, etc.) to get the expected exponential relationships, then using VPython they'll make things move in different ways and we'll model that data. Finally we'll compare that to data for real motion using Tracker, CBRs etc. It might not save time, but it will keep us from getting mired in kinematic equations, and will give students some extra skills. And it's fun.