A) 12
B) zero
C) 3
D) 6
Correct Answer: D
Solution :
Key Idea: The number of beats heard by man is the difference of apparent frequencies of two trains. From Doppler?s effect, the perceived frequency, when train is approaching the man is given by \[f'=\frac{v}{v-{{v}_{s}}}f\] Where \[v\] is speed of sound, \[{{v}_{s}}\] is speed of source. Given, \[v=320\,\,m/s,\,{{v}_{s}}=4\,m/s,\,f=240\,Hz\] \[f'=\frac{320}{320-4}\times 240\] When train is receding the perceiveding frequency is \[f'\,'\,=\frac{v}{v+{{v}_{s}}}f\] \[=\frac{320}{320+4}\times 240\] \[=237\,\,Hz\] Number of bass per second (beat frequency) \[=f'-f'\,'=\]difference of the apparent frequencies of sound sources. \[\Rightarrow \] \[n=240-237=6\]You need to login to perform this action.
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