[Notice that this is unlike the force due to an electric field, qE,
which can have a component parallel (or antiparallel) to motion and thus
can transfer energy in addition to momentum ]
We shall consider motion of a charged particle in a uniform magnetic
field First consider the case of v perpendicular to B The
perpendicular force, q v × B, acts as a centripetal force and
produces a circular motion perpendicular to the magnetic field