Question

The frequency of a car horn encountered a change from 400 Hz to 500 Hz, when the car approaches a vertical wall. If the speed of sound is 330 m/s. Then the speed of car is ______ km/h.

Options

• 132

• 130

• 129

• 131

Answer 1

The frequency of a car horn encountered a change from 400 Hz to 500 Hz, when the car approaches a vertical wall. If the speed of sound is 330 m/s. Then the speed of car is 132 km/h.

Explanation:

Case I:

Whenever the wall is viewed as an observer,

`ν_1 = ν_0[c/(c - v)]` ............(i)

Case II:

As a source, consider the wall,

`ν_2 = ν_1[(c + v)/c]` ..............(ii)

or, `ν_2 = ν_0[(c + v)/(c - v)]` ........using equation (i)

Substituting the given values,

`500 = 400[(c + v)/(c - v)]` (Here, C = 330 m/s)

`v = 330/9 xx 18/5`= 132 km/h