How does a radar gun determine velocity?

[chiddler]

Homework Statement

Radar guns emit radio waves. The radar gun measures the beat frequency between the returning waves. How does the radar gun determine velocity?

A. By measuring beat frequency between two consecutive reflected waves.
B. By measuring the beat frequency between a wave reflected from a stationary object and one reflected from a moving object.
C. By measuring the beat frequency between a wave reflected from two separate objects moving at the same speed.
D. By measuring the beat frequency between a wave reflected from two separate objects moving at different speeds.

Answer is B.

Homework Equations

Fbeat = |F2 - F1|
the doppler equation: Fs / v+or-vs = Fl / v+or-vl

The Attempt at a Solution

My interpretation. Correct me if I'm wrong, please:
A wave is emitted, reflects off object, and goes back to the radar gun. The frequency of the wave changes due to doppler effect. As new waves are emitted, a beat is produced which can measure the difference between the shifted wave and the new wave.

But beyond this, I'm not sure. Been staring at this question for an hour and could use some help. Thanks very much.

 

[berkeman]

Homework Statement

Radar guns emit radio waves. The radar gun measures the beat frequency between the returning waves. How does the radar gun determine velocity?

A. By measuring beat frequency between two consecutive reflected waves.
B. By measuring the beat frequency between a wave reflected from a stationary object and one reflected from a moving object.
C. By measuring the beat frequency between a wave reflected from two separate objects moving at the same speed.
D. By measuring the beat frequency between a wave reflected from two separate objects moving at different speeds.

Answer is B.

Homework Equations

Fbeat = |F2 - F1|
the doppler equation: Fs / v+or-vs = Fl / v+or-vl

The Attempt at a Solution

My interpretation. Correct me if I'm wrong, please:
A wave is emitted, reflects off object, and goes back to the radar gun. The frequency of the wave changes due to doppler effect. As new waves are emitted, a beat is produced which can measure the difference between the shifted wave and the new wave.

But beyond this, I'm not sure. Been staring at this question for an hour and could use some help. Thanks very much.

You have it pretty much correct. The wording of answer B is a bit weird, since the beat frequency is between the reflected wave (which is Doppler shifted) and the original source (not a reflected wave off of a stationary object). But the wording of your answer seems fine.

 

[chiddler]

Thanks for the response.

I'm having difficulty translating B into my answer, though. The emitter is the stationary object. It doesn't really reflect a wave it only emits and receives it, doesn't it?

 

[berkeman]

Thanks for the response.

I'm having difficulty translating B into my answer, though. The emitter is the stationary object. It doesn't really reflect a wave it only emits and receives it, doesn't it?

The difficulty comes because of the wording of answer B. As I said, and you are also saying, there is no reflected wave off of a stationary object involved. You can point to this wikipedia article on Radar Guns, for example:

http://en.wikipedia.org/wiki/Radar_gun

The target object's speed v is proportional to the difference in frequency Δf between the outgoing and the reflected radio waves:

...

After the returning waves are received, a signal with a frequency equal to this difference is created by mixing the received radio signal with a little of the transmitted signal. Just as when two different musical notes are played together they create a "beat note" at the difference in frequency between them, when the two radio signals are mixed they create a "beat" signal (called a heterodyne) at the difference in frequency between the outgoing and reflected waves.

 

[chiddler]

Alright I'm satisfied with my answer then.

Thanks again for your help. Is very appreciated.

edit: Would you mind if I ask another sound related question, please Or ought I start a new thread.