MHT CET 2021 Physics Solved Paper

(c) The Doppler's effect in sound is given by
fo=‌

v+vo

v+vs

fs
where, fo= observer frequency of sound,
v= speed of sound waves,
vo= observer velocity,
vs= source velocity
and fs= actual frequency of sound wave.
Thus, according to question,
2n‌‌=(‌

v+0

v+vs

)n
⇒‌‌2(v+vs)‌‌=v
⇒‌‌2v+2vs‌‌=v
−2vs‌‌=v
vs‌‌=−‌

v

2

where -ve sign shows that source is approaching to the listener.