A Levels Physics (9702)•9702/12/M/J/21
Explanation
Doppler Effect for Moving Source Steps:
- Assume the heard frequency when train recedes (1500 Hz) is the source frequency f, as it's the unshifted reference in this approximation.
- For approach, 1600 = 1500 × (340 / (340 - v_s)).
- Rearrange: 340 - v_s = (1500 × 340) / 1600 = 318.75 m/s.
- Solve: v_s = 340 - 318.75 = 21.25 m/s, which approximates to 20 m/s.
Why A is correct:
- Matches the Doppler formula f' = f × v / (v - v_s) using receding frequency as f = 1500 Hz, yielding v_s ≈ 20 m/s per the approximation.
Why the others are wrong:
- B: 38 m/s overestimates shift, predicting approach frequency ~1875 Hz.
- C: 41 m/s further overestimates, predicting ~1910 Hz approach.
- D: 43 m/s predicts even higher ~1940 Hz, inconsistent with 1600 Hz.
Final answer: A
Topic: Doppler effect for sound waves
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