A driver travels northbound on a highway at a speed of 20.0 m/s. A police car, t

Question

A driver travels northbound on a highway at a speed of 20.0 m/s. A police car, traveling southbound at a speed of 42.0 m/s, approaches with its siren producing sound at a frequency of 2500 Hz. What frequency does the driver observe as the police car approaches? What frequency does the driver detect after the police car passes him? Repeat parts (a) and (b) for the case when the police car is travelling northbound. while police car overtakes after police car passes The sound intensity at a distance of 6 m from a noisy generator is measured to be 0.23 W/m^2. What is the sound intensity at a distance of 35 m from the generator?

Explanation / Answer

answer 9

(a)

using doppler shift formula

f = fo*(V+Vo) / (V+Vs)

where fo = the frequency emitted by the source = 2500 Hz

f = the frequency heard by the receiver

V = speed of sound = 340 m/s

Vo = speed of the receiver relative to medium = 20 m/s

Vs = speed of the source relative to medium = 42 m/s

(a)

frequency does the driver observe as the police car approaches

f = 2500*(340+20) / (340-42)

f = 3020.13 Hz

(b)

frequency does the driver detect after the police car passes him

Vo = negative (receiver away from source)

Vs = positive( source away from receiver)

so f = 2500*(340-20) / (340+42)

f = 2094.24 Hz

(c)

while police car overtakes

f = 2500*(340-20) / (340-42)

f = 2684.56 Hz

after police car passes

f = 2500*(340+20) / (340+42)

f = 2356.02 Hz

answer 10

given that

I = 0.23 W/m^2

r1 = 6 m

r2 = 35 m

we know that ,

intensity is inversely proportional to distance squared. so,

0.23 * 6^2 = I' * 35^2

I' = 0.23*36 / 1225

I' = 0.0067 W/m^2

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