Magnetic fields don't push things forward or backward — they push perpendicular to motion. That's why a charged particle in a uniform field curves into a perfect circle, changing direction constantly but never changing speed. Change the particle's mass and watch how the radius of its path changes.
Raise the particle mass. Watch the radius.
A moving charged particle in a magnetic field experiences a force F = qvB when its velocity is perpendicular to the field. Because this magnetic force is always perpendicular to the particle's velocity, it does no work on the particle — it can't speed it up or slow it down. Instead, it constantly changes the particle's direction, bending its path into a circle. The radius of this circular path is r = mv/(qB), which means heavier or faster particles curve less sharply than lighter or slower ones. This is why particle accelerators use powerful magnets to steer beams: the magnetic force provides the centripetal acceleration needed for circular motion without ever changing the particles' kinetic energy.
A magnetic field pushes a moving charge sideways with force F = qvB, and because this force is always perpendicular to motion, the particle's speed stays constant while its path curves into a circle with radius r = mv/(qB).