Two engines pass each other in opposite directions . One of them blows a whistle of frequency 540 Hz .
Find the freqauencies heard by a passenger sitting on the other engine before and after passing each other .
Velocity of both engines = `72 km h^(-1)` , velocity of sound = `340 ms^(-1)` .

Explanation

Velocity of the engine ,
` u = 72 km h^(-1) = (72 xx100)/(60xx60 ) m. S^(-1) = 20 m s^(-1)`
Velocity of sound , V = 340 m ` S^(-1)` ,
Actual frequency , n = 540 Hz .
At the time of approching each other , velocity of the whistle
` u_(s) = + 20 m S^(-1)`
Velocity of the passenger on the other engine
` u_(0) = + 20 m S^(-1)`
` therefore ` Apparent frequency ,
` n. (V + u_(0))/(V + u_(s)) xx n `
` = (340 + 20 )/(340 - 20) xx 540 = (360)/(320 ) xx 540 = 607 .5 Hz `
Again at the time of receding from each other ,
` u_(s) = - 20 m S^(-1)" and " u_(o) = - 20 S^(-1)`
` therefore ` Apperent frequency ,
` n . = (V + u_(0))/(V- u_(s)) xx n `
` = (340-20)/(340 - (-20)) xx 540 = (320)/(360 ) xx 540 = 480 Hz ` .