Doppler Effect: Source Approaching Stationary Observer

Steps:

• Identify roles: Siren is moving source (vs = 20 m/s towards observer); observer stationary (vo = 0).
• Apply formula: Heard frequency f' = f × (v / (v - vs)), where v = speed of sound, f = source frequency.
• Substitute values: f' = 1000 × (330 / (330 - 20)).
• Result: Expression matches option D, yielding higher frequency due to wave compression.

Why D is correct:

• Matches Doppler formula f' = f × v / (v - vs) for source approaching stationary observer, increasing frequency as waves bunch up.

Why the others are wrong:

• A: Applies to stationary source with observer approaching (f × (v + vo)/v).
• B: Gives decreased frequency, suits observer receding from source (f × (v - vo)/v).
• C: For source receding from stationary observer (f × v / (v + vs)).

Final answer: D