6.5 The Doppler Effect

Critical Questions:

  • Why does the sound of an ambulance or car engine seem to change as the vehicle passes by?
  • Why do we hear a loud bang when a supersonic jet flies by?
  • What does all this have to do with the Big Bang?

I’d like you to do me a favour. This is especially important if you’re in a public place, surrounded by lots of strangers.

I’d like you to imitate, out loud and at a significant volume, the sound of a racecar going by.

You can imagine Ricky Bobby driving if it helps.
You can imagine Ricky Bobby driving if it helps.

The noise you made probably sounded something like, “weeeeeeeeEEEEEOOOOOOOooooo”. It’s a fairly common sound effect, one most people can perform even if they’ve never attended a car race. But why does it sound like that? Why does the car’s engine go from a higher pitch to a lower one as it passes us by?

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6.4 Resonance

Critical Questions:

  • What is resonance?
  • Can you shatter a wine glass just by singing at it?

In the same way that it must be illegal to talk about buoyancy without telling the story of Archimedes and the crown, everyone who teaches resonance brings up the Tacoma Narrows Bridge at one point or another.

This is probably because there is some cool old black-and-white footage of the bridge swaying and rippling impossibly in a stiff breeze, as follows:

It’s clear from the video that this was a windy day, but not tornado-windy. So what made the bridge move like that?

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6.3 Sound

Critical Questions:

  • What is a sound wave?
  • How do we hear sounds?
  • How do multiple sound sources mix together to form one sound signal?
  • Why does a flute sound different than a piano?

The well-known Zen koan asks: if a tree falls in the forest, does it make a sound? If you’re ok with the idea of not reaching enlightenment just now, physics can give a rather bland answer to the question.

Thinkin' real hard about physics right now
Thinkin’ real hard about physics right now [source]
Sound is nothing more than vibrating air. When you speak, for example, your vocal cords are moving rapidly back and forth, striking air molecules with every forward motion. These first few molecules rush forwards until they run into other molecules, at which point they collide and move back, repeating the cycle. This vibration spreads and moves through the air as a wave, causing molecules in all directions to begin vibrating. At some point, the air near someone else’s ear might vibrate as well, and the wave will travel the short distance down the ear canal until it reaches the eardrum (or ‘tympanic membrane’, which sounds much more official).

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6.2 Waves

Critical Questions:

  • What is a wave?
  • How does a wave travel?

There’s a story I heard somewhere – I have no idea whether it’s true or not, but I like it anyway. The story is that a group of people somewhere in the world developed a language that had an interesting way of referring to water waves. Instead of pointing at the ocean and saying, “Hey look, there’s a wave,” they would point at the ocean and say, “There is waving.”

If this is a true story, two things are possible: the first is that these people disliked the expression “Hey look,” and the other is that the speakers of this language understood wave behaviour much better than most.

if you ask me, he's kind of showboating [source]
if you ask me, he’s kind of showboating [source]

The reason “waving” is more appropriate than “a wave” is that waves are best understood not as objects but as actions. A water wave, for example, isn’t a thing in many senses of the word – it is definitely not solid, it may have a beginning but often lacks a clearly definable end, and it can’t be picked up and separated from the water it’s made of.

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6.1 Introduction to Waves and Sound

There is a secret about waves. It is the kind of secret that, if you fully understand it and its implications, may well blow your mind and leave you scribbling paranoid manifestos on attic walls.

Or worse, you might only think you understand this secret, and go off and make some horrible and misleading movie like What The Bleep Do We Know1 based on your flawed understanding.

What the bleep do we know
Augh, I wish I could un-see this movie

It is the kind of secret that has a lot in common with some of the best conspiracy theories: it’s far-fetched and far-reaching, and for many years it attracted only a small handful of dedicated believers trying in vain to convince everyone else that it was true.

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  1. Just don’t even bother googling it, seriously.

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To Stir, Or Not To Stir

In my physics undergrad, we had one prof who seemed a bit strange. Actually, being only a bit strange seems something of an accomplishment for a physics professor, as most of them ran the gamut from odd to unintelligibly bizarre. But the one I’m talking about here was a younger guy who taught us about thermodynamics.

He was odd in a number of ways, but one sticks out in my memory: one day, in class, he said he couldn’t understand why anyone would ever stir a hot cup of tea, because stirring adds energy, which would only make the liquid hotter. He also said that people switched from wooden spoons to metal ones for the sole reason that they conducted heat out of hot liquids, allowing them to cool down more quickly.

That is patently absurd. We use metal spoons because they make a pleasant 'tinking' sound when you stir.

Both of these comments were strange because they seemed to betray a pretty deep misunderstanding of the physical world – something that would cause problems for, say, a physics professor.

Luckily for us, Randall Munroe of xkcd fame has tackled the issue of stirring hot drinks in his latest instalment of What If: Stirring Tea. The question asked:

‘I was absentmindedly stirring a cup of hot tea, when I got to thinking, “aren’t I actually adding kinetic energy into this cup?” I know that stirring does help to cool down the tea, but what if I were to stir it faster? Would I be able to boil a cup of water by stirring?’

Read the whole thing here!

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Wired: The Experiments Most Likely to Shake Up the Future of Physics

LUX Detector
LUX: A dark matter detector and art installation

If you’ve been following Pop Physics (especially if you’ve stuck through the overlong and unannounced hiatus that started with the new school year) and are still in school, there’s an excellent chance that you’ll end up working for one of these projects someday!
The Experiments Most Likely to Shake Up the Future of Physics

Here’s one of my favourites:

“NOvA will attempt to figure out this mass hierarchy by shooting a beam of neutrinos from Fermilab near Chicago 810 kilometers away to a detector in Ash River, Minnesota. A similar experiment in Japan called T2K is also sending neutrinos across 295 kilometers. As they pass through the Earth, neutrinos oscillate between their three different types. By comparing how the neutrinos look when they are first shot out versus how they appear at the distant detector, NOvA and T2K will be able to determine their properties with high precision.”

Science!

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Free Energy! Only $7610

Earlier this year, I posted about why perpetual motion and “Free Energy” machines are impossible. Soon after, I found someone trying to sell their own perpetual motion device on YouTube.

Today, I’m going to do one better: an indiegogo campaign called “Home Quantum Energy Generator” has raised $16,902 (far surpassing its oddly specific goal of $7,610) to build a protoype Free Energy device. This thing features several of the hallmarks of modern physics quackery:

  1. An inventor who is specifically not a trained physicist, but a “career Electronics Engineer”
  2. Layperson explanations that defy logic
  3. The word “Quantum” thrown in for good measure
free energy device video still
Also: magnets, of course.

Here are a few of my favourite quotes: (more…)

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