A Policeman in a Stationary Car Measures the Speed of Approaching

question 18

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

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