A sound of frequency 512 Hz is emitted from a stationary source . A train running at a speed of 72 km . `h^(-1) ` passes the source . What will be the frequency of the sound heard by a passenger of the train before and after passing the source ? Neglect the effect of wind . Velocity of sound is 336 . `ms^(-1)` .

Explanation

Velocity of the train = 72 km . ` h^(-1)`
` = (72 xx1000)/(60xx60) m . S^(-1) = 20 m. S^(-1)`
When the train approaches the source , the velocity of sound relative to the passenger ,
` V. = 336 + 20 = 356 m . S^(-1)`
Since the source is at rest , the wavelength of sound remains the same .
` therefore ` Apparent freuency due to Doppler effect
`(V.) /(lambda ) = (V.) /((V)/(n)) = (V. n)/(V) = (356 xx 512)/(336) = 542 .5 Hz `
Again when the train recedes from the source , the velocity of sound relative to the passenger
` Again when the train recedes from the source , the velocity of sound relative to the passenger
` V. = 336 - 20 = 316 . s^(-1) `
` therefore Apparent freaquency ` = (V. n)/(V) = (316 xx 512)/(336) = 481 .5 Hz `