Given, velocity of car X,
vx=36kmh−1=36×ms−1=10ms−1 Velocity of car
Y,vy=72kmh−1 =72×ms−1=20ms−1 Frequency of sound heard by passenger in car Y emitted by passenger in car X, f′ =1320 Hz
Velocity of sound,
vs = 340 m/s.
By Doppler’s effect,
f′= Velocity of observer is equal to velocity of car,
vo=20ms−1 Velocity of source is equal to velocity of car X,
vs=10ms−1 Substituting the values in above expression, we get
1320= ⇒1320= ⇒f=1320×=1210‌Hz Thus, the actual frequency is 1210 Hz.