A source producing sound of frequency 720 Hz is falling freely from the top of a tower of height 20 m. The frequency of sound heard by an observer on the top of the tower when the source just reaches the ground is
(Acceleration due to gravity, `g=10ms^(-2)` and speed of sound in air `=340ms^(-1)`)

Options

A. 660 Hz
B. 680 Hz
C. 740 Hz
D. 760 Hz

Answer

B

Explanation

`because` Final velocity of source, `v_(s)^(2)=u_(s)^(2)+2gh`
`rArr " "v_(s)=sqrt(2gh) " "(because u_(s)=0)`
or `" "v_(s)=sqrt(2 times 10 times 20)=20 m//s`
`" "` [Given, h = 20 m, g = 10 `m//s^(2)`]
Frequency of sound heard by an observer,
`" "n^(.)=n(v/(v+v_(s)))`
Given, n = 720 Hz, v = 340 `ms^(-1)`
`rArr " "n^(.)=720(340/(340+20))=680 Hz`