# How do I choose u' u and v for relative velocity

1. Mar 26, 2015

### grandpa2390

1. The problem statement, all variables and given/known data
A certain quasar recedes from the earth with a speed of .87c. a jet of material is ejjected towards the earth at .55c relative to the quasar. find speed of ejected material relative to the earth

2. Relevant equations
u = u'+v / 1 + u'v/c^2

3. The attempt at a solution
guessing. that if I draw both reference frames, v is the velocity that stays constant. in this case v=.87c. in one reference frame, the quasar is receding, and in the other the earth is receding from the quasar. either way there is a .87c and I will call it v.

u is the velocity in the reference frame that it is given. .55c is the velocity of the ejected material in the quasar's reference frame. I want it in the Earth's reference frame though, so I call the known velocity in the quasar's reference frame u and the unknown velocity in the earth's reference frame u'.

and as long as I keep my signs straight, this should work right?

scratch that. the signs on u and v must be from the same reference frame.

Last edited: Mar 26, 2015
2. Mar 26, 2015

### Staff: Mentor

You might find this way of writing the velocity addition formula helpful:
Where $V_{a/c}$, for example, means the velocity of a as measured by c. That should allow you to keep the signs straight.