Question

A train moving at a speed of $220m{s}^{-1}$ towards a stationary object, emits a sound of frequency $1000Hz$. Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is- (speed of sound in air is $330m{s}^{-1})$

• A. $4000Hz$
• B. $5000Hz$
• C. $3000Hz$
• D. $3500Hz$

Answer 1

The correct option is B $5000Hz$

By applying Doppler effect,
$n^{\prime }=n\left(\frac{v+v_s}{v-v_s}\right)$
$=1000\left(\frac{330+220}{330-220}\right)$
$=1000\left(\frac{550}{110}\right)$
$=5000Hz$