Two cars are approaching each other at an equal speed of $7.2\, km / hr$. When they see each other, both blow horns having frequency of $676\, Hz$. The beat frequency heard by each driver will be $.....\, Hz$. [Velocity of sound in air is $340\,m / s .]$
JEE MAIN 2021, Diffcult
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Frequency of sound heard by car$-1,$ which comes by reflection from car$-2$
$f _{1}= f _{0}\left(\frac{340+2}{340-2}\right)\left(\frac{340+2}{340-2}\right)$
$= f _{0}\left(\frac{342}{338}\right)^{2}$
Frequency of sound coming directly from car$-2$
$f _{2}= f _{0}\left(\frac{340+2}{340-2}\right)$
$\therefore f _{1}- f _{2}= f _{0}\left(\frac{342}{338}\right)\left(\frac{342}{338}-1\right)=8.09 \simeq 8$
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