Doppler Effect for Approaching Source

Steps:

• Source moves toward stationary observer, so observed frequency increases due to Doppler effect.
• Formula: $f' = f \frac{v}{v - v_s}$, where $v$ is speed of sound, $v_s$ is source speed (positive toward observer).
• As car speed increases, $v_s$ rises, denominator decreases, so $f'$ increases further.
• Thus, heard frequency starts greater than f and keeps rising.

Why A is correct:

• Matches Doppler formula: approaching source causes $f' > f$, and accelerating source makes $f'$ increase over time.

Why the others are wrong:

• B: Wrong direction; approaching raises frequency, doesn't lower it.
• C: Frequency increases with speed, doesn't decrease.
• D: Approaching can't make frequency less than f.

Final answer: A