Question

A person with vibrating tuning fork of frequency $338 \,Hz$ is moving towards a vertical wall with a speed of $2\, m\,s^{-1}$. Velocity of sound in air is $340\, m\,s^{-1}$. The number of beats heard by that person per second is

• A. 2
• B. 4
• C. 6
• D. 8

Answer 1

Answer: (B)

4

Answer 2

As the person having tuning fork is moving towards a wall, therefore,
$v_{s}=v_{0}=2 \,m / s$ (given)
So, $\frac{f'}{f}=\frac{v+v_{0}}{v-v_{0}}$
$\Rightarrow \frac{f'-f}{f}=\frac{v+v_{0}-v+v_{0}}{v-v_{0}}$
$\Rightarrow \frac{\Delta f}{f}=\frac{2 v_{0}}{v-v_{0}}$
or $\Delta f=\frac{2 v_{0}}{v-v_{0}} \times f=\frac{2 \times 2 \times 338}{(340-2)}=4$