 Mach effects are relativistic by nature, and considering a spaceship accelerating with a Mach effect thruster, the propellant is not accelerating with the ship, so the situation should be treated as an accelerating and therefore non-inertial reference frame, where F does not equal "m"a.In order for the effect to generate a useful propulsion it must result in a significant force and be generated in a way that can be controlled.It has been shown that under Hoyle-Narlikar's scalar-tensor theory of gravity the force generated by local mass-energy fluctuations is a higher order term that does not produce a significant force.