Question

A person riding a car moving at 72 km/h sounds a whistle emitting a wave of frequency 1250 Hz. What frequency will be heard by another person standing an the road (a) in front of the car (b) behind the car ? Speed of sound in air = 340 m/s.

Answer 1

(a)Given : v_s =72 km/hour

=20 m/s, F =1250 Hz

Apparent frequency

$=\frac{340+0+0}{340+0-20}\times 1250$

=1328 Hz

(b)For second case, Apparent frequency

$=\left(\frac{340+0+0}{340+0-(-20)}\right)\times 1250$

$=\frac{340}{360}\times 1250=1181Hz$