- #1

k77i

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## Homework Statement

A radar device emits microwaves with a frequency of 3.20e+09 Hz. When the waves are reflected from a van moving directly away from the emitter, the beat frequency between the source wave and the reflected wave is 838 beats per second. What is the speed of the van? (Note: microwaves, like all forms of electromagnetic radiation, propagate at the speed of light c = 3.00e+08 m/s.)

## Homework Equations

frequency of beat = |f1-f2|

f' = [(v-vo)/v]fo

where v is the speed of emitted frequency (the speed of light on this case), vo is the speed of the observer and fo is the frequency of the observer

The above equation is for when the observer is moving away from the source and the source is stationary

## The Attempt at a Solution

First i found the frequency of the observer (van) by using the first equation:

f(beat) = |f1-f2|

838 = |3.20e+09 - f2|

So f2 = 3 199 999 162

Then i used the second equation to find v0 (speed of the van):

f' = [(v-vo)/v]fo

3 199 999 162 = [(3.00e+08 - vo)3.00e+08]3.20e+09

this gave me vo = 78.56m/s, but according to the homework it's not right

Is it beacause im not supposed to consider f2 as f'? If not im not exactly sure how to solve for vo.