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# Help me answer: A motor car blowing a horn of frequency 120 vib/s moves with velocity 90km/hr. The frequency of the sound heard (Reflected) by the driver will be (speed of sound in air is 325 m/s)

A motor car blowing a horn of frequency 120 vib/s moves with velocity 90km/hr. The frequency of the sound heard (Reflected) by the driver will be (speed of sound in air is 325 m/s)

• Option 1)

109 vib/s

• Option 2)

132 vib/s

• Option 3)

140 vib/s

• Option 4)

248 vib/s

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$n'=\left [ \frac{v+v_{0}}{v-v_{s}} \right ]n$

$n'=120\left [ \frac{325+90\times 518}{325-90\times 518} \right ]= 140$

frequency of sound when source is moving towards the observer and observer is moving away from source -

$\nu {}'= \nu _{0}.\frac{C-V_{0}}{C-V_{s}}$

- wherein

$C=$ Speed of sound

$V_{0}=$ Speed of observer

$V_{s}=$ speed of source

$\nu _{0 }=$ Original frequency

$\nu {}'=$ apparent frequency

Option 1)

109 vib/s

This is incorrect

Option 2)

132 vib/s

This is incorrect

Option 3)

140 vib/s

This is correct

Option 4)

248 vib/s

This is incorrect

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