A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer is f1. If the train’s speed is reduced to 17 m/s, the frequency registered is f2. If the speed of sound is 340 m/s then the ratio f1f2 is
By using n′=n(vv−vs)
⇒ f1=n(vv−vs)=n(340340−34)=340306n
and f2=n(340340−17)=n(340323)⇒ f1f2=323306=1918