# Finding Speed using Doppler Shift

## Homework Statement

Light with a frequency of 6.1(1014) Hz is measured from a star known to produce light with a frequency of 6.0(1014). How fast is the star moving toward or away from earth?

## Homework Equations

f = f0(v + vo)/(v-vs.

f=observed frequency
f0 = original frequency
v=wave speed
vo = wave of observer
vs=wave of source

## The Attempt at a Solution

6.1(1014) = 6.0(1014)(3.0x108 + 0)/(3.0x108 + x)

-> (3.0x108 + x) = 6.0(1014)(3.0x108 + 0)/6.1(1014)

-> x = ( 6/6.1 x (3.0(108) ) - 3.0x108

-> -4918032.787 m/s
Moving away at 4918032.787 m/s

I feel that I'm doing it wrong since shouldn't it be moving toward earth since the frequency observed is higher?

NascentOxygen
Staff Emeritus
Your equation was: f = f0(v + vo)/(v-vs)
where vs is the speed towards the observer which gives that higher f.

You changed it, for some reason, and used +vs
which, to still be correct, means your vs is now the negative of its speed towards the observer.

You calculated this to be -4918032.787 m/s,
so take the negative of that and you have the speed towards the observer. 