# Homework Help: Relativistic Velocity Transformations

1. Dec 12, 2011

### jayjay713

A quasar is moving away from the earth with a speed of 0.850C. It emits a proton that eventually reaches earth, and is travelling at a speed of 0.519C relative to the earth. How fast is the proton moving relative to the quasar?

Is this answer as simple as it seems?

is the answer simply 0.850 + 0.519 = 1.369C?
i am confused
thanks for any help

2. Dec 12, 2011

### BetoG93

No, you can't have a speed greater than c.
In relativity, you must use Lorentz's velocity transformation equations.

3. Dec 14, 2011

### jayjay713

but if I used that formula, wouldn't it give me a speed relative to the earth? Or would it be relative to the quasar? :O

4. Dec 14, 2011

### Dovahkiin

Well you know the quasars velocity relative to the earth and you know the protons velocity relative to the earth. So you know the protons velocity in the Earths rest frame, and you can lorentz transform it into the quasar rest frame using the quasar velocity. Don't forget the signs of the velocities! Post your answer when you have worked it out :)