Waves - Result Question 80

82. Two trains move towards each other with the same speed. The speed of sound is 340m/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

(a) 20m/s

(b) 2m/s

(c) 200m/s

(d) 2000m/s

[1991]

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Answer:

Correct Answer: 82. (a)

Solution:

  1. (a) Here, v=98v

Source and observer are moving in opposite direction, therefore, apparent frequency

v=v×(v+u)(vu)

98v=v×(340+u340u)

9×3409u=8×340+8u

17u=340×1u=34017=20m/sec.



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