Question

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

Answer 1

Answer: 340

The hill will be a secondary source.

$f_1=$ frequency of the car w.r.t. the hill $f_1=\left(\frac{v}{v-v_s}\right)f=\left(\frac{330}{320}\right)\times 320=330Hz$
$f_2=$ Frequency of the sound reflected by hill w.r.t. the car (echo)
$f_2=\left(\frac{v+v_0}{v}\right)f_1=\frac{(330+10)}{330}\times 330=340Hz$