The motion in a plane
perpendicular to B is as before a circular one, thereby producing
a helical motion (Fig 4 6) You have already learnt in earlier classes (See Class XI,
Chapter 4) that if r is the radius of the circular path of a particle,
then a force of  m v2 / r, acts perpendicular to the path towards
the centre of the circle, and is called the  centripetal force