Q 11-14-024NEETJEE MainMedium
A train sounding a whistle of $600$ Hz approaches and then passes a stationary listener at $30$ m/s. The change in frequency heard by the listener as the train passes is (speed of sound $= 330$ m/s)
Answer: (D) $110$ Hz
Concept. Doppler effect for a moving source: $f\dfrac{v}{v - v_s}$ approaching and $f\dfrac{v}{v + v_s}$ receding.
Step 1. Approaching. $600 \times \dfrac{330}{300} = 660$ Hz.
Step 2. Receding. $600 \times \dfrac{330}{360} = 550$ Hz.
Step 3. Change. $660 - 550 = 110$ Hz.
Answer. $110$ Hz (Option D).
Solution by Sreeraj P, M.Sc Physics