  An important and useful relation is known to hold in two specific MOND
theories. It pertains to low-acceleration, isolated systems of pointlike
masses, m_p, at positions r_p, subject to gravitational forces F_p. It reads
sum_p r_p.F_p=-(2/3)(Ga0)^{1/2}[(\sum_p m_p)^{3/2}-\sum_p m_p^{3/2}]; a0 is the
MOND acceleration constant. Here I show that this relation holds in the
nonrelativistic limit of any modified-gravity MOND theory. It follows from only
the basic tenets of MOND (as applied to such theories): departure from standard
dynamics at accelerations below a0, and space-time scale invariance in the
nonrelativistic, low-acceleration limit. This implies space-dilatation
invariance of the static, gravitational-field equations, which, in turn, leads
to the above point-mass virial relation. Thus, the various MOND predictions and
tests based on this relation hold in any modified-gravity MOND theory. Since we
do not know that any of the existing MOND theories point in the right
direction, it is important to identify such predictions that hold in a much
larger class of theories. Among these predictions are the MOND two-body force
for arbitrary masses, and a general mass-velocity-dispersion relation of the
form sigma^2=(2/3)(MGa0)^{1/2}[1-\sum_p (m_p/M)^{3/2}], where M is the total
mass.
