Question

A car going towards a fixed platform with speed $60\text{km/hr}$ blows a horn of frequency $400Hz$. A wind of speed $40\text{km/ hr}$ is blowing in the direction of motion of car. Find frequency of horn as heard by a observer standing on that platform (approximate). Velocity of sound $(v=1200\text{km/hr})$

• A. $400Hz$
• B. $420Hz$
• C. $440Hz$
• D. $460Hz$

Answer 1

Answer: (B)

$420Hz$

According to Doppler's effect

Apparent frequency $f^{\prime }=\left\{\frac{(v+w)}{(v+w)-v_s}\right\}\times f$

$f^{\prime }=\left\{\frac{1200+40}{(1200+40)-60}\right\}\times f$

$f^{\prime }=\left(\frac{1240}{1180}\right)\times 400$

$f^{\prime }=420.3Hz$
$f^{\prime }\simeq 420Hz$