Question
A bystander hears a siren vary in frequency from 554 Hz to 390 Hz as a...
A bystander hears a siren vary in frequency from 554 Hz to 390 Hz as a fire truck approaches, passes by, and moves away on a straight street. What is the speed of the truck? (Take the speed of sound in air to be 343 m/s.)
Homework Answers
Answer #1
Use Formula 
where 
is the apparent frequency ,
is the actual frequency,
is the speed of sound
is the speed of observer
is the speed of truck
-----------------------------
Case 1: Fire truck approaching
(observer is standing still)
----------
-------------(1)
--------------------------------
Case 2: Fire truck moves away
(observer is standing still)
(direction of speed of truck and direction of speed of sound is in
opposite direction)
----------
---------------(2)
-----------
From (1) and (2)
ANSWER: Speed of truck =59.65 m/s
===============================
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