Police use radar guns and the Doppler effect to catch speeders. The figure illus
ID: 1348193 • Letter: P
Question
Police use radar guns and the Doppler effect to catch speeders. The figure illustrates a moving car approaching a stationary police car. A radar gun emits an electromagnetic wave that reflects from the oncoming car. The reflected wave returns to the police car with a frequency (measured by on-board equipment) that is different from the emitted frequency. One such radar gun emits a wave whose frequency is emits 9.7 109 Hz.
When the speed of the car is 35 m/s and the approach is essentially head-on, what is the difference between the frequency of the wave returning to the police car and that emitted by the radar gun?
Explanation / Answer
Since police car is emitting radar of frequency of 9.7*109 Hz
And police car is at rest therefore this is case of source at rest and observer moving toward source
Therefore apparenet frequency = ((v+Vo) /v)*9.7*109
where v is velocity ofsound and Vo is the observer velocity
since velocity of sound is not mentioned therefore taking standard value v = 343 m/s
f = ((343+35)/343)*9.7*109 = 10.689*109 Hz