Question 72: Waves

Question

Two cars X and Y are approaching each other with velocities 36 km/h and 72 km/h respectively. The frequency of a whistle sound as emitted by a passenger in car X, heard by the passenger in car Y is 1320 Hz. If the velocity of sound in air is 340 m/s, the actual frequency of the whistle sound produced is .................. Hz.

Accepted Answer

Image
V
x
= 36 km/hr = 10 m/s
V
y
= 72 km/hr = 20 m/s
by doppler's effect
$$F' = {F_0}\left( {{{V \pm {V_0}} \over {V \pm {V_s}}}} \right)$$
$$1320 = {F_0}\left( {{{340 + 20} \over {340 - 10}}} \right) \Rightarrow {F_0} = 1210$$ Hz

Waves

Integer / Numeric Answer Type