Consider N = 2 particles with equal mass m, acted upon by mutually attractive forces. Suppose the particles are at diametrically opposite points of a circular orbit with radius r. The velocities are v 1 (t) and v 2 (t) = − v 1 (t), which are normal to forces F 1 (t) and F 2 (t) = − F 1 (t). The respective magnitudes are fixed at v and F. The average kinetic energy of the system in an interval of time from t 1 to t 2 is