# Relativistic Momentum

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1. May 15, 2017

### Curtis Cleary

1. The problem statement, all variables and given/known data
Hi all, I'm given an electron with momentum 2.0*10-20kgm/s and was asked to convert the momentum into units of Mev/c then calculate the total energy of the electron, the lorentz factor and the speed of the electron, I did this successfully but then the question got confusing, it goes like this.

An observer Z is moving with a a constant speed of 0.772c exactly opposite to the direction of motion. Calculate the momentum of the electron in the observers frame of reference. Compare this value to the value given in the question. I have no clue how to do this, I found a formula online for the lorentz transformation for momentum but don't know how to use it in this situation

2. Relevant equations

p'=gamma*(p-vE/c2)

3. The attempt at a solution

2. May 15, 2017

### TheReal NdGT

Hi Curtis,
I believe you know how to calculate ρ0, or the momentum in the particle's frame of reference. The particle from the observer, or Z's point of view, can be worked out by subtracting the velocity of the particle from the velocity of the observer. Then, plug it into the equation you have above, namely- ρ=γ(ρ-vE/c^2). To compare this with ρ0, I assume you find the ratio- one momentum divided by the other momentum. Hope this helps!