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.