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  4. 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. ]

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Q. 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 $340\,m/s.$ ]

NTA AbhyasNTA Abhyas 2022

Solution:

Solution
Speed $=7.2\,km/h=2\,m/s$
Frequency as heard by $A$
$f_{A}^{'}=f_{B}\left(\frac{v + v_{0}}{v - v_{s}}\right)$
$f_{A}^{'}=676\left(\frac{340 + 2}{340 - 2}\right)$
$f_{A}^{'}=684\,Hz$
$\therefore f_{Beat}=f_{A}^{'}-f_{B}$
$=684-676$
$=8\,Hz$