question_answer

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) [UPSEAT 2002]

A)            600 Hz                                     

B)            585 Hz

C)            645 Hz                                     

D)            666 Hz

Correct Answer: D

Solution :

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