Question

A detector is released from rest over a source of sound of frequency $f_0={10}^{3}Hz$. The frequency observed by the detector at time t is plotted in the graph. The speed of sound in air is: $(g=10m/s^2)$

• A. 330 m/s
• B. 350 m/s
• C. 300 m/s
• D. 310 m/s

Answer 1

The correct option is C 300 m/s

Initially the detector (observer) is at rest.

Velocity of observer after $30s$,

$v_o=0+gt=10\times 3=30$ $m/s$

Given : $f_o=1000$ Hz

$f=1100$ Hz

Using Doppler's effect in case an observer moves towards the stationary source :

$f=f_o[\frac{v+v_o}{v}]$ where $v$ is the speed of sound in air

$\therefore$ $1100=1000[\frac{v+30}{v}]$

$⟹v=300$ m/s