Multiple-Concept Example 6 explores the concepts that areinvolved in this proble
ID: 1741600 • Letter: M
Question
Multiple-Concept Example 6 explores the concepts that areinvolved in this problem. A police car is moving to the right at 30m/s, while a speeder is coming up from behind at a speed 35 m/s,both speeds being with respect to the ground. The police officerpoints a radar gun at the oncoming speeder. Assume that theelectromagnetic wave emitted by the gun has a frequency of 7.3 x109 Hz. Find the difference between the frequency of thewave that returns to the police car after reflecting from thespeeder's car and the frequency emitted by the police car.
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Chapter 24, Problem 46Explanation / Answer
as from the given question we can see that there aretwo doppler frequency changes in the
emitted wave in this case
first, the speeder's car observes the wave frequencycoming from the radar gun to have a
frequency that is different from the emitted(source) frequency fs
the wave then reflects and returns to the police car,where it is observed to have a frequency that is different than its frequency at theinstant of reflection so when the source and the observer of theradar are approaching each other, the magnitude of the difference between frequency ofthe emitted wave and the wave that returns to the police car after reflecting from thespeeder's car is fo' - fs =- 2 fs(vrel / c) where vrel is the relative speedbetween the speeding car and the police car vrel = 35 m / s - 30 m/ s = 5 m /s so the total doppler change in the frequency willbe fo' - fs = 2 (7.3 x109 Hz) (5 m / s / 3.0 x 108 m / s) = ........ Hz
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