# A stupidly simple mnemonic for acceleration graphs

When working with position/time graphs of accelerated motion, it is easy to confuse the dire
ction of velocity with the direction of acceleration. for example, look at the two graphs below:

Many students will instinctively say that the first graph is positive acceleration, while the second graph is negative. In fact, these are the same graph, as you can see here:

It is a single parabola, representing positive acceleration.

So how do I get kids to recognize positive and negative acceleration? Using the following, stupidly simple trick:

Positive face

Negative face

It’s really that simple. Any part part of the smile, be it a corner of the mouth smirk or a full on grin, still looks positive. Any part of the frown still looks negative. And that’s it.

# Is Physics a language class?

When I teach Physics, I like to focus heavily on the conceptual side, as well as the process of problem solving and how to think about problems in general, rather than just the mechanics of the math. After all, the math we do in Physics is typically a year or more behind what they are actually studying in Math class, so they should be pretty good at it. In other words, I like to get metacognitive about the subject.

Physics is not exactly about the real world. Physics is about studying mathematical representations (models) of reality, in hopes that those representations can be predictive. For the cognoscenti, real science hides in the places where the math does not predict what we see in the real world, but we rarely delve into those places in school. Instead, we focus on how the models do match and predict reality. In any event, we need to translate what we see around us into math in order to manipulate the model, and then translate it back from math to real world.

It is that translation piece that got me thinking about the title of this post. When we do physics, we are really “translating” from the language of the real world top the language of mathematics – a bit like translating a question from German to English, answering the question in English, and then translating it back – the grammar doesn’t always match, so we have to be judicious in how we translate. And when we study languages we learn to recognize the nuances of each, and the differences in how they work to express things. Likewise in Physics, we need to recognize how a mathematical representation is similar to, and how it differs from the real world.  There are of course other ways to represent reality – artistic, linguistic, and so on, as well as other analogies for translation (thinking gene expression here…), but I’m not sure they convey the same sense of how Physics operates.

Here’s the thing – though I have learned other languages, I have never taught other languages, so I don’t really have a sense of the metacognition of that process. I think it’s about time I explored that in order to develop a full toolkit to help my students understand more about the process of doing Physics.

# Trouble with Static Kits

My grade 9’s are doing static electricity at the moment, and I am experiencing some weirdness and frustration with the kits I have. Firstly I have a combination of new and old kits, but they all come with a standard collection of items – glass, ebonite and plexiglass rods, fur, felt and silk pads, and a few other odds and ends. My problem is I am finding it hard to reliably, and consistently, get a good charge, especially a positive charge.

The instructions usually suggest rubbing glass with silk to produce a positive charge on the glass. This, in my experience, never works. Last year I discovered that rubbing the glass rod with the plastic baggie the kit comes in worked well – but with the new kits I just received that doesn’t seem to work, no idea why not.

And then there is the mysterious flip-flopping plexiglass. Acrylic when rubbed with fur should become negative – but when I held it up to a negatively charged electroscope, the leaves dropped – indicating a positive charge. I tested it with a positively charged electroscope, and the leaves spread apart, confirming the positive charge. Okay, so it’s positive. But then when I tried to demo that it was positive, it suddenly had the opposite effect. It was bizarre, and my students were totally confused – as was I.

I love a teachable moment as much as the next guy, but when the message seems to be that science is arbitrary and unpredictable, I don’t think it is the right lesson to convey. I hate it when that happens.

# Magnets. How DO they work?

That’s a meme question that has been floating around for a while, but now I’m pleased to say that there is a pretty clear explanation. Here it is in two short video collaborations from Veritassium and Minute Physics:

# That moment when…

We live in a pretty safe world. Or at least, where I am it is pretty safe. Despite the excessive portrayal of violence in the news, the chances of injury from either accident or crime has been on the decline for some time. Parents may become (needlessly?) overprotective, and students can develop a sense of complacent protection. That is, if there is possible danger, that Someone is Taking Care of It.

So yesterday we were talking about the flotsam and jetsam of the solar system – comets, asteroids, meteors etc, and we were discussing the recent bolide over Siberia, the close approach of 2012DA14 on the same day, and the Tunguska event of 1908 – when one of the students asked “So what do they do when they see that one of these is going to hit Earth?”

“Who is ‘they‘?”

“You know, astronomers and NASA and stuff”

“Well, there isn’t really a ‘they’. To look for these requires funding, and with tight budgets, few government bodies want to fund projects that do not have foreseeable, immediate benefits. So there aren’t many people looking.”

“Ya, but, if they do find one, what do they do? Like, just…” I think he was waiting for me to finish his sentence. Which I didn’t. “Just… just…”

“Crash a nuke into it?” another student suggested helpfully.

At this point I had to make their world a little bit less safe, and I explained that, at present, there is nothing we can do. Only if a potential impact is discovered years in advance is there any chance of altering the course of even a small asteroid, and even that requires technology we don’t actually possess. Not that it isn’t theoretically possible, but it would require a very rapid design and construction, and it could only be launched within a narrow window of opportunity. It’s not the movies, and it’s not off-the-shelf parts. In other words, at present, there is nothing we could do.

