[portej05]

Seeing as I've just done this, I thought I'd share.

I saw a sticker in the US with "If this sticker is blue, you're driving too fast" (the sticker was red)
Someone just asked me how fast you'd have to be going for that to occur.

As initial values, assume red is approx 700nm and blue approx 480nm ( 4.29 x 10^14Hz and 6.25 x 10^14Hz repspectively) and c=3 x 10^8
Using the doppler formula f=(v+vr/v+vs)f0, stick in the values, and you get an approx answer of 100 x 10^6 m/s.
Approx 1/3 the speed of light.

That'll get you a speeding ticket for sure 

[Snail]

What car does yo mama have then?

[The E]

Pinto?

[portej05]

I see the word filter has been turned on

[Rian]

At that speed, you probably want to use the relativistic doppler formula for this.

f = f'(1+v/c)/sqrt(1-(v/c)^2)

I think the answer you get is a little larger, depending on how many significant figures you reported in your original answer. I'm getting about 1.08x10^8 m/s.

[The E]

AKA ~100.000.000 m/s AKA 100.000 km/s AKA approximately 1/3 of c.

[Rian]

Yes. As I said, similar answer, but more accurate method.

[High Max]

.

[Scotty]

I'm just gonna stick to using "ye" and "yer" for today.

[portej05]

At that speed, you probably want to use the relativistic doppler formula for this.

f = f'(1+v/c)/sqrt(1-(v/c)^2)

I think the answer you get is a little larger, depending on how many significant figures you reported in your original answer. I'm getting about 1.08x10^8 m/s.

Cheers for the pointer - I wasn't sure whether or not I had to do this at the start since I was assuming v << c, and then it came out a little faster than I expected

[Rian]

Yeah, I thought it sounded like a pretty drastic shift, and the difference with relativity was actually less than I was expecting it to be. I figured I might as well post it anyway, since I'd done the calculation.

The relativistic formula is nice because the direction of movement is built in, (there's a cosine in there that I ignored for forward movement) whereas I seem to recall the classical formula is different depending on whether one is coming or going. Also, I've always thought the transverse doppler effect (from time dilation) is cool – there's no classical analog for that.

[Galemp]

Wow. Now THAT is a physics joke.

[Enigmatic Entity]

Seeing as I've just done this, I thought I'd share.

I saw a sticker in the US with "If this sticker is blue, you're driving too fast" (the sticker was red)
Someone just asked me how fast you'd have to be going for that to occur.

As initial values, assume red is approx 700nm and blue approx 480nm ( 4.29 x 10^14Hz and 6.25 x 10^14Hz repspectively) and c=3 x 10^8
Using the doppler formula f=(v+vr/v+vs)f0, stick in the values, and you get an approx answer of 100 x 10^6 m/s.
Approx 1/3 the speed of light.

That'll get you a speeding ticket for sure 

Quite unlikely - at that speed, the police/cameras wouldn't even notice you going past.

[BloodEagle]

Seeing as I've just done this, I thought I'd share.

I saw a sticker in the US with "If this sticker is blue, you're driving too fast" (the sticker was red)
Someone just asked me how fast you'd have to be going for that to occur.

As initial values, assume red is approx 700nm and blue approx 480nm ( 4.29 x 10^14Hz and 6.25 x 10^14Hz repspectively) and c=3 x 10^8
Using the doppler formula f=(v+vr/v+vs)f0, stick in the values, and you get an approx answer of 100 x 10^6 m/s.
Approx 1/3 the speed of light.

That'll get you a speeding ticket for sure 

Quite unlikely - at that speed, the police/cameras wouldn't even notice you going past.

Which is why the cameras will be set up to be moving at that speed 24/7. 

[Scotty]

Sure, it'll catch them.... When they come around again fast enough again, and again, and again.  Think of a moving helicopter's rotor.  You can still see it.