Question

Two cars are approaching each other at an equal speed of $7.2\text{km/hr}$. When they see each other, both blow horns having frequency of $676\text{Hz}$. The beat frequency heard by each driver will be $\text{Hz}$.

[Velocity of sound in air is $340\text{m/s}$]

Answer 1

Speed = 7.2 $\text{km/h}$ = $2\text{m/s}$
Frequency as heard by $A$,

$f_{\text{A}}^{\prime }=f_B\left(\frac{v+v_0}{v-v_s}\right)$

$f_{\text{A}}^{\prime }=676\left(\frac{340+2}{340-2}\right)$

$f_{\text{A}}^{\prime }=684\text{Hz}$

$\therefore f_{\text{beat}}=f_A^{\prime }-f_B=684-676=8\text{Hz}$