Friday, April 16, 2010

Air friction myths

1. Drag

Ever since I was a young kid, I remember people telling me how air friction slows objects moving through the atmosphere. The problem with this claim is that it's wrong. Well, technically it's partially right: there is a friction force at play and it does have a slowing effect. The thing is that the effect of air friction is so negligible compared to what is really slowing moving objects down that it can safely be ignored in just about all cases. The claim that air friction is what slows moving objects down is akin to saying that the recoil of a rifle is due to the backward force of your finger upon the trigger.

So what is this force that really slows flying objects? It is called aerodynamic drag. When I correct people about this, I often hear the excuse that when they say air friction, that's just shorthand for aerodynamic drag. I don't buy it! Air friction exists and is quite distinct from drag. So what exactly is aerodynamic drag? I shall attempt to explain it here.

A moving object displaces air as it moves through its gaseous medium. In other words, it pushes air out of the spot where it's going and leaves emptiness in the spot where it has been. Air immediately rushes in to fill the void left behind the moving object in much the same way that air or liquid rushes to fill the vacuum within a syringe when you pull back on the plunger. Of course it takes time for this void to fill in and reach equilibrium with the surrounding air. This leaves a region—called the slipstream—of relatively low pressure behind the moving object. The farther from the moving object within the slipstream, the more air which has been replenished and the closer to equilibrium with the atmosphere that it is. Naturally, the faster an object is moving, the larger the size and lower the pressure of its slipstream.

The pressure difference between the air in front of a moving object and the air behind it creates a force upon the object that is against the direction of motion. The faster the object is moving, the stronger this force is (in fact, the force is proportional to the square of the velocity of the object). This repulsive force is what is known as aerodynamic drag.

Racers know about aerodynamic drag and are able to use it to their advantage. Drafting is the technique where a racer will move into the slipstream of another racer in order to lessen the pressure difference between fore and aft, therefore lowering the drag force resulting in energy savings for the racer.

So the next time somebody tries to tell you that flying objects are slowed down due to air friction, be sure to correct them and tell them all about aerodynamic drag.

2. Shock

A very bright meteor was recently captured on video out in the mid-west. Greg Laden pointed out how the CNN report of the event said that air friction caused the meteor to heat up. This reminded me of the old Tom Glazer/Dottie Evans song What is a Shooting Star?

A shooting star is not a star, is not a star at all.
A shooting star's a meteor that's heading for a fall.

A shooting star is not a star; why does it shine so bright?
The friction as it falls through air produces heat and light.

You might recognize this song from the cover version by They Might Be Giants on their recent album Here Comes Science. Unfortunately, the last time that TMBG covered a Tom Glazer/Dottie Evans song, they picked one with a major scientific inaccuracy.

The sun is a mass of incandescent gas,
A gigantic nuclear furnace.

They were forced to write a new song (titled Why Does The Sun Really Shine?) to correct the falsehood.

The sun is a miasma of incandescent plasma,
The sun's not simply made out of gas. No, no, no!

So will TMBG need to write a new retraction song about why a meteorite really gets hot and bright? Alas, I'm afraid that they will. Once again, air friction exists and does indeed produce heat, but is so negligible that it can be ignored. In fact, at subsonic speeds, the dominant heat exchange effect on (warmer than air) objects is wind chill. Warmer than air objects radiate heat into the surrounding air creating an insulating "blanket" around them slowing heat loss. But if the object and the surrounding air are moving relative to each other, then this layer of warm air is stripped away causing the object to radiate heat faster, ergo the familiar chilling effect. However, at supersonic speeds, a new effect comes into play.


As an object moves through the air, it displaces the air in its path by pushing it forward and aside. The air moves away from the object as a series of compression waves (similar to the waves that a boat creates as it moves through the water). Now if you remember back to science class, you'll know that compression waves have another name: sound. So what happens when the object is moving faster than the speed of sound? Aha!

What happens is that the air gets pushed forward and aside faster than it can naturally escape. As long as the object is moving slower than sound, the compression waves outrace it and take most of their energy with them. But at supersonic speeds, the air keeps getting compressed and compressed and compressed as the object pushes it forward faster than the air can get away. So similarly to the freon in a refrigerator's compressor or the fuel mixture in the cylinder of a diesel engine, the leading air gets superheated by the crushing force of the supersonic object.

Basically, the supersonic object creates its own oven by compressing the air in front of it. Furthermore, if the object is made of an oxidizable material (such as iron, aluminum, or carbon) and the air is oxygen rich (such as the earth's atmosphere), then the object may quite literally burn up.

So the next time somebody tries to tell you that air friction causes a meteorite to burn up in the atmosphere, be sure to correct them and tell them all about supersonic shock.

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