Question

Two factories are sounding their sirens at 800 Hz. A man goes from one factory to the other at a speed of 2 m/s. The velocity of sound is 320 m/s. The number of beats heard by the person in 1 s will be

• A. 2
• B. 4
• C. 8
• D. 10

Answer 1

The correct option is D 10

When the man is approaching the factory,
$n^{\prime }=\left(\frac{v+v_0}{v}\right)n=\left(\frac{320+2}{320}\right)800=\left(\frac{322}{320}\right)800$
When the man is going away from the factory,
$n"=\left(\frac{v-v_0}{v}\right)n=\left(\frac{320-2}{320}\right)800=\left(\frac{318}{320}\right)800\therefore n^{\prime }-n"=\left(\frac{320-318}{320}\right)800=10Hz$