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One engine moves with a speed 10 m/s tow...

One engine moves with a speed 10 m/s towards a cliff (high hill). It blows a whistle with frequency 660 Hz. Find the frequency of an echo (from cliff), heard by driver of this engine. Speed of sound in air is 340 m/s.

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Here for reflected sond, we can consider its imaginary source S. moving towards the cliff with a velocity equal in magnitude and opposite in direction to that of real source of sound. Here, according to sign convention out of `v_(L) and v ._(S)` a velocity in a directon from listener to imaginary source is to be taken positive. Hence `v _(L) = 10 m//sand v._(S) =-10 m//s.`
Now, we have,
`(f _( L))/( v + v _(L)) = (f _(S))/( v + v ._(S))`
`therefore ( f _(L))/( 340 + 10) = (660)/(340 -10)`
`therefore f _(L) = 660 xx (350)/(330 ) = 700 Hz`
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