The take away message of the discussion was there is still plenty of opportunity for Science to save the world, and a need for people to step up and make it happen. Though maybe not a worldview change for most students, it was a chip in the safe bubble they see of the world, and a real, practical. and important application of what we have been learning.

And that’s all I can ask for.

# Optics Song

One of my students started this off, and we just had to finish it. Sung to the tune of Frère Jacques:

Denser slower, Denser slower
That’s how light, That’s how light
Undergoes refraction, Undergoes refraction
That’s Snel’s law, That’s Snel’s law

(Note that while it is usually referred to as Snell’s Law, the person for whom it is named was Willebrord Snel, with one “l”. He later went by the latinized “Snellius”, but it is not called Snellius’ Law, so I will be pedantic and stick with Snel’s Law.)

# Eighty minutes well spent

Eric Mazur gives a terrific, evidence based explanation of what is wrong with lecturing as a primary source of knowledge transfer, and what to do about it. I really like his explanation, about 51 minutes in, that the better we understand the material, the harder it is for us to teach, because we become more removed from what it was like to learn it the first time.

# Squishy circuits

I have not tried it yet, but with my intro to electricity in grade 9 this coming year I am SO doing this:

Funny thing is, she says “high school physics teachers have been doing this for years” – where the hell have I been?

Here’s the website with recipes for making the different doughs:

http://courseweb.stthomas.edu/apthomas/SquishyCircuits/index.htm

# A Physics Fairy Tale

Once upon a time, in a magical Physics kingdom, there lived an evil teacher who gave resistor network problems like this:

The Evil Physics Teacher made his students solve all kinds of complex resistor networks.  It was time consuming, but the Evil Teacher thought it was worth it because it proved their ability to manipulate Ohm’s law, Kirchoff’s laws, and the resistor formulas.

And the one day, a student asked a simple question:

“What does this circuit do?”

The Evil Physics Teacher paused, and then somewhat sheepishly admitted that the resistor network didn’t actually do anything.

“Can you show us a resistor network that does do something? Like one that an electrical engineer would actually design for something?”

Again, the teacher paused. Um, no. He explained. Everything has resistance, so it is important to be able to figure out total loads, but no one actually designs resistor networks, per se.

“But like, in a house everything is in parallel, right? So current loads would just be the sum of individual currents. Nothing is ever in series in a house.”

The teacher thought about that, but did not reply. Instead, at the end of the day he went home to ponder. Perhaps, just perhaps, there was something to this thought. Resistor networks did allow students to exercise their understanding of electricity, but in an overly complex, unrealistic and – let’s face it – borderline sadistic way. And so the Evil Physics Teacher mended his ways. He began providing simpler questions that still exercised the same skill sets without consuming a full period for a single problem.

As a result, learning the material was quicker and more readily attainable, and there was more time to move onto richer and more exciting things in the curriculum.

And they all lived happily ever after.

(PS – some parts are truer than others…)

# Great free tools for sound recording and analysis

With my grade 11 Physics class we are currently studying sound, and we have been using a variety of tools. Here are some of the great free tools that we have found useful:

Free Audio Editor

The title of this software pretty much says it all. It is a compact, easy to use and fairly comprehensive piece of software for recording and editing audio. Perhaps not as well known in educational circles as Audacity, but I find it slightly more intuitive to use. By capturing the full audio signal, the user can see the entire envelope of sound, or zoom in to see the actual waveform. I also use it to record voiceovers for videos or presentations. Could be used for podcasts too. The most recent version I downloaded installed a browser toolbar, but this is easily disabled if unwanted.

Free Audio Editor

Visual Analyzer

Developed by Alfredo Accattatis for academic purposes, and just for the love of it, VA is an oscilloscope and spectrum analyzer that uses nothing more than the sound card on your computer. It shows a live waveform on the oscilloscope, and a live frequency spectrum on the analyzer. It even has a “3d” function so you can see how the frequency changes with time. It can record short snapshots of both the waveform and spectrum for detailed analysis.  A very powerful tool.

Visual Analyzer - oscilloscope on top, 3d frequency spectrum below

Here’s a video showing how VA is used:

Raven Lite

Raven is produced by the Cornell Lab of Ornithology Bioacoustics Research Program. It is designed to record, play back, visualize and analyze sounds – whether musical notes, complex bird calls or whale songs. The Lite version is free for hobbyists and educators, though it must be “purchased” through the online store to receive an activation license. This software combines many of the features of both Free Audio Editor and Visual Analyzer, in that it records and displays the amplitude trace (waveform/oscilloscope) as well as a frequency spectrogram. This spectrogram is actually a 3-d graph, showing frequency over time, with intensity or colour representing the amplitude. This makes it more complicated for novices to interpret, but shows changes in frequency in a very visual way. It also aligns the frequency response to the waveform, so the two can be compared together.

Waveform (top) and spectrogram (bottom) of my voice.

Analysis of a Cardinal song (well, my impression of one...)

Since each of these fills a slightly different niche, it is not an either/or thing – I use each of these differently, and I do use all of them. And since they are free, the price is definitely right!