  Nonrelativistic formalism is developed, which allows describing systems with
internal degrees of freedom in the scalar potential field $U$, which is a
function both on relative coordinates and time, and on relative speed and
accelerations. The equation for energy, which is an integral of motion when $U$
satisfies to certain differential condition, is derived for the general case.
For a free mass point all solutions of the equations of motion in the
center-of-inertia reference frame, moving with constant velocity, are found. As
a result, the center of mass follows along helical and more complicated
trajectories round a direction of motion of the center of inertia. This motion
can be interpreted as trembling movement (Zitterbewegung). On this basis a
conclusion is done that Zitterbewegung has purely classical origin, arising
even in a nonrelativistic case if internal degrees of freedom are taken into
account. The general equation of motion for a spin which can be interpreted
from positions of a classical mechanics is written down. Application of the
obtained results to the electron leads to new conception of electric charge,
sign of which corresponds to a sign of spin polarization.
