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Two cars A and B are moving away from each other in opposite directions. Both the cars are moving with a speed of 20 m s⁻¹ with respect to the ground. If an observer in car A detects a frequency 2000 Hz of the sound coming from car B, what is the natural frequency of the sound source in car B? (speed of sound in air = 340 m/s)

1. 2060 Hz
2. 2150 Hz
3. 2300 Hz
4. 2250 Hz

Correct answer: 2250 Hz

Solution

With v=340, vo=vs=20: 2000 = f0*(340-20)/(340+20) -> f0 = 2000*360/320 = 2250 Hz.

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