Question

When a car is approaching the observer, the frequency of horn is $100Hz$. After passing the observer, it is $50Hz$. If the observer moves with the car, the frequency will be $\frac{x}{3}$ $Hz$ where $x=$ ______

Answer 1

Answer: 200

$f_1=100=f_0\left(\frac{C}{C-V_s}\right)$
$C=$ speed of sound
$V_s=$ speed of source
$f_2=50=f_0\left(\frac{C}{C+V_s}\right)$
$\frac{f_1}{f_2}=2=\frac{C+V_s}{C-V_s}$
$2C-2V_s=C+V_s$
$3V_s=C$
$V_S=\frac{C}{3}$
$100=f_0\frac{C}{\frac{2C}{3}}=\frac{3}{2}{f}_0$
$f_0=\frac{200}{3}$