Question

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

• A. 18/19
• B. 44563
• C. 2
• D. 19/18

Answer 1

Answer: (D)

19/18

Answer 2

\hspace15mm f_1=f \bigg(\frac{v}{v-v_s}\bigg)
\hspace15mm f_1=f \bigg(\frac{340}{340-34}\bigg)= f\bigg(\frac{340}{306}\bigg)
and \hspace8mm f_2=f \bigg(\frac{340}{340-17}\bigg)= f\bigg(\frac{340}{323}\bigg)
\therefore\hspace10mm \frac{f_1}{f_2}=\frac{323}{306}= \frac{19}{18}