Question

A traffic policeman standing on a road sounds a whistle emitting the main frequency of 2.00 kHz. What could be the apparent frequency heard by a scooter-driver approaching the policeman at a speed of 36.0 km h⁻¹? Speed of sound in air = 340 m s⁻¹.

Answer 1

Velocity of sound in air v = 340 ms⁻¹
Velocity of scooter-driver $v_o$ ₌ 36 kmh⁻¹ = $36\times \frac{5}{18}=10{\mathrm{ms}}^{-1}$
Frequency of sound of whistle $f_o$ = 2 kHz

Apparent frequency $\left(f\right)$ heard by the scooter-driver approaching the policeman is given by:

$f=\left(\frac{v+v_o}{v}\right)\times f_o$

$$\begin{gathered} f=\left(\frac{340+10}{340}\right)\times 2 \\ =\frac{350\times 2}{340}\mathrm{kHz} \\ =2.06\mathrm{kHz} \end{gathered}$$