Question

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 towards a hill from which an echo is heard by the train driver. Velocity of sound in air is 330 m/s.

Answer 1

vs=$\frac{36\times5}{18}$=10 m/sec
The frequency that reaches the hill will reflect, and this will act as a source to the driver of the train.
The apparent frequency that reaches the hill =$\frac{330}{330-10}$(320)=330Hz, this will act as a source for the driver.
Apparent frequency heard by the driver in train =$\frac{330+10}{330}$(330)=340 Hz