A vehicle sounding a whistle of frequency $256 \mathrm{~Hz}$ is moving on a straight road, towards a hill…
A vehicle sounding a whistle of frequency $256 \mathrm{~Hz}$ is moving on a straight road, towards a hill with a velocity of $10 \mathrm{~ms}^{-1}$. The number of beats per second observed by a person travelling in the vehicle is velocity of sound $=330 \mathrm{~ms}^{-1}$
zero
10
14
16
Solution
Apparent frequency heard by the observer,
$\begin{aligned}
n^{\prime} & =\left(\frac{v+v_s}{v-v_s}\right) \times n \\
& =\left(\frac{330+10}{330-10}\right) \times 256 \\
& =\frac{340}{320} \times 256=272 \mathrm{~Hz}
\end{aligned}$
$\therefore$ Number of beats heard by the observer
$=272-256=16$