 Whealton et al.use the technical definition, Fd("mv)/d"t", which can be expanded to F"m" dv/d"t" + d"m"/d"t" v. This second term has both delta mass and v, which is measured instantaneously; this term will, in general, cancel out the force from the inertial response terms predicted by Woodward.Woodward argued that the d"m"/d"t" v term does not represent a physical force on the device, because it vanishes in a frame where the device is momentarily stationary.