Question

# A boy blowing a whistle, is running away from a wall towards an observer with a speed of  $$1\ ms^{-1}$$. The frequency of whistle is $$680\ Hz$$. The number of beats heard per second by the observer will be $$($$given $$v = 340\ ms^{-1})$$

A
zero
B
2
C
4
D
8

Solution

## The correct option is A 2Here a boy blowing a whistle, is running away from a wall towards an observer hence, the apparent frequency (of approaching) detected by observer is$$n' = \dfrac{v}{v - u} n$$where$$,\ u$$ is the speed of speed of boy running towards observer$$,\ n$$ is frequency of whistle sound$$,\ v$$ is velocity of sound in air.$$n' = \dfrac{340}{340 - 1} (680)$$ .......................(given)$$n' = \dfrac{340}{339} (680)$$$$n' \approx 682 Hz$$Hence, the number of beats per second heard by the observer is  $$682 - 680 = 2$$Physics

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