A person driving car at a constant speed of $15 \mathrm{~m} / \mathrm{s}$ is approaching a vertical wall. The person notices a change of $40 \mathrm{~Hz}$ in the frequency of his car's horn upon reflection from the wall. The frequency of horn is _______________ $\mathrm{Hz}$.
(Given: Speed of sound : $330 \mathrm{~m} / \mathrm{s}$ )
Answer (integer)
420
Solution
$$
\begin{aligned}
& \text { Frequency of reflected sound }=\left(\frac{v+v_{\mathrm{c}}}{v-v_c}\right) f_0 \\\\
& f=\left(\frac{330+15}{330-15}\right) \times f_0 \\\\
& =\frac{345}{315} f_0 \\\\
& \frac{345}{315} f_0-f_0=40 \\\\
& \frac{30}{315} f_0=40 \\\\
& f_0=\frac{4 \times 315}{3}=420 \mathrm{~Hz}
\end{aligned}
$$
About this question
Subject: Physics · Chapter: Waves · Topic: Wave Motion and Types
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