When a particle with charge q and mass m is introduced into a medium with a uniform

Question:

When a particle with charge q and mass m is introduced into a medium with a uniform field B such that the initial velocity of the particle u is perpendicular to B, as shown in Fig. 5-31 (P5.2), the magnetic force exerted on the particle causes it to move in a circle of radius a. By equating Fm to the centripetal force on the particle, determine a in terms of q, m, u, and B