A) 3
B) 6
C) 9
D) zero
Correct Answer: B
Solution :
Key Idea: The number of beats is the difference of apparent frequencies of two trains. From Doppler?s effect of sound when source is moving towards observer perceived frequency. \[n'=n=\frac{v}{v-{{v}_{s}}}\] where, v is velocity of sound,\[{{v}_{s}}\]is velocity of source. \[n'=\frac{320}{320-4}\times 240\] \[=\frac{320}{316}\times 240\] \[=243\text{ }Hz\] when source is moving away from observer, perceived frequency \[n''=n\frac{v}{v+{{v}_{s}}}\] \[=240\times \frac{320}{320+4}\] \[=237\text{ }Hz\] Number of beats per second (beat frequency) \[=n'-n''=\]difference of the frequency of sound source\[\Rightarrow \]\[=243-237=6\]You need to login to perform this action.
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