Question

A man is watching two trains, one leaving and the other coming in with equal speed of 4 m/s. If they sound their whistles, each of frequency 240 Hz, the number of beats heard by the man (velocity of sound in air is 320 m/s) will be equal to

• A. 6
• B. 3
• C. 0.0
• D. 12

Answer 1

The correct option is A 6

Apparent frequency due to train which is coming in is
$m_1=\frac{v}{v-v_s}n$
Apparent frequency due to train which is leaving is
$m_2=\frac{v}{v-v_s}n$
So the number of beats is
$n_1-n_2=(\frac{1}{316}-\frac{1}{324})320\times 240\Rightarrow n_1-n_2=6$