Question

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

• A. $2$
  
• B. $4$
  
• C. $8$
  
• D. $10$
  

Answer 1

Answer: (B)

$4$

Doppler Effect

$v_o=$Velocity of observer$=17m/s$

$v_s=$Velocity of bus blowing horn$=?$

$v=$velocity of sound$=340m/s$

$n=$Actual frequency$=640Hz$

$n_a=$Apparent frequency$=640hz$

$n=n_a$

Observer$⟶⟵$Source

$n_a=\left(\frac{v+v_o}{v-v_s}\right)n$

$v-v_s=v+v_o$

$\therefore v_s=v_o$

There needs to be change in apparent frequency so as to have another value for velocity of bus or else the velocity of bus & observer is same.