A Policeman in a Stationary Car Measures the Speed of Approaching

question 52

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 41.2 kHz. A car is approaching him at a speed of 33.0 m/s. The wave is reflected by the car and interferes with the emitted sound producing beats. What is the frequency of the beats? The speed of sound in air is 330 m/s.

• A) 9.2 kHz
• B) 4.1 kHz
• C) 4.6 kHz
• D) 1.2 kHz
• E) 8.2 kHz

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