Thread: Doppler's effect

I was just reviewing the Doppler effect, and I thought of something :

the formula involves (v-vr)/(v-vs)
let's say v = 300
vr = 30
vs = 0 (stationary observer)

it will be equal to 270/300

however, if we take the opposite, i.e. consider the reference frame to be that the source is moving not the observer we get

330/300

The 2 are obviously not equal, so is the formula just an approximation?

No, if the observer and source are approaching each other, it is always 270/300.

Originally Posted by epok88

I was just reviewing the Doppler effect, and I thought of something :

the formula involves (v-vr)/(v-vs)
let's say v = 300
vr = 30
vs = 0 (stationary observer)

it will be equal to 270/300

however, if we take the opposite, i.e. consider the reference frame to be that the source is moving not the observer we get

330/300

The 2 are obviously not equal, so is the formula just an approximation?

In the first example, the sender is stationary relative to the medium, though you called him the "observer" and the receiver is moving away from the sender so the frequency decreases. If the sender is moving then you have to add to the denominator instead of subtracting from the numerator. It actually does give you a different frequency shift.