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## Homework Statement

Suppose that a police car on the highway is moving to the right at 28 m/s, while a speeder is coming up from almost directly behind at a speed of 34 m/s, both speeds being with respect to the ground. The police officer aims a radar gun at the speeder. Assume that the electromagnetic wave emitted by the radar gun has a frequency of 10.0x10^9 Hz.* Find the magnitude of the difference between the frequency of the emitted wave and the wave that returns to the police car after reflecting from the speeder's car.

[URL=http://img46.imageshack.us/my.php?image=doppleryd8.png][PLAIN]http://img46.imageshack.us/img46/6098/doppleryd8.th.png[/URL][/PLAIN]

## Homework Equations

doppler Effect equation

## The Attempt at a Solution

i know the doppler equation, but where i am confused is relative velocity. i have tried working this problem out numerous ways: i have subtracted to the two speeds, i have added their speeds, i dont know how to come to the right conclusion in terms of relative speed. i noticed my book says that "do not use the speed of the observer or of the source with respect to the ground in the doppler equation." If that is the case, then what am i supposed to use??? Also, one object is moving away, while one is moving towards the other. Do i use the plus or minus sign in the doppler equation. i would say the + sign since the speeder is moving faster than the cop car and will eventually pass it.
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