The time derivative of the Eq.2 is
that is, the sum over all the particles of the dot product of each particle's momentum with its position. A little calculation shows that
dG/dt = 2T + ∑i Fi . ri
where Fi is the total force exerted on the ith particle. Now let's compute the time average of both sides. Integrate both sides from time 0 to time t and then divide by t. Then take the limit as t -> ∞. On the left hand side, we get
limt -> ∞ (G(t) - G(0))/t = 0
since by assumption 2 the function G(t) is bounded. We thus obtain
0 = 2 +
at least if the time averages here are well-defined. We know that is well-defined by assumption 1. Why is that other time average well-defined? Well, the force on the ith particle is caused by all the other particles, so we have
∑i Fi . ri = ∑i ≠ j -grad(Vij) . ri
where Vij is the potential energy for the interaction between the ith and jth particles. Rewriting this a bit, we get