The frequency of echo will be _____ Hz if the train blowing a whistle of frequency 320 Hz is moving with a velocity of 36 km h - 1 towards a hill from which an echo is heard by the train driver. Velocity of sound in air is 330 m s - 1 .

Step-by-step solution

The hill will be a secondary source. When the train moves towards the hill then, f 1 = frequency of the train w.r.t. the hill is f 1 = v v - v s f = 330 320 × 320 = 330   Hz When the train driver hears an echo, then velocity of observer is v 0 = 36 × 5 18 = 10   m   s - 1 . Now, f 2 = Frequency of the sound reflected by hill w.r.t. the car (echo) f 2 = v + v 0 v f 1 = 330 + 10 330 × 330 = 340   Hz