How does that work

Why do satellites fall out of the sky?

Some people may wonder why satellites usually don't fall out of the sky (the moon is a satellite, ya know). The answer to that question is here. But, sometimes satellites do fall out of the sky. I wanted to know why that was. I had heard that there are forces in space that subtly reduce the velocity of satellites so that the Earth's gravity eventually drags them down. One force is friction, there is some matter in space and thus there is some friction. I had also heard that solar radiation slows down satellites, but I couldn't find any evidence for this. I suspect it is true because the solar radiation would simply be another source of friction.

February 21, 2008
A strange turn of events in the satellite-falling-out-of-sky story. After days of experts on news programs assuring the public that there is nothing to fear from a falling satellite because it will burn up in the atmosphere and furthermore has a slim to nil chance of hitting anything, the new word is "fear." Be afraid! The satellite is full of dangerous fuel, but don't be too afraid because your benevolent government is going to spend 60 million taxpayer dollars (according to the NY Times) to shoot down the satellite.

I hold with the cynics who believe this is the opportunistic Pentagon taking advantage of (and stoking) irrational public fear to perform a weapons test and show the Chinese government, "We saw you shoot a satellite out of the sky. We match and call." Ok, enough of that. On to how stuff works.

How the heck do they superimpose first-down lines on football fields?

Man, this one nearly kept me up at night. How in the world do they get that line to appear above the field, but under the players? I thought to myself, the only way they could do it is if the game was played on top of a green screen. Wrong. Actually the answer is quite sophisticated. The short answer is that computers get a lot of data from the cameras including orientation of the camera relative to the field. Then the computers consult a 3-d model of the field to decide where to paint the line. Then they use color data and to distinguish field pixels from player, referee, or ball pixels. Then the field pixels get painted over. So, in theory, if players had jerseys the color of the turf then the line could get painted over them!

How do traffic lights know I've pulled up?

Basically I should just post links to howstuffworks.com, but I'm going to describe stuff here anyway.

This question I guessed the answer to, sort of. Basically, the weight of the car pushes a "button" on the road. By button, all I mean is that part of the road literally sinks down a minute amount to connect a circuit and voila, the light knows my car is there. Have you ever seen a rectangular cut in the road? That was probably the button.

So if I've been waiting at a light a long time it may actually help to move the car a little forward or backwards to activate the sensor / press the button. That really should work, usually.

What lead me to doubt my button pushing theory was that some lights don't have the rectangular cut in the road. They have no button! That is right too. Some lights detect cars with laser sensors and some lights have no detectors at all. They run on timers so there is nothing that can be done to change them sooner. Just have patience.

How do motion sensors work?

I had a theory on this one too. Basically I figured they used radar. It turns out that some do, but that requires energy, and so I thought, are all these automatic lights actually saving energy if they are constantly using energy by beaming out radar waves? The answer to that question would be no, except that many motion sensors for lights aren't actually motion sensors so much as heat sensors. They detect the infrared light of body heat and they do so passively. One might say, they are solar panels that only give off electrons in the presence of infrared radiation, but don't be misled into thinking they are absorbing enough of your infrared radiation to generate electricity.

Does this mean that if a fire started in an empty building then all the lights would turn on? Not that it matters, I'm just wondering.

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