  Newton's second law: "force = time-derivative of momentum", may also be
defined for theories of gravitation endowing space-time with a curved metric.
Thus, Einstein's assumption of a geodesic motion may be rewritten in that form,
and it corresponds to a velocity-dependent gravity acceleration vector g. In
contrast, the investigated theory states that, in the preferred reference frame
assumed by the theory, vector g does not depend on the velocity. It recovers
geodesic motion only for a constant gravitational field. This leads to a
different equation for continuum dynamics, as compared with general relativity.
For a perfect fluid, this alternative dynamics predicts tenuous amounts of
matter production or destruction, by a reversible exchange with the
gravitational field. This exchange is completely determined by the dynamical
equation and the scalar equation of the gravitational field. In contrast, the
usual equation for relativistic continuum dynamics allows matter production
only if some additional field is assumed, and the production rate must be
phenomenologically postulated. With the alternative equation, the mass
conservation is very nearly recovered for a weak field. The explosion
(implosion) of a spherical compact body implies some matter production
(destruction).
