Doppler Effect with Accelerating Source

Steps:

• Vehicle starts near X and accelerates toward Y, so it recedes from X and approaches Y.
• For observer at X, source moves away; initial low speed means frequency near 750 Hz, but increasing speed causes frequency to decrease over time.
• For observer at Y, source moves toward them; initial low speed means frequency slightly above 750 Hz, but increasing speed causes frequency to increase over time.
• Thus, frequency decreases at X and increases at Y.

Why C is correct:

• Doppler formula $f' = f \frac{v}{v \pm v_s}$ (v sound speed, v_s source speed); receding (+ sign) lowers f' more with rising v_s, approaching (- sign) raises f' more with rising v_s.

Why the others are wrong:

• A: Frequencies change dynamically due to acceleration, not fixed above/below 750 Hz.
• B: Reverses the trends; receding decreases at X, approaching increases at Y.
• D: Swaps the static higher/lower assignments, ignoring approach/recede directions.

Final answer: C