KVPY Sample Paper KVPY Stream-SX Model Paper-24

A train moves towards a stationary observers with 34 m/s. The train sounds a whistle and its registered by the observer is \[{{f}_{1}}.\]If the speed \[{{f}_{2}}.\] train is reduced to 17 m/s, the frequency registered is \[{{f}_{2}}.\]If speed of sound is 340 m/s, then the ratio \[{{f}_{\text{1}}}\text{/}{{f}_{2}}\] is:

A) 18/17

B) 19/18

C) 20/19

D) 21/20

Correct Answer: B

Solution :

\[{{f}_{1}}={{f}_{0}}\left( \frac{340}{340-17} \right)\]

\[{{f}_{2}}={{f}_{0}}\left( \frac{340}{340-34} \right)\]

\[\frac{{{f}_{1}}}{{{f}_{2}}}=\frac{306}{323}\]or \[\frac{18\times 17}{19\times 17}\]or\[\frac{18}{19}.\]