A) 529.2 Hz
B) 295.2 Hz
C) 440.5 Hz
D) none of these
Correct Answer: A
Solution :
Key Idea: The apparent frequency of the two cars should be equal for no beats. From Dopplers effect the apparent change in frequency of a wave that is perceived by stationary observer is \[n={{n}_{0}}\left( \frac{v}{v-{{v}_{s}}} \right)\] where v is speed of sound. Since observer is not hearing any beats the apparent frequency of the two cars should be equal. \[\therefore \] \[n=n\] \[504\left( \frac{v}{v-30} \right)=n\left( \frac{v}{v-15} \right)\] \[\therefore \] \[n=\frac{(330-15)\times 504}{(330-30)}\] \[=\frac{315\times 504}{300}=529.2Hz\]
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