Question

(10 pts) A police car siren emits 120W of sound at a frequency of 1200Hz a. What is the intensity of the sound in decibels at a distance of 500m from the siren? b. If the police car, travelling east at 200 km/hr chases a speeding motorist going east at 140 km/hr. What is the siren frequency as heard by the motorist?

Answer 1

7.

(a)

$I=\frac {P}{4\pi r^2}=\frac {120}{4\pi \cdot 500^2}\: W/m^2= 3.82\times 10^{-5}\: W/m^2$

(b)

$f=\frac {V-v_{m}}{V+v_{source}}f_{o}$

$f=\frac {343-140}{343+200}\cdot 1200\: Hz= 448.62\: Hz$

8.

$R=\frac {KA(T_2-T_1)}{l}=\frac {1\cdot 90\cdot 60\cdot 10^{-4}\cdot (288.70-266.48)}{0.004}\: J/s\approx 2990\: W$