Question

Two cars $\text{A}$ and $\text{B}$ are moving away from each other in opposite directions. Both the cars are moving with a speed of $20{\text{ms}}^{-1}$ with respect to the ground. If an observer in car $\text{A}$ detects a frequency $2000\text{Hz}$ of the sound coming from car $\text{B}.$ what is the natural frequency of the sound source in car $\text{B}$ ?
(speed of sound in air $=340{\text{ms}}^{-1}$)

• A. $2300\text{Hz}$
• B. $2150\text{Hz}$
• C. $2250\text{Hz}$
• D. $2060\text{Hz}$

Answer 1

The correct option is C $2250\text{Hz}$

From, Doppler's effect

$f^{\prime }=f_0\left(\frac{v\pm v_0}{v\pm v_S}\right)$

$\Rightarrow 2000=f_0\left(\frac{340-20}{340+20}\right)$

$\therefore f_0=2250\text{Hz}$
Hence, $\left(A\right)$ is the correct answer.