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 Dopplers effect, the perceived frequency, when train is approaching the man is given by \[\Rightarrow \] where v is speed of sound, \[{{n}_{2}}=\frac{50}{49}\times 392=400\] is speed of source. Given, \[={{n}_{2}}-{{n}_{1}}=\] \[=400-392=8\] \[y=2\,a\,\sin \,\frac{2\pi }{\lambda }\,x\cos \frac{2\pi }{\lambda }ct\] When train is receding the perceive ding frequency is \[\lambda \] \[y=5\sin \frac{\pi \,\,x}{3}\,\cos \,40\,\pi \,t\] \[y=5\sin \frac{\pi \,\,x}{3}\,\cos \,40\,\pi \,t\] Number of beats per second (beat frequency) \[m/{{s}^{2}}\]difference of the apparent frequencies of sound sources. \[y=a\cos (\omega t-kx),\] \[\left[ {{M}^{0}}{{L}^{-1}}{{T}^{-1}} \right]\]
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