Waves Ques 81
81. Two trains move towards each other with the same speed. The speed of sound is $340 $ $m / s$. If the height of the tone of the whistle of one of them heard on the other changes $9 / 8$ times, then the speed of each train should be
[1991]
(a) $20 $ $m / s$
(b) $2 $ $m / s$
(c) $200 $ $m / s$
(d) $2000 $ $m / s$
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Answer:
Correct Answer: 81.(a)
Solution:
- (a) Here, $v^{\prime}=\frac{9}{8} v$
Source and observer are moving in opposite direction, therefore, apparent frequency
$v^{\prime}=v \times \frac{(v+u)}{(v-u)}$
$\frac{9}{8} v=v \times(\frac{340+u}{340-u})$
$\Rightarrow 9 \times 340-9 u=8 \times 340+8 u$
$\Rightarrow 17 u=340 \times 1 \Rightarrow u=\frac{340}{17}=20$ $ m / sec$.
BETA
