# Homework Help: Doppler effect

1. Jun 6, 2014

### kelvin macks

1. The problem statement, all variables and given/known data

can anyone explain why reletive velocity is (v-u), based on my understanding, as the obesrvers moving away, the distance of the f th wavefront and first waavefront increases. so i would say it's v+u . correct me if i'm wrong. thanks.

2. Relevant equations

3. The attempt at a solution

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2. Jun 6, 2014

### paisiello2

The distance between wave fronts appears to increase, yes, but that is because the relative speed is also decreasing as the observer accelerates away. If u=v then the wave would never reach you.

3. Jun 6, 2014

### dauto

The wave and the observer are moving in the same direction. Their relative speed is v-u. If they were moving in the opposite direction than the relative speed would be v+u.

Last edited: Jun 6, 2014
4. Jun 6, 2014

### CWatters

If two cars travel on the freeway at similar speeds in the same direction the relative velocity between them is small. It's not going to be the sum of their speeds.