Question

If a rocket moving at a speed of 200 m/s towards a stationary target emitting a wave of frequency 1000 Hz and some of the sound reaching the target gets reflected back to the rocket as an echo, then calculate a) The frequency of the sound detected by the target b) The frequency of the echo detected by the rocket

Answer 1

Given :
The observer is at rest and 

Rocket Speed =vs=200m/s
V=330m/s
Formula :
v=vo[1-vs/V]⁻¹
v=1000[1-200/330]⁻¹
=2540Hz

b)The target will become source and rocket detector is now observer.[Since it is echo]
ν¹=ν(V+vo)/V
=2540x[200+330]/330
=4080Hz

Answer 2

4080hz is the answer

hope this helps you