Homework Help: Why does horizontal velocity of electron slow down when moving in an electric field?

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

This is from "Special Relativity" by A.P. French. Chapter 1.

"An electron is moving with speed 0.5c in the x direction enters a region of space where there is a uniform electric field in the y direction. Show that the x component of the velocity of the particle must decrease. "


3. The attempt at a solution

1. Is it because there is acceleration in the y-direction, the electron will emit radiation, hence losing energy/mass? But what has this got to do with relativity?

 

Re: Why does horizontal velocity of electron slow down when moving in an electric fie

Well..the acceleration is only in the y-direction so the x-component of the velocity remains unchanged while the vertical velocity is increasing?