Doppler Effect for Moving Source

Steps:

• Observed frequency f' = f * v / (v + vs) when source moves away (v = sound speed, vs = source speed); denominator increases with vs, decreasing f'.
• f' = f * v / (v - vs) when source moves toward observer; denominator decreases, increasing f'.
• Smallest f' occurs for largest vs away from observer, as it maximizes denominator.
• Compare speeds: away options are 5 m/s (A) and 10 m/s (B); toward options (C, D) yield f' > f.

Why B is correct:

• 10 m/s away maximizes denominator in Doppler formula, yielding lowest f' among options.

Why the others are wrong:

• A: 5 m/s away gives smaller denominator than B, so higher f'.
• C: 15 m/s toward decreases denominator, increasing f' above original f.
• D: 20 m/s toward further decreases denominator, raising f' even more.

Final answer: B