First slide

Doppler's effect

Question

Two trains are moving towards each other at speeds of 20 m/s and 15 m/s relative to the ground. The first train sounds a whistle of frequency 600 Hz. the frequency of the whistle heard by a passenger in the second train before the train meets is (the speed of sound in air is 340 m/s)

Moderate

A

600 Hz
B

585 Hz
C

645 Hz
D

666 Hz

Solution

The frequency of whistle heard by passenger in the train B, is
$n^{'} = n (\frac{v + v_{0}}{v - v_{s}}) = 600 (\frac{340 + 15}{340 - 20}) \approx 666 Hz$

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