A train approaching a railway plateform with a speed of 20,,m,s^{-1} starts blowing whistle sp

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14.Waves and Sound

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A train approaching a railway plateform with a speed of $20\,\,m\,s^{-1}$ starts blowing the whistle speed of sound in air is $340\,\,ms^{-1}.$ If frequency of the emitted sound from the whistle is $640\,\,Hz,$ the frequency of sound as heard by person standing on the platform is .... $Hz$

A

$600$

B

$640$

C

$680$

D

$720$

Solution

Here,

Speed of source (i.e. train), $\mathrm{v}_{\mathrm{s}}=20 \mathrm{\,m} \mathrm{s}^{-1}$

Speed of sound in air, $\mathrm{v}=340 \mathrm{\,ms}^{-1}$

Frequency of the source, $v_{0}=640 \mathrm{\,Hz}$

The frequency heard by the person standing on the platform is

${u^{\prime}=v_{0}\left[\frac{v}{v-v_{s}}\right]} $

${=640\left[\frac{340}{340-20}\right]=\frac{640 \times 340}{320}=680 \mathrm{\,Hz}}$

Standard 11

Physics

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