Why does perpendicular source motion produce no Doppler shift?

[undividable]

Homework Statement

Why does a source moving perpendicular to the line between it and the observer produce no doppler effect?

Homework Equations

The Attempt at a Solution

I have heard the the radial velocity of the source relative to the observer must be zero but i don't understand why, if the distance between the source and the observer is increasing or decreasing , shouldn't there be a doppler effect since the wave length is changing?

 

[PeroK]

Homework Statement

Why does a source moving perpendicular to the line between it and the observer produce no doppler effect?

Have you tried to figure out what sort of motion that represents?

 

[scottdave]

Try to think of a type of motion, where one object is stationary, and the other object is always moving, yet does not get any closer or farther from the fixed point.

 

[undividable]

i guess my real question is, how does the doppler effect work when the observer and source are not moving in a straight line, what do we take in as the velocity of the observer and the source in the doppler effect equation?