Question

A car is approaching a railway crossing at a speed of 72 kmph. It sounds a horn, when it is 800 m away, at 600 Hz. If the velocity of sound in air is 330 ms${}^{-1}$, the apparent frequency as received by a man at rest near the railway track perpendicular to the road at a distance of 600 m from the crossing is

• A. 653 Hz
• B. 365.5 Hz
• C. 638.5 Hz
• D. 563.5 Hz

Answer 1

The correct option is C 638.5 Hz

Apparent frequency=$f$

Velocity of sound, $v=330m{s}^{-1}$

$f=600Hz$

$v_s=72kmph=20m{s}^{-1}$

$v_0=0$

$f=f_0\frac{v\pm v_0}{v+v_s}$

$=638.71Hz$