# Relativistic Addition of Velocities

1. Oct 29, 2008

### catch-22

1. The problem statement, all variables and given/known data

An electron moves to the right with a speed of 0.90c relative to the laboratory frame. A proton moves to the left with a speed of 0.70c relative to the electron. Find the speed of the proton relative to the laboratory frame.

2. Relevant equations

Vx'=[Vx-V]/[1+(v2/c2)*Vx]

3. The attempt at a solution

I have no idea where to start other than substituting 0.9c in for V.

Last edited: Oct 29, 2008
2. Oct 30, 2008

### Staff: Mentor

Using the Lorentz transformation for velocity is a perfectly fine approach. But that formula isn't correct. (Not even dimensionally.) So look up the correct formula.

Hint: Let the lab frame be the moving frame (primed) and let the proton's frame be the "stationary" frame (unprimed).

You can also just use the "addition of velocity" formula, which is derived from the Lorentz transformation (of course).

3. Oct 30, 2008

### catch-22

Ahh, got it. Our physics teacher gave us a slightly off formula, so it was cleared up today. Thanks!