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Question

Two trains, each moving with a velocity of 30m/s, cross each other. One of the trains gives a whistle whose frequency is 600Hz. If the speed of sound is 330m/s, the apparent frequency for passengers sitting in the other train before crossing would be:


A

600Hz

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B

630Hz

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C

920Hz

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D

720Hz

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Solution

The correct option is D

720Hz


Step 1: Given

Velocity of source (the train that whistles): vs=30m/s

Velocity of observer (the people in other train): vo=-30m/s (negative, because going in different direction)

Frequency of whistle: f=600Hz

Speed of sound: v=330m/s

The apparent frequency for passenger f'

Step 2: Formula Used

f'=fv-vov-vs

Step 3: Calculate the apparent frequency using the formula

f'=600330--30330-30=600360300=600×65=720Hz

Hence, the apparent frequency for passengers is 720Hz. Option D is correct.


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