A perfect circle is also the locus of a particle (in a plane) moving solely under the influence of forces which are perpendicular to its velocity. The radius of this circle is given by:
r = m v^2 / F
where m is the mass of the particle, v is the magnitude of its velocity (i.e. its speed), and F is the magnitude of the net force acting on the particle. The particle will continue to move in the circle so long as the net force F remains perpendicular to the velocity.
See also circular motion.