Question

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

• A. zero
• B. 2
• C. 4
• D. 8

Answer 1

Answer: (B)

2

Here a boy blowing a whistle, is running away from a wall towards an observer hence, the apparent frequency (of approaching) detected by observer is
$n^{\prime }=\frac{v}{v-u}n$
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.
$n^{\prime }=\frac{340}{340-1}\left(680\right)$ .......................(given)
$n^{\prime }=\frac{340}{339}\left(680\right)$
$n^{\prime }\approx 682Hz$
Hence, the number of beats per second heard by the observer is $682-680=2$