# Relativity speed of electrons and momentum

1. Sep 19, 2008

### racarr

I was wondering if someone could help me with a problem I've been struggling with for a bit. I'm just not quite equipped with the concepts to solve it. Two electrons have a relative speed of 0.9c. Calculate the momentum of each electron in the center-of-mass frame, the frame where they have equal and opposite momenta.

I don't really have an attempt at a solution yet, because I just don't know how to start. If someone could set me in the right direction, it would be greatly appreciated.

2. Sep 19, 2008

### olgranpappy

One thing you need to know is the equation for momentum (p) in terms of rest mass (m) and velocity (v). [Hint: It is not p=mv.]

To start off you should draw a picture of what the two electrons (and their respective velocities) look like in their center-of-mass frame. What directions are their velocities? What can you say about the magnitude of the velocity of each electron in the center-of-mass frame.

3. Sep 19, 2008

### tiny-tim

Welcome to PF!

Hi racarr! Welcome to PF!

Imagine that you're in the centre of mass frame (so the electrons have equal and opposite velocities).

What speed would you have to measure for each electron, for their relative speed (as measured in the frame of either electron) to be 0.9c?