Waves Part 1

Given, velocity of car X,
vx=36kmh−1=36×

5

18

ms−1=10ms−1
Velocity of car Y,vy=72kmh−1
=72×

5

18

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′=

f(vs±vO)

(vs∓vs)

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=

f(340+20)

(340−10)

⇒1320=

360f

330

⇒f=1320×

11

12

=1210‌Hz
Thus, the actual frequency is 1210 Hz.