Particle Motion in a Box: Solving for Electric and Magnetic Fields

[tronter]

If a particle moves in the +x direction with velocity $$v$$ in a box that has either a magnetic field or an electric field, and later it is moving in the +y direction with velocity $$v + 5$$, what field is in the box?So it would be an electric field because magnetic fields can't change speed?

 

[tronter]

anyone?