Sunday, August 24, 2014


Being a physicist is apparently one of those things that is always on. I read in various personal material -- blogs, autobiographies, anecdotes, etc. -- that physicists are constantly aware of the physics going on around them.

This is not a unique attitude. I light the stage, and do sound design for it. I've tried to draw comic book art, and I build and render scenes in 3d. These, and more, are all referenced to observation of the real world. And that means any time I'm out walking, I'm also out analyzing; what made that sound? Why is that shadow that color? What are the depth planes of this scene? 

This is almost back-of-the-mind noticing, mind you. You don't go around with a notebook out, stumbling into things as you try to record your observations. You just, plain, notice. 

And since I've rekindled a (strictly amateur) interest in science, I find myself applying that tool as well.

A couple days ago I wandered into the kitchen late at night and filled a water glass without turning on the lights. It was easy to do, but why? A little experiment the next day showed that the key was the material and shape of the glass. Here's how I reconstruct; the turbulence of the water sets up an oscillation in the tuned resonator that is the sides of the glass. As the glass is filled, the length of the resonator becomes shorter. Since each halving of length is perceived as a doubling of the octave, the perceived pitch center rises exponentially.

Presumably, the pitch appears to be rising swiftly, accelerating, towards a point where it can't be perceived as a tone any more. And somewhere a little short of that, you turn off the water.

The free-air wavelength of a 10 KHz tone is over an inch, though. So what you are hearing is clearly not the air column, but the vibration of the walls of the glass. Which are stiff, and just as making a guitar string more tight increases the pitch, so does making a resonating material stiffer. I tested this with plastic, glass, metal, and ceramic vessels of similar dimensions, and on first blush it does look like that describes the phenomena accurately enough.

On Friday my metal arrived for the next crop of grenades. I bought a stick of the tougher 7075 alloy to experiment with. But what if the label comes off? How can I distinguish it from the 2011?

Isn't it obvious? I held one of each by one end and tapped them lightly with a piece of metal. The harder alloy had a higher pitch.

One of the things that so fascinates me about physics in the "real world" is both how close to the surface it is, and how deeply complex you quickly get out of what are relatively simple rules. Because real materials are complex; they are compounds, they have grain, they have non-simple cross-sections, they connect.

So something as simple as observing which surface exposed to sunlight gets hot, is both a simple and obvious exercise in absorption versus radiation, but also can be unpacked into greater and greater detail as you consider interaction between layers, conduction, whether convection cells are forming in still air directly above the surface, etc., etc.

When you get down to it, much of the behavior of a simple reaved block has to do with levers and friction forces. Friction is what makes short fibers into a rope, and friction is what holds a nut on a bolt. Of course, if you unpack those friction forces, you find yourself in a maze of mechanical interlocking and compression forces and Van der Waals and so forth...

(But at the same time, there are very basic physical laws -- rather, mechanics -- going on that can confuse the untrained. Difficult enough for some to understand that a block trades off length for force; if you are moving twice the amount of rope, you are lifting half the amount of weight (plus overall friction, which goes up with the number of elements in the system). Tough for a lot of people to realize that if you lift a weight, the load on the pulley is twice the weight. You have to add the forces; one force for gravity trying to pull the weight to the ground, a second equal force for the rope keeping that from happening. If you don't understand the basic underlying principles, you can put yourself at risk.)

This is why there are so many jokes about physicists complaining that physics describes the entire universe, and specialities like, say, marine biological acoustics is just detail. Thing of it is, these details are so very, very tough to work out from first principles, you really do need the entire framework of the other sciences, with their laws and rules of thumb and categories (however well or poorly those line up back to the underlying physics).

And that's also why you can get close with an approximation -- like the water glass model above -- but recognize that this simplified model will not track that closely to the real behavior. In fact, it can go entirely wrong...

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