Question

A person carrying a whistle emitting continuously a note of 272 Hz is running towards a reflecting surface with a speed of $18km{h}^{-1}$. The speed of sound in air is $345m{s}^{-1}$. The number of beats heard by him is :

• A. 4
• B. 6
• C. 8
• D. zero

Answer 1

The correct option is C 8

Velocity of source $=18$km/h = 5 m/s
(i) Source moves towards listener ($u_s$)
(ii) listener moves towards source ($u_r$)
${\nu }^{\prime }=\frac{v+u_r}{v-u_s}{\nu }_0=\frac{345+5}{345-5}272=280$ Hz
Beats $={\nu }^{\prime }-\nu =8Hz$