Question

Two trains, each moving with a velocity of $30{ms}^{-1}$, cross each other. One of the trains gives a whistle whose frequency is $600Hz$. If the speed of sound is $330{ms}^{-1}$, the apparent frequency for passengers sitting in the other train before crossing would be

• A. $600Hz$
• B. $630Hz$
• C. $920Hz$
• D. $720Hz$

Answer 1

The correct option is D $720Hz$

Here, velocity of source vS=30ms−1vS=30ms−1
velocity of observer vO=30ms−1vO=30ms−1
velocity of sound v=330ms−1;υ=660Hzv=330ms−1;υ=660Hz
as source and observer are moving towards each other with same speed, therefore apparent frequency is
υ′=υ[v+vOv+vS]=600[330+30330−30]=720Hz