The frequency of the radar signal does not change. Be it a cop getting you speed, or a modern car with Brake assist (they use a radar signal to assist keeping a safe distance) it's all timing, how long from the radar source to return to source. If a police car is still, and you are moving away, the increase in return of signal time tells the computer in the officer's car your speed. Even if you are going toward each other, the computer knows the speed of the officer's car and again, signal return time factored in with speed of cop car the computer can give an instant, real-time speed.
It's frequency reduces as the car gets further away.
no...frequency is the property of source....
FM or frequency modulation is a means to vary the frequency withing a set basic frequency and transmit it. FM radio demodulate the frequency extracting the signal that made the frequency shift at the source.
the frequency of a sound can change depending on how fast the source is vibrating
You can't. Once the wave leaves its source, its frequency doesn't change.
To make sound frequency change alter the pitch.
the frequency of a sound can change depending on how fast the source is vibrating
For a low frequency source, the voltage across the inductor tends to zero because its impedance is proportionnal to source frequency, whereas the voltage across the resistor tends to the voltage source value.
No change in frequency or wavelength.
If there is no relative velocity between source and listener, there will be no change in frequency.
Who? Don't you mean what? Your question makes little sense. If the frequency is constant as you say, then it cannot be changed. However, if talking about radio equipment constant frequencies can be changed through a process known as Mixing. That is a second source of constant frequency is mixed in with the other frequency and you will get the sum and the remainder output. In other words, if the frequency of the 1st source is say 5 mhz and the frequency of the other source is 2 mhz, then on the output you would get 7 mhz and 3 mhz. This can be done with other frequencies also. If you are talking about 60hz (USA AC frequency), that can be changed using a VFD or Variable Frequency Drive.
Yes. The observer frequency fo= f( v + vo)/(v -vs) where f is the signal frequency, v is the speed of wave and vo is the speed of the observer towards the signal and vs is the speed of the signal toward the observer.