Doppler Effect for Source Moving Away

Steps:

• Source frequency f = 400 Hz when stationary.
• For train moving away: f' = f × (v / (v + v_s)), where v = 340 m/s, v_s = 50 m/s.
• f' = 400 × (340 / 390) ≈ 349 Hz.
• Difference = 400 Hz - 349 Hz = 51 Hz.

Why A is correct:

• 51 Hz matches the Doppler shift formula for source receding from stationary observer.

Why the others are wrong:

• B: 69 Hz from incorrect formula, like observer motion instead of source.
• C: 350 Hz approximates heard frequency, not the difference.
• D: 470 Hz from erroneous addition of speeds to frequency.

Final answer: A