Let us choose the coordinate system xOy with the origin coinciding with the instantaneous position of the ball (Fig. 222).
The x-axis is "centripetal", while the y-axis is vertical just as the magnetic induction B.
The system of equations describing the motion of the ball (we assume that the ball moves in a circle counterclockwise) will be written in the form
Solving this system of equations, we obtain
The plus sign corresponds to the counterclockwise rotation of the ball, and the minus sign to the clockwise rotation (if we view from the top).