question_answer

Two passenger trains moving with a speed of 108 km/hour cross each other. One of them blows a whistle whose frequency is 750 Hz. If sound speed is 330 m/s, then passengers sitting in the other train, after trains cross each other will hear sound whose frequency will be        [MP PMT 1991]

A)            900 Hz                                     

B)            625 Hz

C)            750 Hz                                     

D)            800 Hz

Correct Answer: B

Solution :

           \[n'=n\,\left( \frac{v-{{v}_{o}}}{v+{{v}_{s}}} \right)=750\,\left( \frac{330-180\times \frac{5}{18}}{330+108\times \frac{5}{18}} \right)=625\]\[Hz\]