Doppler Effect for Approaching Source

Steps:

• Source (vehicle horn) moves toward stationary observer, increasing observed frequency.
• Apply formula: f' = f × (v / (v - v_s)), where f = 400 Hz, v = 340 m/s, v_s = 30 m/s.
• Compute denominator: v - v_s = 340 - 30 = 310 m/s.
• Calculate f' = 400 × (340 / 310) ≈ 400 × 1.097 = 439 Hz.

Why D is correct:

• D matches the result from the Doppler formula, which predicts higher frequency for source approaching observer at 30 m/s.

Why the others are wrong:

• A and B show negligible increase, ignoring source motion.
• C overestimates shift, possibly confusing observer and source motion.

Final answer: D