1. The problem statement, all variables and given/known data Hi. This is the problem I'm trying to solve: Consider the trajectory of a charged particle moving with a speed 0.8c in the x direction when it enters a large region in which there is a uniform electric field in the y direction. Show that the x velocity of the particle must actually decrease. What about the x component of momentum? 2. The attempt at a solution and Relevant Equations This one is for discussion mainly. Thus, these are my conclusions: When the charged particle enters on the Electric Field, F=qE is perpendicular to the Px-component of the momentum (as it is in y-direction). Thus, there's no force acting on the Px-component of the motion. So, we can use Momentum Conservation Principle. This is, Pxi= Pxf m0Vxi/(1-(Vxi2/c2))1/2 = m0Vxf/(1-(Vxf2+Vy2)/(c2))1/2 Solving for Vxf, Vxf2 = Vxi2(1-(Vy/c)2) Thus if Vy increases, then Vxf decreases. Am I correct?