Q 11-14-129JEE MainJEE Main 2019 (12 Apr, Shift 2)Medium
Two sources of sound $S_1$ and $S_2$ produce sound waves of same frequency $660$ Hz. A listener is moving from source $S_1$ towards $S_2$ with a constant speed $u_0$ m/s and he hears $10$ beats/s. The velocity of sound is $330$ m/s. Then, $u_0$ equals:
Answer: (B) $2.5$ m/s
Moving away from $S_1$: $f_1 = f\dfrac{v - u_0}{v}$. Moving towards $S_2$: $f_2 = f\dfrac{v + u_0}{v}$.
$$f_2 - f_1 = \frac{2fu_0}{v} = 10 \Rightarrow u_0 = \frac{10\times330}{2\times660} = 2.5\ \text{m/s}$$
Solution by Sreeraj P, M.Sc Physics