Waves - Result Question 74

76. A car is moving towards a high cliff. The car driver sounds a horn of frequency f. The reflected sound heard by the driver has frequency 2f. If v be the velocity of sound, then the velocity of the car, in the same velocity units, will be

[2004]

(a) v/2

(b) v/2

(c) v/3

(d) v/4

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Answer:

Correct Answer: 76. (c)

Solution:

  1. (c) Let f be the frequency of sound heard by cliff.

(1)f=vfvvc

Now, for the reflected wave, cliff acts as a source,

2f=f(v+vc)v

2f=(v+vc)fvvc

2v2vc=v+vc or v3=vc



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