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  4. A sound source S is moving along a straight track with speed v , and is emitting sound of frequency v0 (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)

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Q. 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 :
$\left( t _{0}\right.$ represents the instant when the distance between the source and observer is minimum)

JEE MainJEE Main 2020Waves

Solution:

image
$f _{\text {observed }} \Rightarrow \left(\frac{ v _{\text {sound }}}{ v _{\text {sound }}- v \cos \theta}\right) f _{0}$
initially $\theta$ will be less $\Rightarrow \cos \theta$ more
$\therefore f _{\text {observed }}$ more, then it will decrease.