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. Drag the slider to adjust the particle's mass and watch how the radius of its path changes.
Heavier particles curve less in the same field
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).