Question

Two trains, one coming towards and another going away from an observer both at 4 m/s produce a whistle simultaneously, of frequency 300 Hz. The number of beats heard by observer will be (velocity of sound = 340 m/s)

• A. 5
• B. 6
• C. 7
• D. 12

Answer 1

The correct option is C 7

Number of beats $=f\left(\frac{v}{v-v_{\text{s}}}\right)-f\left(\frac{v}{v+v_{\text{s}}}\right)\approx \frac{2f{v}_{\text{s}}}{v^2}$ $(\because v_{\text{s}}<<v)$
$=\frac{2\times 300\times 4}{340}\approx 7$