A Policeman in a Stationary Car Measures the Speed of Approaching

Multiple Choice

A policeman in a stationary car measures the speed of approaching cars by means of an ultrasonic device that emits a sound with a frequency of 39.6 kHz. A car is approaching him at a speed of 35.0
M/s) The wave is reflected by the car and interferes with the emitted sound producing beats. What
Is the frequency of the beats when the speed of sound in air is 343 m/s?

A) 8490 Hz
B) 5000 Hz
C) 4250 Hz
D) 9000 Hz
E) 4500 Hz