Question

A source of sound is travelling at $\frac{100}{3}m/s$ along a road, towards a point A. When the source is $3m$ away from A, a person standing at a point O on a road perpendicular to the track hears a sound of frequency $f$. The distance of O from A at that time is $4m$. If the original frequency is $640Hz$, then the value of $f$ is (given velocity of sound = $340m/s$)

• A. $620Hz$
• B. $680Hz$
• C. $720Hz$
• D. $840Hz$

Answer 1

The correct option is B $680Hz$

Component of velocity of source towards $O=v_s\cos \theta =\frac{100}{3}\times \frac{3}{5}m/s=20m/s$

Hence, the frequency as observed at $O=f_0\frac{v}{v-v_s\cos \theta }=640\times \frac{340}{340-20}Hz=680Hz$