A source emits a sound of frequency 400 Hz. If the source moves toward a stationary observer at 20 m/s, what frequency does the observer hear? (Speed of sound = 343 m/s)
Correct answer: C. 425 Hz
- A. 375 Hz
- B. 400 Hz
- C. 425 Hz
- D. 450 Hz
Explanation
Doppler effect: f' = f × (v / (v - vs )) = 400 × (343 / (343 - 20)) = 400 × (343 / 323) ≈ 425 Hz.
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Progressive waves transfer energy through oscillations, with wavelength, frequency, amplitude and wave speed related by v = fλ; sound speed depends on the medium. Superposition produces interference and stationary waves in strings and organ pipes, while simple harmonic motion describes the oscillation itself and must be distinguished from wave propagation.
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