The frequency of echo will be _____ Hz if the train blowing a whistle of frequency 320 Hz is moving with a velocity of km/h towards a hill from which an echo is heard by the train driver. Velocity of sound in air is m/s.
Answer: 340
Solution
This is a double Doppler effect problem. First, the hill acts as a stationary observer receiving sound from the moving source (train). Then, the hill reflects the sound, acting as a stationary source, and the train is now a moving observer.
Train velocity: km/h m/s
Step 1: Frequency received by the hill (stationary observer, moving source):
Step 2: Frequency of echo heard by train (moving observer, stationary source):
The echo frequency is 340 Hz.
Train velocity: km/h m/s
Step 1: Frequency received by the hill (stationary observer, moving source):
Step 2: Frequency of echo heard by train (moving observer, stationary source):
The echo frequency is 340 Hz.
JEE Main · 2022 · 28 Jul 2022 · Shift 1 · Integer answer