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A musician using an open flute of length 50 cm produces second harmonic sound waves. A person runs towards the musician from another end of a hall at a speed of 10 km/h. If the wave speed is 330 m/s, the frequency heard by the running person shall be close to:

1. 753 Hz
2. 666 Hz
3. 500 Hz
4. 333 Hz

Correct answer: 666 Hz

Solution

Open flute 2nd harmonic: f = 2*v/(2L) = 2*330/(2*0.5) = 660 Hz. Observer moves toward source at 10 km/h = 2.78 m/s: f' = f*(v+vo)/v = 660*(330+2.78)/330 ~ 666 Hz.

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