A train producing a siren with a frequency of 1500 Hz approaches a stationary observr with a speed of 72 kmph. What is apparent frequency of sound heard by him when it crossing him ?( Given velocity of sound `v = 340ms^(-1)` )

Explanation

`f_(0) =1500 Hz`
`v_(s) = 0`
`v_(0) = 72 kmph =72 xx ( 5)/( 18) ms^(-1)`
i.e, `v_(0)=20ms^(-1)`
`v =340 ms^(-1)`
(i) Source approaching the stationary observer
Apparent frequency, `f =f_(0)""(v)/(v-v_(s)) = (1500 xx340)/( ( 340 - 20)) = (1500 xx 340 )/( 320)`
`f=1593 .8 Hz`
(ii) Source receding the stationary observer,
apparent frequency, `f = (f_(0)v)/( v+ v_(2)) = ( 1500 xx 340)/( 340-20)= (1500 xx340 )/(360)`
`f = 1416 Hz`.