Question 103: Waves

Question

An employee of a factory moving away from his workplace by a car listens to the siren of the factory. He drives the car at the speed of 72 kmh$$-$$1 in the direction of wind which is blowing at 72 kmh$$-$$1 speed. Frequency of siren is 720 Hz. The employee hears an apparent frequency of ____________ Hz.
(Assume speed of sound to be 340 ms$$-$$1)

Accepted Answer

Here, the apparent frequency is given by
$$
\begin{aligned}
&f^{\prime}=f\left(\frac{V-V_{0}}{V+V_{s}}ight)=720\left(\frac{(340+20)-20}{(340+20)-0}ight) \\
&=\frac{720 	imes 340}{360}=680 \mathrm{~Hz}
\end{aligned}
$$

Waves

Choose the correct answer: