Wave Motion 4 Question 14
13. 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 $f _1$. If the train’s speed is reduced to $17 m / s$, the frequency registered is $f _2$. If the speed of sound is $340 m / s$, then the ratio $f _1 / f _2$ is
$(2000,2 M)$
(a) $18 / 19$
(b) $1 / 2$
(c) 2
(d) $19 / 18$
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Answer:
Correct Answer: 13. (c)
Solution:
- $f _1=f\left(\frac{v}{v-v _s}\right) \Rightarrow f _1=f\left(\frac{340}{340-34}\right)=f\left(\frac{340}{306}\right)$
$$ \begin{array}{ll} \text { and } & f _2=f\left(\frac{340}{340-17}\right)=f\left(\frac{340}{323}\right) \\ \therefore & \frac{f _1}{f _2}=\frac{323}{306}=\frac{19}{18} \end{array} $$
