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 f1/f2 is

(1) 18/19

(2) 1/2

(3) 2

(4) 19/18

(4) By using $n\text{'}=n\text{\hspace{0.17em}\hspace{0.17em}}\left(\frac{v}{v-{v}_{S}}\right)$

${f}_{1}=n\text{\hspace{0.17em}}\left(\frac{v}{v-{v}_{S}}\right)=n\text{\hspace{0.17em}}\left(\frac{340}{340-34}\right)=\frac{340}{306}n$

and ${f}_{2}=n\text{\hspace{0.17em}}\left(\frac{340}{340-17}\right)=n\text{\hspace{0.17em}}\left(\frac{340}{323}\right)$

$⇒\frac{{f}_{1}}{{f}_{2}}=\frac{323}{306}=\frac{19}{18}$

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