JEE PYQ: Waves And Sound Question 17
Question 17 - 2020 (06 Sep Shift 1)
A sound source $S$ is moving along a straight track with speed $v$, and is emitting sound of frequency $v_0$ (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:
($t_0$ represents the instant when the distance between the source and observer is minimum)
(1) Frequency starts high, drops sharply at $t_0$ and stays low
(2) Frequency gradually increases then gradually decreases, smooth curve
(3) Frequency starts high, dips at $t_0$, then rises back
(4) Frequency constant above $v_0$ then drops to constant below $v_0$
Show Answer
Answer: (2)
Solution
Frequency heard by the observer:
$v_{\text{observed}} = \left(\frac{v_{\text{sound}}}{v_{\text{sound}} - v\cos\theta}\right) v_0$
Initially $\theta$ will be less so $\cos\theta$ more.
$\therefore v_{\text{observed}}$ more, then it will decrease.
BETA
