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

Question

Quizplus · Accepted Answer

The beat frequency is calculated using the Doppler effect. The frequency of the reflected wave is higher than the emitted frequency due to the car's approach. The beat frequency is the difference between the frequency of the reflected wave and the emitted frequency. Calculating this gives a beat frequency of approximately 9000 Hz.

A Policeman in a Stationary Car Measures the Speed of Approaching