[moderator note: Thread title changed to make it descriptivie of the problem] 1. The problem statement, all variables and given/known data A particle having mass m and charge q is released at the origin in a region in which magnetic field and electric field are given by B= -B' j and E= E' k where j and k are unit vectors along y axis and z axis respectively. Find the speed of the particle as a function of z co ordinate. 2. Relevant questions: 3. The attempt at a solution The particle is released at the origin so at t=0 only electric force will act on the particle along z axis. As the particle gains some velocity, magnetic force will start acting and will push the particle in xz plane. Here the velocity has two components, x and z, so magnetic force will act on both these components. I calculted the net force in both x and z directions as: Fx= qBvz Fz= q(E-Bvx) As can be seen from the equations, the acc. in the both the directions is variable , so how come in the solution(picture), he has used the equation v2 - u2= 2as?