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Concepts Of Physics MCQ Edition [Volume 1]PhysicsSound Waves

A person travelling in a car moving at 72 km/h blows a whistle that emits a wave of frequency 1250 Hz . Determine the frequency heard by another person standing on the road behind the car. The speed of sound in air is 340 m/s .

Options

  1. A1181 Hz
  2. B1324 Hz
  3. C1250 Hz
  4. D1328 Hz

Correct answer

A. 1181 Hz

Step-by-step solution

Speed of the car, v_s = 72 km/h = 72 5 18 m/s = 20 m/s Speed of sound, v = 340 m/s Actual frequency, f = 1250 Hz Since the car is moving away from the stationary observer, the apparent frequency f' is given by the Doppler effect formula: f' = f ( v v + v_s ) f' = 1250 ( 340 340 + 20 ) f' = 1250 ( 340 360 ) f' = 1250 17 18 f' 1180.55 Hz 1181 Hz

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