Question

A car with a horn of frequency 620Hz travels towards a large wall at a speed of 20m/s. If the velocity of sound is 330m/s, the frequency of echo of sound of horn observed by the driver is

• A. 700Hz
• B. 660Hz
• C. 620Hz
• D. 580Hz

Answer 1

The correct option is A 700Hz

Due to doppler effect the change of frequency is given by:

$\overline{n}=n\frac{V-V_0}{V-V_s}$. In this situation $V_s=V_0=20m/s$ but

opposite in direction. So using this in the above formula:

$\overline{n}=n\frac{V+20}{V-20}=700Hz$.