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songoku
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Homework Statement
An ambulance is running on an expressway at a speed of 60 km/h from east to west (from A to B) with a siren of 880 Hz. Let the frequency of the siren sound detected by an observer located at a point O be Va and Vb when the vehicle just passes point A and point B respectively. Point C is just north of O and OC = AC = BC = 100 m. Here the sound velocity in calm air is 340 m/s.
a. In the case of calm weather (no wind), what is the approximate value of (Va / Vb) - 1
b. In the case of an east wind of 5 m/s, what is the approximate value of (Va / Vb) - 1
c. In the case of an north wind of 5 m/s, what is the approximate value of (Va / Vb) - 1
Homework Equations
[tex]f ' = \frac{V sound \pm V observer}{V sound \mp Vsource} f [/tex]
The Attempt at a Solution
Angle CAO = 45 degree
V source = 60 km/h cos 45 = 50/3 cos 45 m/s
V observer = 0
a. [tex]Va = \frac{340}{340 - 50/3 cos 45} 880 [/tex]
[tex]Vb = \frac{340}{340 + 50/3 cos 45} 880 [/tex]
(Va / Vb) - 1 = 0.07
b. Because east wind is from A to B as well, so V source = (50/3 + 5) cos 45 ?
c. Taking OA as x-axis, I break the velocity of north wind to other components and get the velocity along x-axis
V source = (50/3 - 5) cos 45 ?
thx