A siren emitting a sound of frequency `1000 H_(Z)` moves away from you towards a cliff at a speed of `10 m//s` .
(a) What is the frequency of the sound you hear coming directly from the sirven ?
(b) What is the frequency of sounds you hear reflected off the cliffb ?
(c) What beat frequency would you hear ? Take the speed of sound in air as `330 m//s` .

Answer

A, C

Explanation

The situation is as shows in figure.
(a) Frequency of sound reaching directly to us (by `S`)
`f_(1) = ((upsilon)/(upsilon + upsilon _(s)))`
`f= ((330)/ (330 +10))(1000)`
= `970. 6 H_(Z)`
Frequency of sound which is reflected from the cliff (from `S')`
`f_(2) = ((upsilon)/(upsilon - upsilon_(s)))f = ((300)/(330 - 10))(1000)`
= `1031.3 H_(Z)`
(c ) Beat frequency = `f_(2) - f_(1)`
= `60.7 H_(Z)`