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A car is moving with a speed of 72 km . ...

A car is moving with a speed of 72 km . `h^(-1)` towards a roadside source that emits sound at a frequency of 850 Hz. The car driver listens to the sound while approaching the source and again while moving away from the source after crossing it. If the velocity of sound is 340 m . `s^(-1)`, the difference of the two frequencies, the driver hears is

A

50 Hz

B

85 Hz

C

100 Hz

D

150 Hz

Text Solution

Verified by Experts

The correct Answer is:
C
Velocity of the car, `u_(o)=72 km cdot h^(-1) = 20 m cdot s^(-1)`
When the car is approaching the source, then apparent frequency is,
`n_(1) = ((V+u_(o))/V)n=((340+20)/340)850=900" Hz " `
When the car moves away from the source, then apparent frequency is,
`n_(2) = ((V+u_(o))/V)n=((340+20)/340)850=800" Hz " `
`therefore n_(1) - n_(2) =900-800=100` Hz
the option © is correct.
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