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Standard XII

Physics

Apparent Frequency:Listener Moving

A train appro...

Question

A train approaching a railway platform with a speed of $20 m s^{- 1}$ , starts blowing the whistle. Speed of sound in air is $340 m s^{- 1}$ . If the frequency of the emitted sound from the whistle is $640 H z$ , the frequency of sound to a person standing on the platform will appear to be:

600 H z

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640 H z

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680 H z

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720 H z

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Solution

The correct option is B $680 H z$
Velocity of source (train) $v_{s} = 20 m / s$
Speed of sound $v_{s o u n d} = 340 m / s$
Original frequency $f_{o} = 640 H z$
Using Doppler effect when a source is approaching towards the stationary observer :
Apparent frequency heard $f^{'} = f_{o} [\frac{v_{s o u n d}}{v_{s o u n d} - v_{s}}]$
$∴$ $f^{'} = 640 [\frac{340}{340 - 20}] = 680 H z$

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A train approaching a railway platform with a speed of 20 ms−1, starts blowing the whistle. Speed of sound in air is 340 ms−1. If the frequency of the emitted sound from the whistle is 640 Hz, the frequency of sound to a person standing on the platform will appear to be:

The correct option is B 680 HzVelocity of source (train) vs=20 m/sSpeed of sound vsound=340 m/sOriginal frequency fo=640 HzUsing Doppler effect when a source is approaching towards the stationary observer :Apparent frequency heard f′=fo[vsoundvsound−vs]∴ f′=640[340340−20]=680 Hz

A train approaching a railway platform with a speed of $20 m s^{- 1}$ , starts blowing the whistle. Speed of sound in air is $340 m s^{- 1}$ . If the frequency of the emitted sound from the whistle is $640 H z$ , the frequency of sound to a person standing on the platform will appear to be: