Question

# Sound of frequency 1000 Hz from a stationary source is reflected from an object approaching the source at $$30\, ms^{-1}$$, back to a stationary observer located at the source.The speed of sound in air is $$330 ms^{-1}$$.The frequency of the sound heard by the observer is

A
1200Hz
B
1000Hz
C
1090Hz
D
1100Hz

Solution

## The correct option is A $$1200 Hz$$Frequency recieved by approaching car$$f_1=f_0(1+\cfrac{30}{330})Hz$$where $$f_0=1000Hz$$$$f_1=1000(1+\cfrac{1}{11})Hz$$Frequency recieved by source again$$f_2=\cfrac{f_1}{1-\cfrac{30}{330}}=\cfrac{1000(1+\cfrac{1}{11})}{1-\cfrac{1}{11}}\\ \quad\quad=1200Hz$$Physics

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