Question

Two aeroplanes A and B are moving away from one another with a speed of 720 kmph. The frequency of the whistle emitted by A is 1100 Hz. The apparent frequency of the whistle as heard by the passenger on the aeroplane B is (velocity of sound in air is 350 ms${}^{-1}$ )

• A. 300Hz
• B. 400Hz
• C. 500Hz
• D. 600H

Answer 1

The correct option is A 300Hz

Given : $V_0=720km/h=720\times \frac{1000}{3600}=200m/s$

$v_s=-720km/h=200m/s$

$v=350m/s$

$\delta =1100hz$

By Doppler effect formula

${\delta }_1=\delta \left(\frac{V-V_0}{V+V_s}\right)$(source and listener are moving away from each other)

$=1100\left(\frac{350-200}{350+200}\right)$

$=\frac{1100\times 150}{550}$

$=300Hz$