Topic awaiting preservation: Those things on the highway near weigh stations...

My brother-in-law and I were having a conversation about a month ago while driving down a highway. We passed one of those new things that resembles a radar gun and is posted about a mile up the road from a weigh station. These things somehow weigh the trucks.

How do they work?

The only thing we could come up with was that they use the momentum of some sort of particle to get a close estimate of the mass of the truck.

Anyone ever heard an explanation of these things? I have no idea what they are called so I'm not sure where I'd begin a google search...

Well, just by measuring speed, you can't tell what the mass of something is. In fact, size doesn't help you either.

Yeah, it must be the momentum thing. The mass of certain particles in the air *is* known, so if you know their velocity, then you can get their momentum (m*v), and by knowing something about whether the collision of that particle with the truck was perfectly elastic, and by knowing the truck's velocity, you can find its mass.

Not that I've seen these things...

Yeah, that's what I mean. These things would have to measure velocity too.

That's pretty scary if you consider the spin-offs that could result.

I'd still love to see a diagram of one of these things and how it works.

Ohhhhh, here we go. This makes more sense. The trucks have to have some kind of emitter doohickey installed in them...
http://www.prepass.com/f_what_is.html

I never even thought about that...

heh, that is pretty cool.
...and so is the internet, for having sites like that out there.

Don't you know that all truckers just let a computer drive their truck for them? Easiest job in the world.</simpsons>

On a more serious note... If you're interested in how radar or laser guns work, you could just check out a science site [or book..]. They're pretty simple, and they use echos/reflection and the Doppler effect to determine an object's position, distance, shape, and through two checks it can determine speed. Different types of radar use it in different ways with different levels of accuracy.. The radar technology of satelites needs to be more accurate than that of an airport's control tower.. or of a weigh station..

I've found a couple resources for information on these subjects, if you have no idea what I'm rambling about..

http://www.howstuffworks.com/radar.htm
http://www.howstuffworks.com/question396.htm

hope I've helped..