Question

The driver of a car approaching a vertical wall notices that the frequency of the horn of his car changes from 400 Hz to 450 Hz after being reflected from the wall. Assuming speed of sound to be 340 m/s, the speed of approach of car towards the wall is

• A. 10 m/s
• B. 20 m/s
• C. 30 m/s
• D. 40 m/s

Answer 1

The correct option is B 20 m/s

$450=400\left(\frac{340+v_s}{340-v_s}\right)\Rightarrow \frac{9}{8}=\frac{340+v_s}{340-v_s}\Rightarrow 9\left(340\right)-9v_s=8\left(340\right)+8v_s\Rightarrow 17v_s=340\Rightarrow v_s=20m/s$