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.

One engine moves with a speed 10 m/s towards a cliff (high hill). It b

1.	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.
Answer» SOLUTION : 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`