Question

A sound source $S$ is moving along a straight track with speed $v$, and is emitting, the sound of frequency $v_o$ (see figure). An observer is standing at a finite distance, at the point $O$, from the track. The time variation of frequency heard by the observer is best represented by ($t_o$ represents the instant when the distance between the source and observer is minimum).

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

Step 1: Draw a diagram

when source moves towards observer,

Step 2: Find time variation of frequency

By doppler effect,

$f_o\Rightarrow \frac{v_s+v_o}{V_s–v\cos \theta }{f}_s$

Initially, $\theta$ will be less

⇒ $\cos \theta$ is more.

Therefore, $f_o$is initially more, and then it will decrease.

Hence, option D is correct.