Question

Two cars are approaching each other at an equal speed of $7.2km/hr.$ When they see each other, both blow horns having frequency of $676Hz.$ The beat frequency heard by each driver will be _________ $Hz.$ [Velocity of sound in air is $340m/s.$ ]

Answer 1

Answer: 8

Speed $=7.2km/h=2m/s$
Frequency as heard by $A$
$f_A^{{}^{\prime }}=f_B\left(\frac{v+v_0}{v-v_s}\right)$
$f_A^{{}^{\prime }}=676\left(\frac{340+2}{340-2}\right)$
$f_A^{{}^{\prime }}=684Hz$
$\therefore f_{Beat}=f_A^{{}^{\prime }}-f_B$
$=684-676$
$=8Hz$