Question

A car moving at a velocity of 17 $m{s}^{-1}$ towards an approaching bus that blows a horn at a frequency of $640Hz$ on a straight track. The frequency of this horn appears to be $680Hz$ to the car driver. If the velocity of sound in air is $340m{s}^{-1}$, then velocity of the approaching bus is

• A. 2 $m{s}^{-1}$
• B. 4 $m{s}^{-1}$
• C. 8 $m{s}^{-1}$
• D. 10 $m{s}^{-1}$

Answer 1

Answer: (B)

Given, velocity of sound, $v=340m/s$
Velocity of listner, $v_L=17m/s$
Velocity of source
$=v_S$ Frequency of horn emitted
$v=640Hz$

The apparent frequency
$v^{\prime }=v\frac{\left(v+v_L\right)}{v-v_s}$
$680=640\left(\frac{340+17}{340-v_S}\right)$
On solving we get $v_S=4m/s$