1) Solid mechanic 
Even if the average of the time derivative of G is only approximately zero, the virial theorem holds to the same degree of approximation. ...
For gravitationally bound systems in equilibrium, the total energy is equal to one half the time averaged potential energy. The virial theorem provides a way of estimating the mass of star clusters, galaxies, or clusters of galaxies from observations of the movement of individual members.
virial theorem 1. A theorem stating that the average kinetic and potential energies of a system are related to the system's total energy and internuclear separation. 2. The relation between the binding energy and kinetic energy of a system in equilibrium.