Q73 / 120 · NEET 2012 (Mains) · Physics
A train moving at a speed of $220 \mathrm{~ms}^{-1}$ towards a stationary object, emits a sound of frequency $1000 \mathrm{~Hz}$. Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is (speed of sound in air is $330 \mathrm{~ms}^{-1}$ )
- A$3500 \mathrm{~Hz}$
- B$4000 \mathrm{~Hz}$
- Correct answerC$5000 \mathrm{~Hz}$
- D$3000 \mathrm{~Hz}$
Solution
From Doppler's shift, we know for this case
$$
\begin{aligned}
n^{\prime} & =n\left(\frac{v+v_s}{v-v_s}\right) \\
& =1000\left(\frac{330+220}{330-220}\right) \\
& =1000\left(\frac{550}{110}\right)=5000 \mathrm{~Hz}
\end{aligned}
$$
Hence, the correct answer is option: 'C'.