At the beginning of the year, students in their first-ever physics class are not ready to deal with the full, unadulterated complexities of the physical universe. So we cause significant confusion when we discuss complexities.
When they ask, "what about air resistance?", we can't go into a discussion of the transition from constant acceleration to constant velocity; we can't talk about resistive forces varying as v or v-squared; we can't talk about how even in a vacuum chamber, still some particles of air remain and exert a teeny-tiny resistive force.
Even though these are all fully correct statements with interesting physical consequences.
Pedagogically, we absolutely must say "air resistance is utterly ignorable within this classroom, and in any simple demonstration I can do. See here, I drop this 1-kg object and a crumpled-up piece of paper. They hit the ground at the same time!* And the kinematics equations make correct predictions in these live experiments I'm doing. So don't make the world more complicated than it is.
*At least as far as our eyes or a stopwatch can discern. It's a really, really bad idea to explain that "Well, actually, if we take high-speed video, the 1-kg object hits a very wee bit before the paper." Every time you're tempted in the first half of your first-year physics course to start a sentence with "actually" or "technically", put $5 in the Swear Jar.
Let me tell you an allegorical story.
In 1995, in the first few weeks of graduate school, I got to know some students who had grown up in Germany. I very much enjoyed them; they invited me to the official "international student" events*, they invited me later that school year to watch the Champions League final (1.5 decades before I had any clue what a "Champions League" even was).
*I wasn't out of place - my nametag listed my home country as "Kentucky", which was as foreign to most university folks as was Berlin or Cologne.
One day in October, a group of three German students came to me in a panic. A professor had posed a numerical simulation / programming challenge to their class as a major project: Consider a baseball game that's going into the top of the 9th inning. Given the current score and some basic statistics about each team's batters, write a program to estimate the probability of each team winning the game.
To me, this assignment seemed straightforward. When I was 10, I used dice, playing cards, and the "random number" generator on my TRS-80 to simulate way, way more complicated baseball situations than was posed here.
But these German graduate students had even simpler questions for me than I had thought about at age 10. "What's a 9th inning?" for example.
I invited these folks, and a few long-time baseball fans, to my apartment to watch some postseason baseball. And yes, the Germans appreciated my friendliness and hospitality, and I did help them a bit. But I realize that I could have helped them so much more if I had understood basic pedagogy.
I should have told them "The batting average gives the probability of reaching base. As your first pass at the problem, promulgate the probabilities: For each batter after the third who reaches base, count it as a run. But when three cumulative batters DON'T reach base, end the inning."
That approach is insufficient in a thousand ways. But it would have given a first-order approximation to their assignment. It could have given these German students a route to a finished project, and quickly ended their difficult foray into understanding baseball (though they would have learned some basic vocabulary about the game). It could also have piqued curiosity to encourage them to learn more - for example, if they use the slugging percentage in addition to the batting average, they'd have a probability not just of getting on base at all, but of how many bases a batter might take, improving their simulation.
Remember, though, that these folks at first did not have the context for me to differentiate between "batting average" and "slugging percentage." They were still stuck on what, exactly, it meant to "score a run."
What I *did* was, we all watched a game together while one of them looked at some sort of encyclopedia of baseball they had grabbed from the library. They asked me questions about the terms in the encyclopedia: "What's a passed ball?" "What does a 'dropped third strike' mean?" I answered in simple language, but without getting thorough comprehension. Responses to follow-up questions made it clear that I was trying to explain complexities of changing planes at O'Hare... to a rural Kansan transplanted in time from 1838.
I was of little help to my colleagues, though I was in fact an expert in baseball statistics. They would have been better off NOT coming to my apartment. I only confused them more than they were already confused.
I've internalized my failure from 1995 into my approach to the first weeks of a new physics topic, making the cleanest simplifying assumptions I can. Speed and velocity are synonyms. Orbits are circular. Voltage is defined as "what a battery provides." Keep physics simple while students wrap their heads around concepts. Then, you can add complexities, bit by bit over months or years.
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