So, a friend asked me this question in school and I've come up with an answer, I'm just not sure that it is right. Say you have a charged particle moving perpendicular to a magnetic field. There will be a force acting on it given by F=qvB. Now suppose your'e moving along at the same velocity as said particle. It's velocity respective to you is zero, so you should not observe any magnetic force even though there is one. My answer is this: From an inertial frame, the charged particle will be moving in a non-inertial reference frame. So if you move along at the same velocity as the particle, yours is an accelerated reference frame too and you don't observe any force on the particle because of pseudo forces. Is this right?