Q: 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 ration f1/f2 is
(a) 18/19
(b) 1/2
(c) 2
(d) 19/18
Ans: (d)
Sol: Using formula ,
$\large f’ = \frac{v-v_o}{v-v_s}\times f$
Here observer is at rest ,
$\large f’ = \frac{v}{v-v_s}\times f$
In first case ,
$\large f_1 = \frac{340}{340 – 34}\times f$ …(i)
In second case ,
$\large f_2 = \frac{340}{340 – 17}\times f$ …(ii)
on dividing (i) by (ii)
$\large \frac{f_1}{f_2} = \frac{19}{18}$