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Question:

A submarine (A) travelling at 18 km/hr is being chased along the line of its velocity by another submarine (B) travelling at 27 km/hr. B sends a sonar signal of 500 Hz to detect A and receives a reflected sound of frequency v. The value of v is close to :(Speed of sound in water=1500 m/s)

502Hz

504Hz

499Hz

507Hz

Solution:

Correct option is B.
502Hz
f0=500Hz
frequency recieved by A →(1500−5/1500−7.5)f0=f1
and frquency recived By B again = ←1500(B) (A) →7.5m/s→→5m/s
f2=(1500+7.5/1500+5)×(1500−5/1500−7.5)f0=502Hz.