A car moving at 20 m/s sounds a horn of frequency 320 Hz. A…
Question
A car moving at 20 m/s sounds a horn of frequency 320 Hz. A stationary observer stands behind the car (the car moves directly away from the observer and directly toward a large vertical cliff ahead). The observer hears two sounds: the direct horn sound and the echo reflected from the cliff. Taking the speed of sound as 340 m/s, find the number of beats per second heard by the stationary observer.
Step-by-step solution
Direct sound: source recedes from the observer, so heard frequency f_direct = f*v/(v+u) = 320*340/360 = 302.222 Hz.
Echo: the source approaches the cliff, which receives f*v/(v-u); the cliff re-emits this frequency and the stationary observer hears it unchanged, f_echo = 320*340/320 = 340 Hz.
Final answer37.778
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