A train moving at a speed of 220 ~ms^ -1 towards a stationary object, emits a sound of frequency 1000 ~Hz . Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as

Correct answer: C. 5000 ~Hz From Doppler's shift, we know for this case aligned n^ & =n((v+v_s)/(v-v_s)) & =1000((330+220)/(330-220)) & =1000((550)/(110))=5000 ~Hz aligned

NEET 2012

Exam: NEET
Year: 2012
Subject: Physics
Chapter: Waves and Sound

NEET 2012 · Physics · Waves and Sound

A train moving at a speed of 220 ~ms^ -1 towards a stationary...

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}

)

Options

A $3500 \mathrm{~Hz}$
B $4000 \mathrm{~Hz}$
C $5000 \mathrm{~Hz}$
D $3000 \mathrm{~Hz}$

Correct answer

C. $5000 \mathrm{~Hz}$

Step-by-step 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}

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