Question
A siren emitting a sound of frequency $$800\,Hz$$ moves away from an observer towards a cliff at a speed of $$15\,m{s^{ - 1}}.$$ Then, the frequency of sound that the observer hears in the echo reflected from the cliff is
A siren emitting a sound of frequency $$800\,Hz$$ moves away from an observer towards a cliff at a speed of $$15\,m{s^{ - 1}}.$$ Then, the frequency of sound that the observer hears in the echo reflected from the cliff is
(Take, velocity of sound in air $$ = 330\,m{s^{ - 1}}$$ )
A.
$$800\,Hz$$
B.
$$838\,Hz$$
C.
$$885\,Hz$$
D.
$$765\,Hz$$
Answer :
$$838\,Hz$$
Solution :
According to question, situation can be drawn as follows.
Frequency of sound that the observer hear in the echo reflected from the cliff is given by
$$f' = \left( {\frac{v}{{v - {v_s}}}} \right)$$
where
$$f =$$ original frequency of source;
$$v =$$ velocity of sound
$${{v_s}} = $$ velocity of source
So, $$f' = \left( {\frac{{330}}{{330 - 15}}} \right)800 = 838\,Hz$$
According to question, situation can be drawn as follows.
Frequency of sound that the observer hear in the echo reflected from the cliff is given by
$$f' = \left( {\frac{v}{{v - {v_s}}}} \right)$$
where
$$f =$$ original frequency of source;
$$v =$$ velocity of sound
$${{v_s}} = $$ velocity of source
So, $$f' = \left( {\frac{{330}}{{330 - 15}}} \right)800 = 838\,Hz$$