Understanding the Doppler Effect: Relative Velocity Calculation Explained

[kelvin macks]

Homework Statement

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.

Homework Equations

The Attempt at a Solution

 

[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.

 

[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.

 

[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.