Two passenger trains moving with a speed of 108 km/hour cross each other. One of them blows a whistle whose frequency is 1500 Hz. If sound speed is 330 m/s, then passengers sitting in the other train, after trains cross each other will hear sound whose frequency will be

  • Option 1)

    1250 Hz

  • Option 2)

    625 Hz

  • Option 3)

    1500 Hz

  • Option 4)

    900 Hz

 

Answers (1)

As we learnt in 

Frequency of sound when source & observer are moving away from each other -

\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

 

 \nu_{o}\:=\:1500

Here both are moving away from each other

\Rightarrow \nu=\nu_{o}\:.\:\left(\frac{C-V_{o}}{C+V_{s}} \right )=1500\:\left(\frac{330-30}{330+30} \right )

\because\:V=108\:Km/hr\:=\:30\:m/s

\nu=1500\times\frac{300}{360}\:Hz\:=1250\:Hz

 


Option 1)

1250 Hz

This option is correct.

Option 2)

625 Hz

This option is incorrect.

Option 3)

1500 Hz

This option is incorrect.

Option 4)

900 Hz

This option is incorrect.

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