= Empirical testing of Newton's laws of motion
Testing requires independently constrained <force laws>, <mass> assignments, <initial conditions> and a common family of <inertial frames>. Defining the net <force> separately for every observation by $\mathbf F=m\mathbf a$ cannot test <Newton's second law>. With interaction parameters fixed, the same equation predicts new <trajectories>. Likewise, zero net <force> implies uniform <motion> within the second-law framework, but the existence and empirical identification of its <inertial frames> are extra content. Pairwise <Newton's third law> adds a constraint not supplied by the individual second-law equations.
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