If a proton flies into a magnetic field I've seen the trajectory but I'm not sure about the directions of all the forces involved. In this picture [PLAIN]http://img7.imageshack.us/img7/7065/helicinbfld.jpg [Broken] I drew in a red, blue and yellow arrow. I assume the proton is moving in the direction the red arrow which is along the magnetic field lines but I have no idea what direction it was travelling before it entered the magnetic field. Was it travelling in the direction of the yellow arrow before entering the magnetic field then once it entered got stuck in that helicoidal trajectory because of the force exerted by the magnetic field? Those green lines I assume are the magnetic field lines. If so why is the proton travelling in their direction?