Question

Two cars $X$ and $Y$ are approaching each other with velocities $36\text{km/h}$ and $72\text{km/h}$ respectively. The frequency of a whistle sound as emitted by a passenger in car $X$, heard by the passenger in car $Y$ is $1320\text{Hz}$. If the velocity of sound in air is $340\text{m/s}$, the actual frequency of the whistle sound produced is $\text{Hz}$.

• A. 1210
• B. 1210.0

Answer 1

Answer: (A), (B)

Given :-
$V_s=V_x=36\text{km/h}=36\times \frac{5}{18}=10\text{m/s}$
$V_o=V_y=72\text{km/h}=72\times \frac{5}{18}=20\text{m/s}$
$V=340\text{m/s}$
$f=1320\text{Hz}$
Using Doppler's law :
$f=f_o\left(\frac{V+V_o}{V-V_s}\right)$
$1320=f_o\left(\frac{340+20}{340-10}\right)$
$\Rightarrow f_o=1210\text{Hz}$