# doppler effect and sound waves

• May 20th 2008, 01:44 PM
checkmarks
doppler effect and sound waves
A sound wave of frequency 300Hz is emitted towards an appproaching car. The wave is reflected from the car and then received back at the emitter at a frequency of 315 Hz. If the speed of sound is 240 m/s, what is the velocity of the car?
• May 20th 2008, 01:54 PM
CaptainBlack
Quote:

Originally Posted by checkmarks
A sound wave of frequency 300Hz is emitted towards an appproaching car. The wave is reflected from the car and then received back at the emitter at a frequency of 315 Hz. If the speed of sound is 240 m/s, what is the velocity of the car?

If the range rate of the car is $v$, then the recieved frequency will be $f_0 (1-2v/c)$, where $c$ is the speed of sound and $f_0$ is the source frequency, the $2$ is because this is a two way doppler.

Now an approaching car has negative range rate, so $v=-v_{car}$ , where $v_{car}$ is the car speed.

RonL