Question

A source and a detector move away from each other in absence of wind with a speed of 20 m/s with respect to the ground. If the detector detects a frequency of 1800 Hz of the sound coming from the source, then the original frequency of the source considering the speed of sound in air 340 m/s will be ______ Hz.

Options

• 2000

• 2021

• 2011

• 2025

Answer 1

A source and a detector move away from each other in absence of wind with a speed of 20 m/s with respect to the ground. If the detector detects a frequency of 1800 Hz of the sound coming from the source, then the original frequency of the source considering the speed of sound in air 340 m/s will be 2025 Hz.

Explanation:

Using the Doppler formula,

`f_"app" = f_0((V - V_D)/(V - V_S))`

Using the sign convention here,

V_S = -20 m/s ..(moving in the opposite direction of the sound)

V_D = 20 m/s ...(moving in the direction of the sound)

`f_"app" = f((340 - 20)/(340 + 20))`

⇒ `1800 = f(320/360)`

f = 2025 Hz.